rework enums and unions and their relationship to each other

 * @enumTagName renamed to @tagName and it works on enums and
   union-enums
 * Remove the EnumTag type. Now there is only enum and union,
   and the tag type of a union is always an enum.
 * unions support specifying the tag enum type, and they support
   inferring an enum tag type.
 * Enums no longer support field types but they do support
   setting the tag values. Likewise union-enums when inferring
   an enum tag type support setting the tag values.
 * It is now an error for enums and unions to have 0 fields.
 * switch statements support union-enums

closes #618

4 files changed, 40 insertions, 42 deletions

diff --git a/std/build.zig b/std/build.zig
index 009295c6ad..9bdc4b3076 100644
--- a/std/build.zig
+++ b/std/build.zig
@@ -69,8 +69,8 @@ pub const Builder = struct {
         used: bool,
     };
 
-    const UserValue = enum {
-        Flag,
+    const UserValue = union(enum) {
+        Flag: void,
         Scalar: []const u8,
         List: ArrayList([]const u8),
     };
@@ -450,7 +450,7 @@ pub const Builder = struct {
     pub fn addUserInputOption(self: &Builder, name: []const u8, value: []const u8) -> bool {
         if (%%self.user_input_options.put(name, UserInputOption {
             .name = name,
-            .value = UserValue.Scalar { value },
+            .value = UserValue { .Scalar = value },
             .used = false,
         })) |*prev_value| {
             // option already exists
@@ -462,7 +462,7 @@ pub const Builder = struct {
                     %%list.append(value);
                     _ = %%self.user_input_options.put(name, UserInputOption {
                         .name = name,
-                        .value = UserValue.List { list },
+                        .value = UserValue { .List = list },
                         .used = false,
                     });
                 },
@@ -471,7 +471,7 @@ pub const Builder = struct {
                     %%list.append(value);
                     _ = %%self.user_input_options.put(name, UserInputOption {
                         .name = name,
-                        .value = UserValue.List { *list },
+                        .value = UserValue { .List = *list },
                         .used = false,
                     });
                 },
@@ -487,7 +487,7 @@ pub const Builder = struct {
     pub fn addUserInputFlag(self: &Builder, name: []const u8) -> bool {
         if (%%self.user_input_options.put(name, UserInputOption {
             .name = name,
-            .value = UserValue.Flag,
+            .value = UserValue {.Flag = {} },
             .used = false,
         })) |*prev_value| {
             switch (prev_value.value) {
@@ -685,8 +685,8 @@ const CrossTarget = struct {
     environ: builtin.Environ,
 };
 
-const Target = enum {
-    Native,
+const Target = union(enum) {
+    Native: void,
     Cross: CrossTarget,
 
     pub fn oFileExt(self: &const Target) -> []const u8 {
@@ -844,7 +844,7 @@ pub const LibExeObjStep = struct {
             .kind = kind,
             .root_src = root_src,
             .name = name,
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .linker_script = null,
             .link_libs = BufSet.init(builder.allocator),
             .frameworks = BufSet.init(builder.allocator),
@@ -879,7 +879,7 @@ pub const LibExeObjStep = struct {
             .kind = kind,
             .version = *version,
             .static = static,
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .cflags = ArrayList([]const u8).init(builder.allocator),
             .source_files = ArrayList([]const u8).init(builder.allocator),
             .object_files = ArrayList([]const u8).init(builder.allocator),
@@ -948,8 +948,8 @@ pub const LibExeObjStep = struct {
     pub fn setTarget(self: &LibExeObjStep, target_arch: builtin.Arch, target_os: builtin.Os,
         target_environ: builtin.Environ)
     {
-        self.target = Target.Cross {
-            CrossTarget {
+        self.target = Target {
+            .Cross = CrossTarget {
                 .arch = target_arch,
                 .os = target_os,
                 .environ = target_environ,
@@ -1186,13 +1186,13 @@ pub const LibExeObjStep = struct {
             Target.Native => {},
             Target.Cross => |cross_target| {
                 %%zig_args.append("--target-arch");
-                %%zig_args.append(@enumTagName(cross_target.arch));
+                %%zig_args.append(@tagName(cross_target.arch));
 
                 %%zig_args.append("--target-os");
-                %%zig_args.append(@enumTagName(cross_target.os));
+                %%zig_args.append(@tagName(cross_target.os));
 
                 %%zig_args.append("--target-environ");
-                %%zig_args.append(@enumTagName(cross_target.environ));
+                %%zig_args.append(@tagName(cross_target.environ));
             },
         }
 
@@ -1553,7 +1553,7 @@ pub const TestStep = struct {
             .name_prefix = "",
             .filter = null,
             .link_libs = BufSet.init(builder.allocator),
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .exec_cmd_args = null,
         }
     }
@@ -1581,8 +1581,8 @@ pub const TestStep = struct {
     pub fn setTarget(self: &TestStep, target_arch: builtin.Arch, target_os: builtin.Os,
         target_environ: builtin.Environ)
     {
-        self.target = Target.Cross {
-            CrossTarget {
+        self.target = Target {
+            .Cross = CrossTarget {
                 .arch = target_arch,
                 .os = target_os,
                 .environ = target_environ,
@@ -1620,13 +1620,13 @@ pub const TestStep = struct {
             Target.Native => {},
             Target.Cross => |cross_target| {
                 %%zig_args.append("--target-arch");
-                %%zig_args.append(@enumTagName(cross_target.arch));
+                %%zig_args.append(@tagName(cross_target.arch));
 
                 %%zig_args.append("--target-os");
-                %%zig_args.append(@enumTagName(cross_target.os));
+                %%zig_args.append(@tagName(cross_target.os));
 
                 %%zig_args.append("--target-environ");
-                %%zig_args.append(@enumTagName(cross_target.environ));
+                %%zig_args.append(@tagName(cross_target.environ));
             },
         }
 
diff --git a/std/debug.zig b/std/debug.zig
index 50322024c3..5bfa436614 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -280,7 +280,7 @@ const AbbrevAttr = struct {
     form_id: u64,
 };
 
-const FormValue = enum {
+const FormValue = union(enum) {
     Address: u64,
     Block: []u8,
     Const: Constant,
@@ -475,7 +475,7 @@ fn readAllocBytes(allocator: &mem.Allocator, in_stream: &io.InStream, size: usiz
 
 fn parseFormValueBlockLen(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
     const buf = %return readAllocBytes(allocator, in_stream, size);
-    return FormValue.Block { buf };
+    return FormValue { .Block = buf };
 }
 
 fn parseFormValueBlock(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
@@ -484,7 +484,7 @@ fn parseFormValueBlock(allocator: &mem.Allocator, in_stream: &io.InStream, size:
 }
 
 fn parseFormValueConstant(allocator: &mem.Allocator, in_stream: &io.InStream, signed: bool, size: usize) -> %FormValue {
-    FormValue.Const { Constant {
+    FormValue { .Const = Constant {
         .signed = signed,
         .payload = %return readAllocBytes(allocator, in_stream, size),
     }}
@@ -510,7 +510,7 @@ fn parseFormValueTargetAddrSize(in_stream: &io.InStream) -> %u64 {
 
 fn parseFormValueRefLen(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
     const buf = %return readAllocBytes(allocator, in_stream, size);
-    return FormValue.Ref { buf };
+    return FormValue { .Ref = buf };
 }
 
 fn parseFormValueRef(allocator: &mem.Allocator, in_stream: &io.InStream, comptime T: type) -> %FormValue {
@@ -520,7 +520,7 @@ fn parseFormValueRef(allocator: &mem.Allocator, in_stream: &io.InStream, comptim
 
 fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u64, is_64: bool) -> %FormValue {
     return switch (form_id) {
-        DW.FORM_addr => FormValue.Address { %return parseFormValueTargetAddrSize(in_stream) },
+        DW.FORM_addr => FormValue { .Address = %return parseFormValueTargetAddrSize(in_stream) },
         DW.FORM_block1 => parseFormValueBlock(allocator, in_stream, 1),
         DW.FORM_block2 => parseFormValueBlock(allocator, in_stream, 2),
         DW.FORM_block4 => parseFormValueBlock(allocator, in_stream, 4),
@@ -540,13 +540,11 @@ fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u
         DW.FORM_exprloc => {
             const size = %return readULeb128(in_stream);
             const buf = %return readAllocBytes(allocator, in_stream, size);
-            return FormValue.ExprLoc { buf };
-        },
-        DW.FORM_flag => FormValue.Flag { (%return in_stream.readByte()) != 0 },
-        DW.FORM_flag_present => FormValue.Flag { true },
-        DW.FORM_sec_offset => FormValue.SecOffset {
-            %return parseFormValueDwarfOffsetSize(in_stream, is_64)
+            return FormValue { .ExprLoc = buf };
         },
+        DW.FORM_flag => FormValue { .Flag = (%return in_stream.readByte()) != 0 },
+        DW.FORM_flag_present => FormValue { .Flag = true },
+        DW.FORM_sec_offset => FormValue { .SecOffset = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
 
         DW.FORM_ref1 => parseFormValueRef(allocator, in_stream, u8),
         DW.FORM_ref2 => parseFormValueRef(allocator, in_stream, u16),
@@ -557,11 +555,11 @@ fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u
             parseFormValueRefLen(allocator, in_stream, ref_len)
         },
 
-        DW.FORM_ref_addr => FormValue.RefAddr { %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
-        DW.FORM_ref_sig8 => FormValue.RefSig8 { %return in_stream.readIntLe(u64) },
+        DW.FORM_ref_addr => FormValue { .RefAddr = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
+        DW.FORM_ref_sig8 => FormValue { .RefSig8 = %return in_stream.readIntLe(u64) },
 
-        DW.FORM_string => FormValue.String { %return readStringRaw(allocator, in_stream) },
-        DW.FORM_strp => FormValue.StrPtr { %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
+        DW.FORM_string => FormValue { .String = %return readStringRaw(allocator, in_stream) },
+        DW.FORM_strp => FormValue { .StrPtr = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
         DW.FORM_indirect => {
             const child_form_id = %return readULeb128(in_stream);
             parseFormValue(allocator, in_stream, child_form_id, is_64)
diff --git a/std/os/child_process.zig b/std/os/child_process.zig
index 005d9772e4..3a4c9410c5 100644
--- a/std/os/child_process.zig
+++ b/std/os/child_process.zig
@@ -58,7 +58,7 @@ pub const ChildProcess = struct {
     err_pipe: if (is_windows) void else [2]i32,
     llnode: if (is_windows) void else LinkedList(&ChildProcess).Node,
 
-    pub const Term = enum {
+    pub const Term = union(enum) {
         Exited: i32,
         Signal: i32,
         Stopped: i32,
@@ -281,13 +281,13 @@ pub const ChildProcess = struct {
 
     fn statusToTerm(status: i32) -> Term {
         return if (posix.WIFEXITED(status)) {
-            Term.Exited { posix.WEXITSTATUS(status) }
+            Term { .Exited = posix.WEXITSTATUS(status) }
         } else if (posix.WIFSIGNALED(status)) {
-            Term.Signal { posix.WTERMSIG(status) }
+            Term { .Signal = posix.WTERMSIG(status) }
         } else if (posix.WIFSTOPPED(status)) {
-            Term.Stopped { posix.WSTOPSIG(status) }
+            Term { .Stopped = posix.WSTOPSIG(status) }
         } else {
-            Term.Unknown { status }
+            Term { .Unknown = status }
         };
     }
 
diff --git a/std/os/path.zig b/std/os/path.zig
index a372b5b077..3fd7b5e2db 100644
--- a/std/os/path.zig
+++ b/std/os/path.zig
@@ -1016,7 +1016,7 @@ pub fn real(allocator: &Allocator, pathname: []const u8) -> %[]u8 {
 
             return os.readLink(allocator, proc_path);
         },
-        else => @compileError("TODO implement os.path.real for " ++ @enumTagName(builtin.os)),
+        else => @compileError("TODO implement os.path.real for " ++ @tagName(builtin.os)),
     }
 }