eliminate stderr usage in std.Build make() functions

* Eliminate all uses of `std.debug.print` in make() functions, instead
  properly using the step failure reporting mechanism.
* Introduce the concept of skipped build steps. These do not cause the
  build to fail, and they do allow their dependants to run.
* RunStep gains a new flag, `skip_foreign_checks` which causes the
  RunStep to be skipped if stdio mode is `check` and the binary cannot
  be executed due to it being a foreign executable.
  - RunStep is improved to automatically use known interpreters to
    execute binaries if possible (integrating with flags such as
    -fqemu and -fwasmtime). It only does this after attempting a native
    execution and receiving a "exec file format" error.
  - Update RunStep to use an ArrayList for the checks rather than this
    ad-hoc reallocation/copying mechanism.
  - `expectStdOutEqual` now also implicitly adds an exit_code==0 check
    if there is not already an expected termination. This matches
    previously expected behavior from older API and can be overridden by
    directly setting the checks array.
* Add `dest_sub_path` to `InstallArtifactStep` which allows choosing an
  arbitrary subdirectory relative to the prefix, as well as overriding
  the basename.
  - Delete the custom InstallWithRename step that I found deep in the
    test/ directory.
* WriteFileStep will now update its step display name after the first
  file is added.
* Add missing stdout checks to various standalone test case build
  scripts.

10 files changed, 459 insertions, 233 deletions

diff --git a/lib/std/Build/CheckFileStep.zig b/lib/std/Build/CheckFileStep.zig
index f70a29840e..03b23d0b03 100644
--- a/lib/std/Build/CheckFileStep.zig
+++ b/lib/std/Build/CheckFileStep.zig
@@ -42,15 +42,14 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
 
     for (self.expected_matches) |expected_match| {
         if (mem.indexOf(u8, contents, expected_match) == null) {
-            std.debug.print(
+            return step.fail(
                 \\
-                \\========= Expected to find: ===================
+                \\========= expected to find: ===================
                 \\{s}
-                \\========= But file does not contain it: =======
+                \\========= but file does not contain it: =======
                 \\{s}
                 \\
             , .{ expected_match, contents });
-            return error.TestFailed;
         }
     }
 }
diff --git a/lib/std/Build/CheckObjectStep.zig b/lib/std/Build/CheckObjectStep.zig
index 57d280da0e..2a58850fab 100644
--- a/lib/std/Build/CheckObjectStep.zig
+++ b/lib/std/Build/CheckObjectStep.zig
@@ -133,7 +133,8 @@ const Action = struct {
     /// Will return true if the `phrase` is correctly parsed into an RPN program and
     /// its reduced, computed value compares using `op` with the expected value, either
     /// a literal or another extracted variable.
-    fn computeCmp(act: Action, gpa: Allocator, global_vars: anytype) !bool {
+    fn computeCmp(act: Action, step: *Step, global_vars: anytype) !bool {
+        const gpa = step.owner.allocator;
         var op_stack = std.ArrayList(enum { add, sub, mod, mul }).init(gpa);
         var values = std.ArrayList(u64).init(gpa);
 
@@ -150,11 +151,11 @@ const Action = struct {
             } else {
                 const val = std.fmt.parseInt(u64, next, 0) catch blk: {
                     break :blk global_vars.get(next) orelse {
-                        std.debug.print(
+                        try step.addError(
                             \\
-                            \\========= Variable was not extracted: ===========
+                            \\========= variable was not extracted: ===========
                             \\{s}
-                            \\
+                            \\=================================================
                         , .{next});
                         return error.UnknownVariable;
                     };
@@ -186,11 +187,11 @@ const Action = struct {
 
         const exp_value = switch (act.expected.?.value) {
             .variable => |name| global_vars.get(name) orelse {
-                std.debug.print(
+                try step.addError(
                     \\
-                    \\========= Variable was not extracted: ===========
+                    \\========= variable was not extracted: ===========
                     \\{s}
-                    \\
+                    \\=================================================
                 , .{name});
                 return error.UnknownVariable;
             },
@@ -323,14 +324,12 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     );
 
     const output = switch (self.obj_format) {
-        .macho => try MachODumper.parseAndDump(contents, .{
-            .gpa = gpa,
+        .macho => try MachODumper.parseAndDump(step, contents, .{
             .dump_symtab = self.dump_symtab,
         }),
         .elf => @panic("TODO elf parser"),
         .coff => @panic("TODO coff parser"),
-        .wasm => try WasmDumper.parseAndDump(contents, .{
-            .gpa = gpa,
+        .wasm => try WasmDumper.parseAndDump(step, contents, .{
             .dump_symtab = self.dump_symtab,
         }),
         else => unreachable,
@@ -346,54 +345,50 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
                     while (it.next()) |line| {
                         if (try act.match(line, &vars)) break;
                     } else {
-                        std.debug.print(
+                        return step.fail(
                             \\
-                            \\========= Expected to find: ==========================
+                            \\========= expected to find: ==========================
                             \\{s}
-                            \\========= But parsed file does not contain it: =======
+                            \\========= but parsed file does not contain it: =======
                             \\{s}
-                            \\
+                            \\======================================================
                         , .{ act.phrase, output });
-                        return error.TestFailed;
                     }
                 },
                 .not_present => {
                     while (it.next()) |line| {
                         if (try act.match(line, &vars)) {
-                            std.debug.print(
+                            return step.fail(
                                 \\
-                                \\========= Expected not to find: ===================
+                                \\========= expected not to find: ===================
                                 \\{s}
-                                \\========= But parsed file does contain it: ========
+                                \\========= but parsed file does contain it: ========
                                 \\{s}
-                                \\
+                                \\===================================================
                             , .{ act.phrase, output });
-                            return error.TestFailed;
                         }
                     }
                 },
                 .compute_cmp => {
-                    const res = act.computeCmp(gpa, vars) catch |err| switch (err) {
+                    const res = act.computeCmp(step, vars) catch |err| switch (err) {
                         error.UnknownVariable => {
-                            std.debug.print(
-                                \\========= From parsed file: =====================
+                            return step.fail(
+                                \\========= from parsed file: =====================
                                 \\{s}
-                                \\
+                                \\=================================================
                             , .{output});
-                            return error.TestFailed;
                         },
                         else => |e| return e,
                     };
                     if (!res) {
-                        std.debug.print(
+                        return step.fail(
                             \\
-                            \\========= Comparison failed for action: ===========
+                            \\========= comparison failed for action: ===========
                             \\{s} {}
-                            \\========= From parsed file: =======================
+                            \\========= from parsed file: =======================
                             \\{s}
-                            \\
+                            \\===================================================
                         , .{ act.phrase, act.expected.?, output });
-                        return error.TestFailed;
                     }
                 },
             }
@@ -402,7 +397,6 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
 }
 
 const Opts = struct {
-    gpa: ?Allocator = null,
     dump_symtab: bool = false,
 };
 
@@ -410,8 +404,8 @@ const MachODumper = struct {
     const LoadCommandIterator = macho.LoadCommandIterator;
     const symtab_label = "symtab";
 
-    fn parseAndDump(bytes: []align(@alignOf(u64)) const u8, opts: Opts) ![]const u8 {
-        const gpa = opts.gpa orelse unreachable; // MachO dumper requires an allocator
+    fn parseAndDump(step: *Step, bytes: []align(@alignOf(u64)) const u8, opts: Opts) ![]const u8 {
+        const gpa = step.owner.allocator;
         var stream = std.io.fixedBufferStream(bytes);
         const reader = stream.reader();
 
@@ -693,8 +687,8 @@ const MachODumper = struct {
 const WasmDumper = struct {
     const symtab_label = "symbols";
 
-    fn parseAndDump(bytes: []const u8, opts: Opts) ![]const u8 {
-        const gpa = opts.gpa orelse unreachable; // Wasm dumper requires an allocator
+    fn parseAndDump(step: *Step, bytes: []const u8, opts: Opts) ![]const u8 {
+        const gpa = step.owner.allocator;
         if (opts.dump_symtab) {
             @panic("TODO: Implement symbol table parsing and dumping");
         }
@@ -715,20 +709,24 @@ const WasmDumper = struct {
         const writer = output.writer();
 
         while (reader.readByte()) |current_byte| {
-            const section = std.meta.intToEnum(std.wasm.Section, current_byte) catch |err| {
-                std.debug.print("Found invalid section id '{d}'\n", .{current_byte});
-                return err;
+            const section = std.meta.intToEnum(std.wasm.Section, current_byte) catch {
+                return step.fail("Found invalid section id '{d}'", .{current_byte});
             };
 
             const section_length = try std.leb.readULEB128(u32, reader);
-            try parseAndDumpSection(section, bytes[fbs.pos..][0..section_length], writer);
+            try parseAndDumpSection(step, section, bytes[fbs.pos..][0..section_length], writer);
             fbs.pos += section_length;
         } else |_| {} // reached end of stream
 
         return output.toOwnedSlice();
     }
 
-    fn parseAndDumpSection(section: std.wasm.Section, data: []const u8, writer: anytype) !void {
+    fn parseAndDumpSection(
+        step: *Step,
+        section: std.wasm.Section,
+        data: []const u8,
+        writer: anytype,
+    ) !void {
         var fbs = std.io.fixedBufferStream(data);
         const reader = fbs.reader();
 
@@ -751,7 +749,7 @@ const WasmDumper = struct {
             => {
                 const entries = try std.leb.readULEB128(u32, reader);
                 try writer.print("\nentries {d}\n", .{entries});
-                try dumpSection(section, data[fbs.pos..], entries, writer);
+                try dumpSection(step, section, data[fbs.pos..], entries, writer);
             },
             .custom => {
                 const name_length = try std.leb.readULEB128(u32, reader);
@@ -760,7 +758,7 @@ const WasmDumper = struct {
                 try writer.print("\nname {s}\n", .{name});
 
                 if (mem.eql(u8, name, "name")) {
-                    try parseDumpNames(reader, writer, data);
+                    try parseDumpNames(step, reader, writer, data);
                 } else if (mem.eql(u8, name, "producers")) {
                     try parseDumpProducers(reader, writer, data);
                 } else if (mem.eql(u8, name, "target_features")) {
@@ -776,7 +774,7 @@ const WasmDumper = struct {
         }
     }
 
-    fn dumpSection(section: std.wasm.Section, data: []const u8, entries: u32, writer: anytype) !void {
+    fn dumpSection(step: *Step, section: std.wasm.Section, data: []const u8, entries: u32, writer: anytype) !void {
         var fbs = std.io.fixedBufferStream(data);
         const reader = fbs.reader();
 
@@ -786,19 +784,18 @@ const WasmDumper = struct {
                 while (i < entries) : (i += 1) {
                     const func_type = try reader.readByte();
                     if (func_type != std.wasm.function_type) {
-                        std.debug.print("Expected function type, found byte '{d}'\n", .{func_type});
-                        return error.UnexpectedByte;
+                        return step.fail("expected function type, found byte '{d}'", .{func_type});
                     }
                     const params = try std.leb.readULEB128(u32, reader);
                     try writer.print("params {d}\n", .{params});
                     var index: u32 = 0;
                     while (index < params) : (index += 1) {
-                        try parseDumpType(std.wasm.Valtype, reader, writer);
+                        try parseDumpType(step, std.wasm.Valtype, reader, writer);
                     } else index = 0;
                     const returns = try std.leb.readULEB128(u32, reader);
                     try writer.print("returns {d}\n", .{returns});
                     while (index < returns) : (index += 1) {
-                        try parseDumpType(std.wasm.Valtype, reader, writer);
+                        try parseDumpType(step, std.wasm.Valtype, reader, writer);
                     }
                 }
             },
@@ -812,9 +809,8 @@ const WasmDumper = struct {
                     const name = data[fbs.pos..][0..name_len];
                     fbs.pos += name_len;
 
-                    const kind = std.meta.intToEnum(std.wasm.ExternalKind, try reader.readByte()) catch |err| {
-                        std.debug.print("Invalid import kind\n", .{});
-                        return err;
+                    const kind = std.meta.intToEnum(std.wasm.ExternalKind, try reader.readByte()) catch {
+                        return step.fail("invalid import kind", .{});
                     };
 
                     try writer.print(
@@ -831,11 +827,11 @@ const WasmDumper = struct {
                             try parseDumpLimits(reader, writer);
                         },
                         .global => {
-                            try parseDumpType(std.wasm.Valtype, reader, writer);
+                            try parseDumpType(step, std.wasm.Valtype, reader, writer);
                             try writer.print("mutable {}\n", .{0x01 == try std.leb.readULEB128(u32, reader)});
                         },
                         .table => {
-                            try parseDumpType(std.wasm.RefType, reader, writer);
+                            try parseDumpType(step, std.wasm.RefType, reader, writer);
                             try parseDumpLimits(reader, writer);
                         },
                     }
@@ -850,7 +846,7 @@ const WasmDumper = struct {
             .table => {
                 var i: u32 = 0;
                 while (i < entries) : (i += 1) {
-                    try parseDumpType(std.wasm.RefType, reader, writer);
+                    try parseDumpType(step, std.wasm.RefType, reader, writer);
                     try parseDumpLimits(reader, writer);
                 }
             },
@@ -863,9 +859,9 @@ const WasmDumper = struct {
             .global => {
                 var i: u32 = 0;
                 while (i < entries) : (i += 1) {
-                    try parseDumpType(std.wasm.Valtype, reader, writer);
+                    try parseDumpType(step, std.wasm.Valtype, reader, writer);
                     try writer.print("mutable {}\n", .{0x01 == try std.leb.readULEB128(u1, reader)});
-                    try parseDumpInit(reader, writer);
+                    try parseDumpInit(step, reader, writer);
                 }
             },
             .@"export" => {
@@ -875,9 +871,8 @@ const WasmDumper = struct {
                     const name = data[fbs.pos..][0..name_len];
                     fbs.pos += name_len;
                     const kind_byte = try std.leb.readULEB128(u8, reader);
-                    const kind = std.meta.intToEnum(std.wasm.ExternalKind, kind_byte) catch |err| {
-                        std.debug.print("invalid export kind value '{d}'\n", .{kind_byte});
-                        return err;
+                    const kind = std.meta.intToEnum(std.wasm.ExternalKind, kind_byte) catch {
+                        return step.fail("invalid export kind value '{d}'", .{kind_byte});
                     };
                     const index = try std.leb.readULEB128(u32, reader);
                     try writer.print(
@@ -892,7 +887,7 @@ const WasmDumper = struct {
                 var i: u32 = 0;
                 while (i < entries) : (i += 1) {
                     try writer.print("table index {d}\n", .{try std.leb.readULEB128(u32, reader)});
-                    try parseDumpInit(reader, writer);
+                    try parseDumpInit(step, reader, writer);
 
                     const function_indexes = try std.leb.readULEB128(u32, reader);
                     var function_index: u32 = 0;
@@ -908,7 +903,7 @@ const WasmDumper = struct {
                 while (i < entries) : (i += 1) {
                     const index = try std.leb.readULEB128(u32, reader);
                     try writer.print("memory index 0x{x}\n", .{index});
-                    try parseDumpInit(reader, writer);
+                    try parseDumpInit(step, reader, writer);
                     const size = try std.leb.readULEB128(u32, reader);
                     try writer.print("size {d}\n", .{size});
                     try reader.skipBytes(size, .{}); // we do not care about the content of the segments
@@ -918,11 +913,10 @@ const WasmDumper = struct {
         }
     }
 
-    fn parseDumpType(comptime WasmType: type, reader: anytype, writer: anytype) !void {
+    fn parseDumpType(step: *Step, comptime WasmType: type, reader: anytype, writer: anytype) !void {
         const type_byte = try reader.readByte();
-        const valtype = std.meta.intToEnum(WasmType, type_byte) catch |err| {
-            std.debug.print("Invalid wasm type value '{d}'\n", .{type_byte});
-            return err;
+        const valtype = std.meta.intToEnum(WasmType, type_byte) catch {
+            return step.fail("Invalid wasm type value '{d}'", .{type_byte});
         };
         try writer.print("type {s}\n", .{@tagName(valtype)});
     }
@@ -937,11 +931,10 @@ const WasmDumper = struct {
         }
     }
 
-    fn parseDumpInit(reader: anytype, writer: anytype) !void {
+    fn parseDumpInit(step: *Step, reader: anytype, writer: anytype) !void {
         const byte = try std.leb.readULEB128(u8, reader);
-        const opcode = std.meta.intToEnum(std.wasm.Opcode, byte) catch |err| {
-            std.debug.print("invalid wasm opcode '{d}'\n", .{byte});
-            return err;
+        const opcode = std.meta.intToEnum(std.wasm.Opcode, byte) catch {
+            return step.fail("invalid wasm opcode '{d}'", .{byte});
         };
         switch (opcode) {
             .i32_const => try writer.print("i32.const {x}\n", .{try std.leb.readILEB128(i32, reader)}),
@@ -953,14 +946,13 @@ const WasmDumper = struct {
         }
         const end_opcode = try std.leb.readULEB128(u8, reader);
         if (end_opcode != std.wasm.opcode(.end)) {
-            std.debug.print("expected 'end' opcode in init expression\n", .{});
-            return error.MissingEndOpcode;
+            return step.fail("expected 'end' opcode in init expression", .{});
         }
     }
 
-    fn parseDumpNames(reader: anytype, writer: anytype, data: []const u8) !void {
+    fn parseDumpNames(step: *Step, reader: anytype, writer: anytype, data: []const u8) !void {
         while (reader.context.pos < data.len) {
-            try parseDumpType(std.wasm.NameSubsection, reader, writer);
+            try parseDumpType(step, std.wasm.NameSubsection, reader, writer);
             const size = try std.leb.readULEB128(u32, reader);
             const entries = try std.leb.readULEB128(u32, reader);
             try writer.print(
diff --git a/lib/std/Build/CompileStep.zig b/lib/std/Build/CompileStep.zig
index f1a8f71334..99d99694c3 100644
--- a/lib/std/Build/CompileStep.zig
+++ b/lib/std/Build/CompileStep.zig
@@ -538,8 +538,7 @@ pub fn run(cs: *CompileStep) *RunStep {
 }
 
 pub fn checkObject(self: *CompileStep, obj_format: std.Target.ObjectFormat) *CheckObjectStep {
-    const b = self.step.owner;
-    return CheckObjectStep.create(b, self.getOutputSource(), obj_format);
+    return CheckObjectStep.create(self.step.owner, self.getOutputSource(), obj_format);
 }
 
 pub fn setLinkerScriptPath(self: *CompileStep, source: FileSource) void {
diff --git a/lib/std/Build/ConfigHeaderStep.zig b/lib/std/Build/ConfigHeaderStep.zig
index 8b4c05bab7..37b04e75a4 100644
--- a/lib/std/Build/ConfigHeaderStep.zig
+++ b/lib/std/Build/ConfigHeaderStep.zig
@@ -192,13 +192,13 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
             try output.appendSlice(c_generated_line);
             const src_path = file_source.getPath(b);
             const contents = try std.fs.cwd().readFileAlloc(gpa, src_path, self.max_bytes);
-            try render_autoconf(contents, &output, self.values, src_path);
+            try render_autoconf(step, contents, &output, self.values, src_path);
         },
         .cmake => |file_source| {
             try output.appendSlice(c_generated_line);
             const src_path = file_source.getPath(b);
             const contents = try std.fs.cwd().readFileAlloc(gpa, src_path, self.max_bytes);
-            try render_cmake(contents, &output, self.values, src_path);
+            try render_cmake(step, contents, &output, self.values, src_path);
         },
         .blank => {
             try output.appendSlice(c_generated_line);
@@ -234,8 +234,7 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
         output_dir;
 
     var dir = std.fs.cwd().makeOpenPath(sub_dir_path, .{}) catch |err| {
-        std.debug.print("unable to make path {s}: {s}\n", .{ output_dir, @errorName(err) });
-        return err;
+        return step.fail("unable to make path '{s}': {s}", .{ output_dir, @errorName(err) });
     };
     defer dir.close();
 
@@ -247,6 +246,7 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
 }
 
 fn render_autoconf(
+    step: *Step,
     contents: []const u8,
     output: *std.ArrayList(u8),
     values: std.StringArrayHashMap(Value),
@@ -273,7 +273,7 @@ fn render_autoconf(
         }
         const name = it.rest();
         const kv = values_copy.fetchSwapRemove(name) orelse {
-            std.debug.print("{s}:{d}: error: unspecified config header value: '{s}'\n", .{
+            try step.addError("{s}:{d}: error: unspecified config header value: '{s}'", .{
                 src_path, line_index + 1, name,
             });
             any_errors = true;
@@ -283,15 +283,17 @@ fn render_autoconf(
     }
 
     for (values_copy.keys()) |name| {
-        std.debug.print("{s}: error: config header value unused: '{s}'\n", .{ src_path, name });
+        try step.addError("{s}: error: config header value unused: '{s}'", .{ src_path, name });
+        any_errors = true;
     }
 
     if (any_errors) {
-        return error.HeaderConfigFailed;
+        return error.MakeFailed;
     }
 }
 
 fn render_cmake(
+    step: *Step,
     contents: []const u8,
     output: *std.ArrayList(u8),
     values: std.StringArrayHashMap(Value),
@@ -317,14 +319,14 @@ fn render_cmake(
             continue;
         }
         const name = it.next() orelse {
-            std.debug.print("{s}:{d}: error: missing define name\n", .{
+            try step.addError("{s}:{d}: error: missing define name", .{
                 src_path, line_index + 1,
             });
             any_errors = true;
             continue;
         };
         const kv = values_copy.fetchSwapRemove(name) orelse {
-            std.debug.print("{s}:{d}: error: unspecified config header value: '{s}'\n", .{
+            try step.addError("{s}:{d}: error: unspecified config header value: '{s}'", .{
                 src_path, line_index + 1, name,
             });
             any_errors = true;
@@ -334,7 +336,8 @@ fn render_cmake(
     }
 
     for (values_copy.keys()) |name| {
-        std.debug.print("{s}: error: config header value unused: '{s}'\n", .{ src_path, name });
+        try step.addError("{s}: error: config header value unused: '{s}'", .{ src_path, name });
+        any_errors = true;
     }
 
     if (any_errors) {
diff --git a/lib/std/Build/InstallArtifactStep.zig b/lib/std/Build/InstallArtifactStep.zig
index 7e35d0a5ee..377cba301c 100644
--- a/lib/std/Build/InstallArtifactStep.zig
+++ b/lib/std/Build/InstallArtifactStep.zig
@@ -12,6 +12,9 @@ artifact: *CompileStep,
 dest_dir: InstallDir,
 pdb_dir: ?InstallDir,
 h_dir: ?InstallDir,
+/// If non-null, adds additional path components relative to dest_dir, and
+/// overrides the basename of the CompileStep.
+dest_sub_path: ?[]const u8,
 
 pub fn create(owner: *std.Build, artifact: *CompileStep) *InstallArtifactStep {
     if (artifact.install_step) |s| return s;
@@ -40,6 +43,7 @@ pub fn create(owner: *std.Build, artifact: *CompileStep) *InstallArtifactStep {
             }
         } else null,
         .h_dir = if (artifact.kind == .lib and artifact.emit_h) .header else null,
+        .dest_sub_path = null,
     };
     self.step.dependOn(&artifact.step);
     artifact.install_step = self;
@@ -71,7 +75,9 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     const self = @fieldParentPtr(InstallArtifactStep, "step", step);
     const dest_builder = self.dest_builder;
 
-    const full_dest_path = dest_builder.getInstallPath(self.dest_dir, self.artifact.out_filename);
+    const dest_sub_path = if (self.dest_sub_path) |sub_path| sub_path else self.artifact.out_filename;
+    const full_dest_path = dest_builder.getInstallPath(self.dest_dir, dest_sub_path);
+
     try src_builder.updateFile(
         self.artifact.getOutputSource().getPath(src_builder),
         full_dest_path,
diff --git a/lib/std/Build/ObjCopyStep.zig b/lib/std/Build/ObjCopyStep.zig
index 7199431ee6..13046b3efe 100644
--- a/lib/std/Build/ObjCopyStep.zig
+++ b/lib/std/Build/ObjCopyStep.zig
@@ -95,8 +95,7 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     const full_dest_path = try b.cache_root.join(b.allocator, &.{ "o", &digest, self.basename });
     const cache_path = "o" ++ fs.path.sep_str ++ digest;
     b.cache_root.handle.makePath(cache_path) catch |err| {
-        std.debug.print("unable to make path {s}: {s}\n", .{ cache_path, @errorName(err) });
-        return err;
+        return step.fail("unable to make path {s}: {s}", .{ cache_path, @errorName(err) });
     };
 
     var argv = std.ArrayList([]const u8).init(b.allocator);
diff --git a/lib/std/Build/RunStep.zig b/lib/std/Build/RunStep.zig
index 087483fea8..9304cc758e 100644
--- a/lib/std/Build/RunStep.zig
+++ b/lib/std/Build/RunStep.zig
@@ -10,6 +10,7 @@ const ArrayList = std.ArrayList;
 const EnvMap = process.EnvMap;
 const Allocator = mem.Allocator;
 const ExecError = std.Build.ExecError;
+const assert = std.debug.assert;
 
 const RunStep = @This();
 
@@ -54,6 +55,8 @@ rename_step_with_output_arg: bool = true,
 /// Command-line arguments such as -fqemu and -fwasmtime may affect whether a
 /// binary is detected as foreign, as well as system configuration such as
 /// Rosetta (macOS) and binfmt_misc (Linux).
+/// If this RunStep is considered to have side-effects, then this flag does
+/// nothing.
 skip_foreign_checks: bool = false,
 
 /// If stderr or stdout exceeds this amount, the child process is killed and
@@ -79,7 +82,7 @@ pub const StdIo = union(enum) {
     /// conditions.
     /// Note that an explicit check for exit code 0 needs to be added to this
     /// list if such a check is desireable.
-    check: []const Check,
+    check: std.ArrayList(Check),
 
     pub const Check = union(enum) {
         expect_stderr_exact: []const u8,
@@ -214,14 +217,20 @@ pub fn setEnvironmentVariable(self: *RunStep, key: []const u8, value: []const u8
     env_map.put(b.dupe(key), b.dupe(value)) catch @panic("unhandled error");
 }
 
+/// Adds a check for exact stderr match. Does not add any other checks.
 pub fn expectStdErrEqual(self: *RunStep, bytes: []const u8) void {
     const new_check: StdIo.Check = .{ .expect_stderr_exact = self.step.owner.dupe(bytes) };
     self.addCheck(new_check);
 }
 
+/// Adds a check for exact stdout match as well as a check for exit code 0, if
+/// there is not already an expected termination check.
 pub fn expectStdOutEqual(self: *RunStep, bytes: []const u8) void {
     const new_check: StdIo.Check = .{ .expect_stdout_exact = self.step.owner.dupe(bytes) };
     self.addCheck(new_check);
+    if (!self.hasTermCheck()) {
+        self.expectExitCode(0);
+    }
 }
 
 pub fn expectExitCode(self: *RunStep, code: u8) void {
@@ -229,19 +238,21 @@ pub fn expectExitCode(self: *RunStep, code: u8) void {
     self.addCheck(new_check);
 }
 
+pub fn hasTermCheck(self: RunStep) bool {
+    for (self.stdio.check.items) |check| switch (check) {
+        .expect_term => return true,
+        else => continue,
+    };
+    return false;
+}
+
 pub fn addCheck(self: *RunStep, new_check: StdIo.Check) void {
-    const arena = self.step.owner.allocator;
     switch (self.stdio) {
         .infer_from_args => {
-            const list = arena.create([1]StdIo.Check) catch @panic("OOM");
-            list.* = .{new_check};
-            self.stdio = .{ .check = list };
-        },
-        .check => |checks| {
-            const new_list = arena.alloc(StdIo.Check, checks.len + 1) catch @panic("OOM");
-            std.mem.copy(StdIo.Check, new_list, checks);
-            new_list[checks.len] = new_check;
+            self.stdio = .{ .check = std.ArrayList(StdIo.Check).init(self.step.owner.allocator) };
+            self.stdio.check.append(new_check) catch @panic("OOM");
         },
+        .check => |*checks| checks.append(new_check) catch @panic("OOM"),
         else => @panic("illegal call to addCheck: conflicting helper method calls. Suggest to directly set stdio field of RunStep instead"),
     }
 }
@@ -298,14 +309,15 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     _ = prog_node;
 
     const b = step.owner;
+    const arena = b.allocator;
     const self = @fieldParentPtr(RunStep, "step", step);
     const has_side_effects = self.hasSideEffects();
 
-    var argv_list = ArrayList([]const u8).init(b.allocator);
+    var argv_list = ArrayList([]const u8).init(arena);
     var output_placeholders = ArrayList(struct {
         index: usize,
         output: Arg.Output,
-    }).init(b.allocator);
+    }).init(arena);
 
     var man = b.cache.obtain();
     defer man.deinit();
@@ -357,7 +369,7 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
             const digest = man.final();
             for (output_placeholders.items) |placeholder| {
                 placeholder.output.generated_file.path = try b.cache_root.join(
-                    b.allocator,
+                    arena,
                     &.{ "o", &digest, placeholder.output.basename },
                 );
             }
@@ -367,30 +379,21 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
         const digest = man.final();
 
         for (output_placeholders.items) |placeholder| {
-            const output_path = try b.cache_root.join(
-                b.allocator,
-                &.{ "o", &digest, placeholder.output.basename },
-            );
-            const output_dir = fs.path.dirname(output_path).?;
-            fs.cwd().makePath(output_dir) catch |err| {
-                std.debug.print("unable to make path {s}: {s}\n", .{ output_dir, @errorName(err) });
-                return err;
+            const output_components = .{ "o", &digest, placeholder.output.basename };
+            const output_sub_path = try fs.path.join(arena, &output_components);
+            const output_sub_dir_path = fs.path.dirname(output_sub_path).?;
+            b.cache_root.handle.makePath(output_sub_dir_path) catch |err| {
+                return step.fail("unable to make path '{}{s}': {s}", .{
+                    b.cache_root, output_sub_dir_path, @errorName(err),
+                });
             };
-
+            const output_path = try b.cache_root.join(arena, &output_components);
             placeholder.output.generated_file.path = output_path;
             argv_list.items[placeholder.index] = output_path;
         }
     }
 
-    try runCommand(
-        step,
-        self.cwd,
-        argv_list.items,
-        self.env_map,
-        self.stdio,
-        has_side_effects,
-        self.max_stdio_size,
-    );
+    try runCommand(self, argv_list.items, has_side_effects);
 
     if (!has_side_effects) {
         try man.writeManifest();
@@ -442,92 +445,150 @@ fn termMatches(expected: ?std.ChildProcess.Term, actual: std.ChildProcess.Term)
     };
 }
 
-fn runCommand(
-    step: *Step,
-    opt_cwd: ?[]const u8,
-    argv: []const []const u8,
-    env_map: ?*EnvMap,
-    stdio: StdIo,
-    has_side_effects: bool,
-    max_stdio_size: usize,
-) !void {
+fn runCommand(self: *RunStep, argv: []const []const u8, has_side_effects: bool) !void {
+    const step = &self.step;
     const b = step.owner;
     const arena = b.allocator;
-    const cwd = if (opt_cwd) |cwd| b.pathFromRoot(cwd) else b.build_root.path;
 
-    try step.handleChildProcUnsupported(opt_cwd, argv);
-    try Step.handleVerbose(step.owner, opt_cwd, argv);
-
-    var child = std.ChildProcess.init(argv, arena);
-    child.cwd = cwd;
-    child.env_map = env_map orelse b.env_map;
-
-    child.stdin_behavior = switch (stdio) {
-        .infer_from_args => if (has_side_effects) .Inherit else .Ignore,
-        .inherit => .Inherit,
-        .check => .Close,
-    };
-    child.stdout_behavior = switch (stdio) {
-        .infer_from_args => if (has_side_effects) .Inherit else .Ignore,
-        .inherit => .Inherit,
-        .check => |checks| if (checksContainStdout(checks)) .Pipe else .Ignore,
-    };
-    child.stderr_behavior = switch (stdio) {
-        .infer_from_args => if (has_side_effects) .Inherit else .Pipe,
-        .inherit => .Inherit,
-        .check => .Pipe,
-    };
-
-    child.spawn() catch |err| return step.fail("unable to spawn {s}: {s}", .{
-        argv[0], @errorName(err),
-    });
+    try step.handleChildProcUnsupported(self.cwd, argv);
+    try Step.handleVerbose(step.owner, self.cwd, argv);
 
     var stdout_bytes: ?[]const u8 = null;
     var stderr_bytes: ?[]const u8 = null;
 
-    if (child.stdout) |stdout| {
-        if (child.stderr) |stderr| {
-            var poller = std.io.poll(arena, enum { stdout, stderr }, .{
-                .stdout = stdout,
-                .stderr = stderr,
-            });
-            defer poller.deinit();
+    const term = spawnChildAndCollect(self, argv, &stdout_bytes, &stderr_bytes, has_side_effects) catch |err| term: {
+        if (err == error.InvalidExe) interpret: {
+            // TODO: learn the target from the binary directly rather than from
+            // relying on it being a CompileStep. This will make this logic
+            // work even for the edge case that the binary was produced by a
+            // third party.
+            const exe = switch (self.argv.items[0]) {
+                .artifact => |exe| exe,
+                else => break :interpret,
+            };
+            if (exe.kind != .exe) break :interpret;
+
+            var interp_argv = std.ArrayList([]const u8).init(b.allocator);
+            defer interp_argv.deinit();
+
+            const need_cross_glibc = exe.target.isGnuLibC() and exe.is_linking_libc;
+            switch (b.host.getExternalExecutor(exe.target_info, .{
+                .qemu_fixes_dl = need_cross_glibc and b.glibc_runtimes_dir != null,
+                .link_libc = exe.is_linking_libc,
+            })) {
+                .native, .rosetta => {
+                    if (self.stdio == .check and self.skip_foreign_checks)
+                        return error.MakeSkipped;
+
+                    break :interpret;
+                },
+                .wine => |bin_name| {
+                    if (b.enable_wine) {
+                        try interp_argv.append(bin_name);
+                    } else {
+                        return failForeign(self, "-fwine", argv[0], exe);
+                    }
+                },
+                .qemu => |bin_name| {
+                    if (b.enable_qemu) {
+                        const glibc_dir_arg = if (need_cross_glibc)
+                            b.glibc_runtimes_dir orelse return
+                        else
+                            null;
+
+                        try interp_argv.append(bin_name);
+
+                        if (glibc_dir_arg) |dir| {
+                            // TODO look into making this a call to `linuxTriple`. This
+                            // needs the directory to be called "i686" rather than
+                            // "x86" which is why we do it manually here.
+                            const fmt_str = "{s}" ++ fs.path.sep_str ++ "{s}-{s}-{s}";
+                            const cpu_arch = exe.target.getCpuArch();
+                            const os_tag = exe.target.getOsTag();
+                            const abi = exe.target.getAbi();
+                            const cpu_arch_name: []const u8 = if (cpu_arch == .x86)
+                                "i686"
+                            else
+                                @tagName(cpu_arch);
+                            const full_dir = try std.fmt.allocPrint(b.allocator, fmt_str, .{
+                                dir, cpu_arch_name, @tagName(os_tag), @tagName(abi),
+                            });
+
+                            try interp_argv.append("-L");
+                            try interp_argv.append(full_dir);
+                        }
+                    } else {
+                        return failForeign(self, "-fqemu", argv[0], exe);
+                    }
+                },
+                .darling => |bin_name| {
+                    if (b.enable_darling) {
+                        try interp_argv.append(bin_name);
+                    } else {
+                        return failForeign(self, "-fdarling", argv[0], exe);
+                    }
+                },
+                .wasmtime => |bin_name| {
+                    if (b.enable_wasmtime) {
+                        try interp_argv.append(bin_name);
+                        try interp_argv.append("--dir=.");
+                    } else {
+                        return failForeign(self, "-fwasmtime", argv[0], exe);
+                    }
+                },
+                .bad_dl => |foreign_dl| {
+                    if (self.stdio == .check and self.skip_foreign_checks)
+                        return error.MakeSkipped;
+
+                    const host_dl = b.host.dynamic_linker.get() orelse "(none)";
 
-            while (try poller.poll()) {
-                if (poller.fifo(.stdout).count > max_stdio_size)
-                    return error.StdoutStreamTooLong;
-                if (poller.fifo(.stderr).count > max_stdio_size)
-                    return error.StderrStreamTooLong;
+                    return step.fail(
+                        \\the host system is unable to execute binaries from the target
+                        \\  because the host dynamic linker is '{s}',
+                        \\  while the target dynamic linker is '{s}'.
+                        \\  consider setting the dynamic linker or enabling skip_foreign_checks in the Run step
+                    , .{ host_dl, foreign_dl });
+                },
+                .bad_os_or_cpu => {
+                    if (self.stdio == .check and self.skip_foreign_checks)
+                        return error.MakeSkipped;
+
+                    const host_name = try b.host.target.zigTriple(b.allocator);
+                    const foreign_name = try exe.target.zigTriple(b.allocator);
+
+                    return step.fail("the host system ({s}) is unable to execute binaries from the target ({s})", .{
+                        host_name, foreign_name,
+                    });
+                },
             }
 
-            stdout_bytes = try poller.fifo(.stdout).toOwnedSlice();
-            stderr_bytes = try poller.fifo(.stderr).toOwnedSlice();
-        } else {
-            stdout_bytes = try stdout.reader().readAllAlloc(arena, max_stdio_size);
-        }
-    } else if (child.stderr) |stderr| {
-        stderr_bytes = try stderr.reader().readAllAlloc(arena, max_stdio_size);
-    }
+            if (exe.target.isWindows()) {
+                // On Windows we don't have rpaths so we have to add .dll search paths to PATH
+                RunStep.addPathForDynLibsInternal(&self.step, b, exe);
+            }
 
-    if (stderr_bytes) |stderr| if (stderr.len > 0) {
-        const stderr_is_diagnostic = switch (stdio) {
-            .check => |checks| !checksContainStderr(checks),
-            else => true,
-        };
-        if (stderr_is_diagnostic) {
-            try step.result_error_msgs.append(arena, stderr);
+            try interp_argv.append(argv[0]);
+
+            try Step.handleVerbose(step.owner, self.cwd, interp_argv.items);
+
+            assert(stdout_bytes == null);
+            assert(stderr_bytes == null);
+            break :term spawnChildAndCollect(self, interp_argv.items, &stdout_bytes, &stderr_bytes, has_side_effects) catch |inner_err| {
+                return step.fail("unable to spawn {s}: {s}", .{
+                    interp_argv.items[0], @errorName(inner_err),
+                });
+            };
         }
-    };
 
-    const term = child.wait() catch |err| {
-        return step.fail("unable to wait for {s}: {s}", .{ argv[0], @errorName(err) });
+        return step.fail("unable to spawn {s}: {s}", .{ argv[0], @errorName(err) });
     };
 
-    switch (stdio) {
-        .check => |checks| for (checks) |check| switch (check) {
+    switch (self.stdio) {
+        .check => |checks| for (checks.items) |check| switch (check) {
             .expect_stderr_exact => |expected_bytes| {
                 if (!mem.eql(u8, expected_bytes, stderr_bytes.?)) {
                     return step.fail(
+                        \\
                         \\========= expected this stderr: =========
                         \\{s}
                         \\========= but found: ====================
@@ -537,13 +598,14 @@ fn runCommand(
                     , .{
                         expected_bytes,
                         stderr_bytes.?,
-                        try Step.allocPrintCmd(arena, opt_cwd, argv),
+                        try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stderr_match => |match| {
                 if (mem.indexOf(u8, stderr_bytes.?, match) == null) {
                     return step.fail(
+                        \\
                         \\========= expected to find in stderr: =========
                         \\{s}
                         \\========= but stderr does not contain it: =====
@@ -553,13 +615,14 @@ fn runCommand(
                     , .{
                         match,
                         stderr_bytes.?,
-                        try Step.allocPrintCmd(arena, opt_cwd, argv),
+                        try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stdout_exact => |expected_bytes| {
                 if (!mem.eql(u8, expected_bytes, stdout_bytes.?)) {
                     return step.fail(
+                        \\
                         \\========= expected this stdout: =========
                         \\{s}
                         \\========= but found: ====================
@@ -569,13 +632,14 @@ fn runCommand(
                     , .{
                         expected_bytes,
                         stdout_bytes.?,
-                        try Step.allocPrintCmd(arena, opt_cwd, argv),
+                        try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
             .expect_stdout_match => |match| {
                 if (mem.indexOf(u8, stdout_bytes.?, match) == null) {
                     return step.fail(
+                        \\
                         \\========= expected to find in stdout: =========
                         \\{s}
                         \\========= but stdout does not contain it: =====
@@ -585,7 +649,7 @@ fn runCommand(
                     , .{
                         match,
                         stdout_bytes.?,
-                        try Step.allocPrintCmd(arena, opt_cwd, argv),
+                        try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
@@ -594,17 +658,89 @@ fn runCommand(
                     return step.fail("the following command {} (expected {}):\n{s}", .{
                         fmtTerm(term),
                         fmtTerm(expected_term),
-                        try Step.allocPrintCmd(arena, opt_cwd, argv),
+                        try Step.allocPrintCmd(arena, self.cwd, argv),
                     });
                 }
             },
         },
         else => {
-            try step.handleChildProcessTerm(term, opt_cwd, argv);
+            try step.handleChildProcessTerm(term, self.cwd, argv);
         },
     }
 }
 
+fn spawnChildAndCollect(
+    self: *RunStep,
+    argv: []const []const u8,
+    stdout_bytes: *?[]const u8,
+    stderr_bytes: *?[]const u8,
+    has_side_effects: bool,
+) !std.ChildProcess.Term {
+    const b = self.step.owner;
+    const arena = b.allocator;
+    const cwd = if (self.cwd) |cwd| b.pathFromRoot(cwd) else b.build_root.path;
+
+    var child = std.ChildProcess.init(argv, arena);
+    child.cwd = cwd;
+    child.env_map = self.env_map orelse b.env_map;
+
+    child.stdin_behavior = switch (self.stdio) {
+        .infer_from_args => if (has_side_effects) .Inherit else .Ignore,
+        .inherit => .Inherit,
+        .check => .Close,
+    };
+    child.stdout_behavior = switch (self.stdio) {
+        .infer_from_args => if (has_side_effects) .Inherit else .Ignore,
+        .inherit => .Inherit,
+        .check => |checks| if (checksContainStdout(checks.items)) .Pipe else .Ignore,
+    };
+    child.stderr_behavior = switch (self.stdio) {
+        .infer_from_args => if (has_side_effects) .Inherit else .Pipe,
+        .inherit => .Inherit,
+        .check => .Pipe,
+    };
+
+    child.spawn() catch |err| return self.step.fail("unable to spawn {s}: {s}", .{
+        argv[0], @errorName(err),
+    });
+
+    if (child.stdout) |stdout| {
+        if (child.stderr) |stderr| {
+            var poller = std.io.poll(arena, enum { stdout, stderr }, .{
+                .stdout = stdout,
+                .stderr = stderr,
+            });
+            defer poller.deinit();
+
+            while (try poller.poll()) {
+                if (poller.fifo(.stdout).count > self.max_stdio_size)
+                    return error.StdoutStreamTooLong;
+                if (poller.fifo(.stderr).count > self.max_stdio_size)
+                    return error.StderrStreamTooLong;
+            }
+
+            stdout_bytes.* = try poller.fifo(.stdout).toOwnedSlice();
+            stderr_bytes.* = try poller.fifo(.stderr).toOwnedSlice();
+        } else {
+            stdout_bytes.* = try stdout.reader().readAllAlloc(arena, self.max_stdio_size);
+        }
+    } else if (child.stderr) |stderr| {
+        stderr_bytes.* = try stderr.reader().readAllAlloc(arena, self.max_stdio_size);
+    }
+
+    if (stderr_bytes.*) |stderr| if (stderr.len > 0) {
+        const stderr_is_diagnostic = switch (self.stdio) {
+            .check => |checks| !checksContainStderr(checks.items),
+            else => true,
+        };
+        if (stderr_is_diagnostic) {
+            try self.step.result_error_msgs.append(arena, stderr);
+        }
+    };
+
+    return child.wait();
+}
+
 fn addPathForDynLibs(self: *RunStep, artifact: *CompileStep) void {
     addPathForDynLibsInternal(&self.step, self.step.owner, artifact);
 }
@@ -624,3 +760,29 @@ pub fn addPathForDynLibsInternal(step: *Step, builder: *std.Build, artifact: *Co
         }
     }
 }
+
+fn failForeign(
+    self: *RunStep,
+    suggested_flag: []const u8,
+    argv0: []const u8,
+    exe: *CompileStep,
+) error{ MakeFailed, MakeSkipped, OutOfMemory } {
+    switch (self.stdio) {
+        .check => {
+            if (self.skip_foreign_checks)
+                return error.MakeSkipped;
+
+            const b = self.step.owner;
+            const host_name = try b.host.target.zigTriple(b.allocator);
+            const foreign_name = try exe.target.zigTriple(b.allocator);
+
+            return self.step.fail(
+                \\unable to spawn foreign binary '{s}' ({s}) on host system ({s})
+                \\  consider using {s} or enabling skip_foreign_checks in the Run step
+            , .{ argv0, foreign_name, host_name, suggested_flag });
+        },
+        else => {
+            return self.step.fail("unable to spawn foreign binary '{s}'", .{argv0});
+        },
+    }
+}
diff --git a/lib/std/Build/Step.zig b/lib/std/Build/Step.zig
index 3219588050..928e7e8735 100644
--- a/lib/std/Build/Step.zig
+++ b/lib/std/Build/Step.zig
@@ -26,6 +26,9 @@ pub const State = enum {
     dependency_failure,
     success,
     failure,
+    /// This state indicates that the step did not complete, however, it also did not fail,
+    /// and it is safe to continue executing its dependencies.
+    skipped,
 };
 
 pub const Id = enum {
@@ -106,13 +109,15 @@ pub fn init(options: Options) Step {
 /// If the Step's `make` function reports `error.MakeFailed`, it indicates they
 /// have already reported the error. Otherwise, we add a simple error report
 /// here.
-pub fn make(s: *Step, prog_node: *std.Progress.Node) error{MakeFailed}!void {
-    return s.makeFn(s, prog_node) catch |err| {
-        if (err != error.MakeFailed) {
+pub fn make(s: *Step, prog_node: *std.Progress.Node) error{ MakeFailed, MakeSkipped }!void {
+    return s.makeFn(s, prog_node) catch |err| switch (err) {
+        error.MakeFailed => return error.MakeFailed,
+        error.MakeSkipped => return error.MakeSkipped,
+        else => {
             const gpa = s.dependencies.allocator;
             s.result_error_msgs.append(gpa, @errorName(err)) catch @panic("OOM");
-        }
-        return error.MakeFailed;
+            return error.MakeFailed;
+        },
     };
 }
 
@@ -192,10 +197,14 @@ pub fn evalChildProcess(s: *Step, argv: []const []const u8) !void {
 }
 
 pub fn fail(step: *Step, comptime fmt: []const u8, args: anytype) error{ OutOfMemory, MakeFailed } {
+    try step.addError(fmt, args);
+    return error.MakeFailed;
+}
+
+pub fn addError(step: *Step, comptime fmt: []const u8, args: anytype) error{OutOfMemory}!void {
     const arena = step.owner.allocator;
     const msg = try std.fmt.allocPrint(arena, fmt, args);
     try step.result_error_msgs.append(arena, msg);
-    return error.MakeFailed;
 }
 
 /// Assumes that argv contains `--listen=-` and that the process being spawned
@@ -398,5 +407,5 @@ fn failWithCacheError(s: *Step, man: *const std.Build.Cache.Manifest, err: anyer
     const i = man.failed_file_index orelse return err;
     const pp = man.files.items[i].prefixed_path orelse return err;
     const prefix = man.cache.prefixes()[pp.prefix].path orelse "";
-    return s.fail("{s}: {s}/{s}\n", .{ @errorName(err), prefix, pp.sub_path });
+    return s.fail("{s}: {s}/{s}", .{ @errorName(err), prefix, pp.sub_path });
 }
diff --git a/lib/std/Build/WriteFileStep.zig b/lib/std/Build/WriteFileStep.zig
index d554d6efc1..b5164d5730 100644
--- a/lib/std/Build/WriteFileStep.zig
+++ b/lib/std/Build/WriteFileStep.zig
@@ -37,7 +37,7 @@ pub fn init(owner: *std.Build) WriteFileStep {
     return .{
         .step = Step.init(.{
             .id = .write_file,
-            .name = "writefile",
+            .name = "WriteFile",
             .owner = owner,
             .makeFn = make,
         }),
@@ -56,6 +56,8 @@ pub fn add(wf: *WriteFileStep, sub_path: []const u8, bytes: []const u8) void {
         .contents = .{ .bytes = b.dupe(bytes) },
     };
     wf.files.append(gpa, file) catch @panic("OOM");
+
+    wf.maybeUpdateName();
 }
 
 /// Place the file into the generated directory within the local cache,
@@ -75,6 +77,8 @@ pub fn addCopyFile(wf: *WriteFileStep, source: std.Build.FileSource, sub_path: [
         .contents = .{ .copy = source },
     };
     wf.files.append(gpa, file) catch @panic("OOM");
+
+    wf.maybeUpdateName();
 }
 
 /// A path relative to the package root.
@@ -101,6 +105,15 @@ pub fn getFileSource(wf: *WriteFileStep, sub_path: []const u8) ?std.Build.FileSo
     return null;
 }
 
+fn maybeUpdateName(wf: *WriteFileStep) void {
+    if (wf.files.items.len == 1) {
+        // First time adding a file; update name.
+        if (std.mem.eql(u8, wf.step.name, "WriteFile")) {
+            wf.step.name = wf.step.owner.fmt("WriteFile {s}", .{wf.files.items[0].sub_path});
+        }
+    }
+}
+
 fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     _ = prog_node;
     const b = step.owner;
@@ -110,14 +123,39 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     // WriteFileStep - arguably it should be a different step. But anyway here
     // it is, it happens unconditionally and does not interact with the other
     // files here.
+    var any_miss = false;
     for (wf.output_source_files.items) |output_source_file| {
-        const basename = fs.path.basename(output_source_file.sub_path);
         if (fs.path.dirname(output_source_file.sub_path)) |dirname| {
-            var dir = try b.build_root.handle.makeOpenPath(dirname, .{});
-            defer dir.close();
-            try writeFile(wf, dir, output_source_file.contents, basename);
-        } else {
-            try writeFile(wf, b.build_root.handle, output_source_file.contents, basename);
+            b.build_root.handle.makePath(dirname) catch |err| {
+                return step.fail("unable to make path '{}{s}': {s}", .{
+                    b.build_root, dirname, @errorName(err),
+                });
+            };
+        }
+        switch (output_source_file.contents) {
+            .bytes => |bytes| {
+                b.build_root.handle.writeFile(output_source_file.sub_path, bytes) catch |err| {
+                    return step.fail("unable to write file '{}{s}': {s}", .{
+                        b.build_root, output_source_file.sub_path, @errorName(err),
+                    });
+                };
+                any_miss = true;
+            },
+            .copy => |file_source| {
+                const source_path = file_source.getPath(b);
+                const prev_status = fs.Dir.updateFile(
+                    fs.cwd(),
+                    source_path,
+                    b.build_root.handle,
+                    output_source_file.sub_path,
+                    .{},
+                ) catch |err| {
+                    return step.fail("unable to update file from '{s}' to '{}{s}': {s}", .{
+                        source_path, b.build_root, output_source_file.sub_path, @errorName(err),
+                    });
+                };
+                any_miss = any_miss or prev_status == .stale;
+            },
         }
     }
 
@@ -164,19 +202,52 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     const cache_path = "o" ++ fs.path.sep_str ++ digest;
 
     var cache_dir = b.cache_root.handle.makeOpenPath(cache_path, .{}) catch |err| {
-        std.debug.print("unable to make path {s}: {s}\n", .{ cache_path, @errorName(err) });
-        return err;
+        return step.fail("unable to make path '{}{s}': {s}", .{
+            b.cache_root, cache_path, @errorName(err),
+        });
     };
     defer cache_dir.close();
 
     for (wf.files.items) |file| {
-        const basename = fs.path.basename(file.sub_path);
         if (fs.path.dirname(file.sub_path)) |dirname| {
-            var dir = try b.cache_root.handle.makeOpenPath(dirname, .{});
-            defer dir.close();
-            try writeFile(wf, dir, file.contents, basename);
-        } else {
-            try writeFile(wf, cache_dir, file.contents, basename);
+            cache_dir.makePath(dirname) catch |err| {
+                return step.fail("unable to make path '{}{s}{c}{s}': {s}", .{
+                    b.cache_root, cache_path, fs.path.sep, dirname, @errorName(err),
+                });
+            };
+        }
+        switch (file.contents) {
+            .bytes => |bytes| {
+                cache_dir.writeFile(file.sub_path, bytes) catch |err| {
+                    return step.fail("unable to write file '{}{s}{c}{s}': {s}", .{
+                        b.cache_root, cache_path, fs.path.sep, file.sub_path, @errorName(err),
+                    });
+                };
+            },
+            .copy => |file_source| {
+                const source_path = file_source.getPath(b);
+                const prev_status = fs.Dir.updateFile(
+                    fs.cwd(),
+                    source_path,
+                    cache_dir,
+                    file.sub_path,
+                    .{},
+                ) catch |err| {
+                    return step.fail("unable to update file from '{s}' to '{}{s}{c}{s}': {s}", .{
+                        source_path,
+                        b.cache_root,
+                        cache_path,
+                        fs.path.sep,
+                        file.sub_path,
+                        @errorName(err),
+                    });
+                };
+                // At this point we already will mark the step as a cache miss.
+                // But this is kind of a partial cache hit since individual
+                // file copies may be avoided. Oh well, this information is
+                // discarded.
+                _ = prev_status;
+            },
         }
 
         file.generated_file.path = try b.cache_root.join(
@@ -188,19 +259,6 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     try man.writeManifest();
 }
 
-fn writeFile(wf: *WriteFileStep, dir: fs.Dir, contents: Contents, basename: []const u8) !void {
-    const b = wf.step.owner;
-    // TODO after landing concurrency PR, improve error reporting here
-    switch (contents) {
-        .bytes => |bytes| return dir.writeFile(basename, bytes),
-        .copy => |file_source| {
-            const source_path = file_source.getPath(b);
-            const prev_status = try fs.Dir.updateFile(fs.cwd(), source_path, dir, basename, .{});
-            _ = prev_status; // TODO logging (affected by open PR regarding concurrency)
-        },
-    }
-}
-
 const std = @import("../std.zig");
 const Step = std.Build.Step;
 const fs = std.fs;
diff --git a/lib/std/child_process.zig b/lib/std/child_process.zig
index f8cba85874..3bdef3177a 100644
--- a/lib/std/child_process.zig
+++ b/lib/std/child_process.zig
@@ -185,7 +185,6 @@ pub const ChildProcess = struct {
     }
 
     /// Blocks until child process terminates and then cleans up all resources.
-    /// TODO: set the pid to undefined in this function.
     pub fn wait(self: *ChildProcess) !Term {
         const term = if (builtin.os.tag == .windows)
             try self.waitWindows()