stage2: implement error notes and regress -femit-zir

 * Implement error notes
   - note: other symbol exported here
   - note: previous else prong is here
   - note: previous '_' prong is here
 * Add Compilation.CObject.ErrorMsg. This object properly converts to
   AllErrors.Message when the time comes.
 * Add Compilation.CObject.failure_retryable. Properly handles
   out-of-memory and other transient failures.
 * Introduce Module.SrcLoc which has not only a byte offset but also
   references the file which the byte offset applies to.
 * Scope.Block now contains both a pointer to the "owner" Decl and the
   "source" Decl. As an example, during inline function call, the
   "owner" will be the Decl of the caller and the "source" will be the
   Decl of the callee.
 * Module.ErrorMsg now sports a `file_scope` field so that notes can
   refer to source locations in a file other than the parent error
   message.
 * Some instances where a `*Scope` was stored, now store a
   `*Scope.Container`.
 * Some methods in the `Scope` namespace were moved to the more specific
   type, since there was only an implementation for one particular tag.
   - `removeDecl` moved to `Scope.Container`
   - `destroy` moved to `Scope.File`
 * Two kinds of Scope deleted:
   - zir_module
   - decl
 * astgen: properly use DeclVal / DeclRef. DeclVal was incorrectly
   changed to be a reference; this commit fixes it. Fewer ZIR
   instructions processed as a result.
   - declval_in_module is renamed to declval
   - previous declval ZIR instruction is deleted; it was only for .zir
     files.
 * Test harness: friendlier diagnostics when an unexpected set of errors
   is encountered.
 * zir_sema: fix analyzeInstBlockFlat by properly calling resolvingInst
   on the last zir instruction in the block.

Compile log implementation:
 * Write to a buffer rather than directly to stderr.
 * Only keep track of 1 callsite per Decl.
 * No longer mutate the ZIR Inst struct data.
 * "Compile log statement found" errors are only emitted when there are
   no other compile errors.

-femit-zir and support for .zir source files is regressed. If we wanted
to support this again, outputting .zir would need to be done as yet
another backend rather than in the haphazard way it was previously
implemented.

For parsing .zir, it was implemented previously in a way that was not
helpful for debugging. We need tighter integration with the test harness
for it to be useful; so clearly a rewrite is needed. Given that a
rewrite is needed, and it was getting in the way of progress and
organization of the rest of stage2, I regressed the feature.

4 files changed, 28 insertions, 46 deletions

diff --git a/src/link/Coff.zig b/src/link/Coff.zig
index f7cd9b69ce..981d4ec3a3 100644
--- a/src/link/Coff.zig
+++ b/src/link/Coff.zig
@@ -670,7 +670,7 @@ pub fn updateDecl(self: *Coff, module: *Module, decl: *Module.Decl) !void {
     var code_buffer = std.ArrayList(u8).init(self.base.allocator);
     defer code_buffer.deinit();
 
-    const res = try codegen.generateSymbol(&self.base, decl.src(), typed_value, &code_buffer, .none);
+    const res = try codegen.generateSymbol(&self.base, decl.srcLoc(), typed_value, &code_buffer, .none);
     const code = switch (res) {
         .externally_managed => |x| x,
         .appended => code_buffer.items,
@@ -732,7 +732,7 @@ pub fn updateDeclExports(self: *Coff, module: *Module, decl: *const Module.Decl,
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
                     exp,
-                    try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: ExportOptions.section", .{}),
+                    try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: ExportOptions.section", .{}),
                 );
                 continue;
             }
@@ -743,7 +743,7 @@ pub fn updateDeclExports(self: *Coff, module: *Module, decl: *const Module.Decl,
             try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
             module.failed_exports.putAssumeCapacityNoClobber(
                 exp,
-                try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: Exports other than '_start'", .{}),
+                try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: Exports other than '_start'", .{}),
             );
             continue;
         }
diff --git a/src/link/Elf.zig b/src/link/Elf.zig
index 8c76a4e967..ee50eb5d94 100644
--- a/src/link/Elf.zig
+++ b/src/link/Elf.zig
@@ -2189,22 +2189,14 @@ pub fn updateDecl(self: *Elf, module: *Module, decl: *Module.Decl) !void {
         try dbg_line_buffer.ensureCapacity(26);
 
         const line_off: u28 = blk: {
-            if (decl.scope.cast(Module.Scope.Container)) |container_scope| {
-                const tree = container_scope.file_scope.contents.tree;
-                const file_ast_decls = tree.root_node.decls();
-                // TODO Look into improving the performance here by adding a token-index-to-line
-                // lookup table. Currently this involves scanning over the source code for newlines.
-                const fn_proto = file_ast_decls[decl.src_index].castTag(.FnProto).?;
-                const block = fn_proto.getBodyNode().?.castTag(.Block).?;
-                const line_delta = std.zig.lineDelta(tree.source, 0, tree.token_locs[block.lbrace].start);
-                break :blk @intCast(u28, line_delta);
-            } else if (decl.scope.cast(Module.Scope.ZIRModule)) |zir_module| {
-                const byte_off = zir_module.contents.module.decls[decl.src_index].inst.src;
-                const line_delta = std.zig.lineDelta(zir_module.source.bytes, 0, byte_off);
-                break :blk @intCast(u28, line_delta);
-            } else {
-                unreachable;
-            }
+            const tree = decl.container.file_scope.contents.tree;
+            const file_ast_decls = tree.root_node.decls();
+            // TODO Look into improving the performance here by adding a token-index-to-line
+            // lookup table. Currently this involves scanning over the source code for newlines.
+            const fn_proto = file_ast_decls[decl.src_index].castTag(.FnProto).?;
+            const block = fn_proto.getBodyNode().?.castTag(.Block).?;
+            const line_delta = std.zig.lineDelta(tree.source, 0, tree.token_locs[block.lbrace].start);
+            break :blk @intCast(u28, line_delta);
         };
 
         const ptr_width_bytes = self.ptrWidthBytes();
@@ -2268,7 +2260,7 @@ pub fn updateDecl(self: *Elf, module: *Module, decl: *Module.Decl) !void {
     } else {
         // TODO implement .debug_info for global variables
     }
-    const res = try codegen.generateSymbol(&self.base, decl.src(), typed_value, &code_buffer, .{
+    const res = try codegen.generateSymbol(&self.base, decl.srcLoc(), typed_value, &code_buffer, .{
         .dwarf = .{
             .dbg_line = &dbg_line_buffer,
             .dbg_info = &dbg_info_buffer,
@@ -2642,7 +2634,7 @@ pub fn updateDeclExports(
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
                     exp,
-                    try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: ExportOptions.section", .{}),
+                    try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: ExportOptions.section", .{}),
                 );
                 continue;
             }
@@ -2660,7 +2652,7 @@ pub fn updateDeclExports(
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
                     exp,
-                    try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: GlobalLinkage.LinkOnce", .{}),
+                    try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: GlobalLinkage.LinkOnce", .{}),
                 );
                 continue;
             },
@@ -2703,8 +2695,7 @@ pub fn updateDeclLineNumber(self: *Elf, module: *Module, decl: *const Module.Dec
 
     if (self.llvm_ir_module) |_| return;
 
-    const container_scope = decl.scope.cast(Module.Scope.Container).?;
-    const tree = container_scope.file_scope.contents.tree;
+    const tree = decl.container.file_scope.contents.tree;
     const file_ast_decls = tree.root_node.decls();
     // TODO Look into improving the performance here by adding a token-index-to-line
     // lookup table. Currently this involves scanning over the source code for newlines.
diff --git a/src/link/MachO.zig b/src/link/MachO.zig
index d913a82328..1017405255 100644
--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -1148,7 +1148,7 @@ pub fn updateDecl(self: *MachO, module: *Module, decl: *Module.Decl) !void {
     }
 
     const res = if (debug_buffers) |*dbg|
-        try codegen.generateSymbol(&self.base, decl.src(), typed_value, &code_buffer, .{
+        try codegen.generateSymbol(&self.base, decl.srcLoc(), typed_value, &code_buffer, .{
             .dwarf = .{
                 .dbg_line = &dbg.dbg_line_buffer,
                 .dbg_info = &dbg.dbg_info_buffer,
@@ -1156,7 +1156,7 @@ pub fn updateDecl(self: *MachO, module: *Module, decl: *Module.Decl) !void {
             },
         })
     else
-        try codegen.generateSymbol(&self.base, decl.src(), typed_value, &code_buffer, .none);
+        try codegen.generateSymbol(&self.base, decl.srcLoc(), typed_value, &code_buffer, .none);
 
     const code = switch (res) {
         .externally_managed => |x| x,
@@ -1316,7 +1316,7 @@ pub fn updateDeclExports(
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
                     exp,
-                    try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: ExportOptions.section", .{}),
+                    try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: ExportOptions.section", .{}),
                 );
                 continue;
             }
@@ -1334,7 +1334,7 @@ pub fn updateDeclExports(
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
                     exp,
-                    try Compilation.ErrorMsg.create(self.base.allocator, 0, "Unimplemented: GlobalLinkage.LinkOnce", .{}),
+                    try Module.ErrorMsg.create(self.base.allocator, decl.srcLoc(), "Unimplemented: GlobalLinkage.LinkOnce", .{}),
                 );
                 continue;
             },
diff --git a/src/link/MachO/DebugSymbols.zig b/src/link/MachO/DebugSymbols.zig
index 81a016ce42..fb7488a12c 100644
--- a/src/link/MachO/DebugSymbols.zig
+++ b/src/link/MachO/DebugSymbols.zig
@@ -906,8 +906,7 @@ pub fn updateDeclLineNumber(self: *DebugSymbols, module: *Module, decl: *const M
     const tracy = trace(@src());
     defer tracy.end();
 
-    const container_scope = decl.scope.cast(Module.Scope.Container).?;
-    const tree = container_scope.file_scope.contents.tree;
+    const tree = decl.container.file_scope.contents.tree;
     const file_ast_decls = tree.root_node.decls();
     // TODO Look into improving the performance here by adding a token-index-to-line
     // lookup table. Currently this involves scanning over the source code for newlines.
@@ -951,22 +950,14 @@ pub fn initDeclDebugBuffers(
             try dbg_line_buffer.ensureCapacity(26);
 
             const line_off: u28 = blk: {
-                if (decl.scope.cast(Module.Scope.Container)) |container_scope| {
-                    const tree = container_scope.file_scope.contents.tree;
-                    const file_ast_decls = tree.root_node.decls();
-                    // TODO Look into improving the performance here by adding a token-index-to-line
-                    // lookup table. Currently this involves scanning over the source code for newlines.
-                    const fn_proto = file_ast_decls[decl.src_index].castTag(.FnProto).?;
-                    const block = fn_proto.getBodyNode().?.castTag(.Block).?;
-                    const line_delta = std.zig.lineDelta(tree.source, 0, tree.token_locs[block.lbrace].start);
-                    break :blk @intCast(u28, line_delta);
-                } else if (decl.scope.cast(Module.Scope.ZIRModule)) |zir_module| {
-                    const byte_off = zir_module.contents.module.decls[decl.src_index].inst.src;
-                    const line_delta = std.zig.lineDelta(zir_module.source.bytes, 0, byte_off);
-                    break :blk @intCast(u28, line_delta);
-                } else {
-                    unreachable;
-                }
+                const tree = decl.container.file_scope.contents.tree;
+                const file_ast_decls = tree.root_node.decls();
+                // TODO Look into improving the performance here by adding a token-index-to-line
+                // lookup table. Currently this involves scanning over the source code for newlines.
+                const fn_proto = file_ast_decls[decl.src_index].castTag(.FnProto).?;
+                const block = fn_proto.getBodyNode().?.castTag(.Block).?;
+                const line_delta = std.zig.lineDelta(tree.source, 0, tree.token_locs[block.lbrace].start);
+                break :blk @intCast(u28, line_delta);
             };
 
             dbg_line_buffer.appendSliceAssumeCapacity(&[_]u8{