compiler: remove anonymous struct types, unify all tuples

This commit reworks how anonymous struct literals and tuples work.

Previously, an untyped anonymous struct literal
(e.g. `const x = .{ .a = 123 }`) was given an "anonymous struct type",
which is a special kind of struct which coerces using structural
equivalence. This mechanism was a holdover from before we used
RLS / result types as the primary mechanism of type inference. This
commit changes the language so that the type assigned here is a "normal"
struct type. It uses a form of equivalence based on the AST node and the
type's structure, much like a reified (`@Type`) type.

Additionally, tuples have been simplified. The distinction between
"simple" and "complex" tuple types is eliminated. All tuples, even those
explicitly declared using `struct { ... }` syntax, use structural
equivalence, and do not undergo staged type resolution. Tuples are very
restricted: they cannot have non-`auto` layouts, cannot have aligned
fields, and cannot have default values with the exception of `comptime`
fields. Tuples currently do not have optimized layout, but this can be
changed in the future.

This change simplifies the language, and fixes some problematic
coercions through pointers which led to unintuitive behavior.

Resolves: #16865

1 files changed, 17 insertions, 78 deletions

diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index cc373cd8b1..283c8dbc4a 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -1013,84 +1013,6 @@ test "struct with 0-length union array field" {
     try expectEqual(@as(usize, 0), s.zero_length.len);
 }
 
-test "type coercion of anon struct literal to struct" {
-    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
-
-    const S = struct {
-        const S2 = struct {
-            A: u32,
-            B: []const u8,
-            C: void,
-            D: Foo = .{},
-        };
-
-        const Foo = struct {
-            field: i32 = 1234,
-        };
-
-        fn doTheTest() !void {
-            var y: u32 = 42;
-            _ = &y;
-            const t0 = .{ .A = 123, .B = "foo", .C = {} };
-            const t1 = .{ .A = y, .B = "foo", .C = {} };
-            const y0: S2 = t0;
-            const y1: S2 = t1;
-            try expect(y0.A == 123);
-            try expect(std.mem.eql(u8, y0.B, "foo"));
-            try expect(y0.C == {});
-            try expect(y0.D.field == 1234);
-            try expect(y1.A == y);
-            try expect(std.mem.eql(u8, y1.B, "foo"));
-            try expect(y1.C == {});
-            try expect(y1.D.field == 1234);
-        }
-    };
-    try S.doTheTest();
-    try comptime S.doTheTest();
-}
-
-test "type coercion of pointer to anon struct literal to pointer to struct" {
-    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
-
-    const S = struct {
-        const S2 = struct {
-            A: u32,
-            B: []const u8,
-            C: void,
-            D: Foo = .{},
-        };
-
-        const Foo = struct {
-            field: i32 = 1234,
-        };
-
-        fn doTheTest() !void {
-            var y: u32 = 42;
-            _ = &y;
-            const t0 = &.{ .A = 123, .B = "foo", .C = {} };
-            const t1 = &.{ .A = y, .B = "foo", .C = {} };
-            const y0: *const S2 = t0;
-            const y1: *const S2 = t1;
-            try expect(y0.A == 123);
-            try expect(std.mem.eql(u8, y0.B, "foo"));
-            try expect(y0.C == {});
-            try expect(y0.D.field == 1234);
-            try expect(y1.A == y);
-            try expect(std.mem.eql(u8, y1.B, "foo"));
-            try expect(y1.C == {});
-            try expect(y1.D.field == 1234);
-        }
-    };
-    try S.doTheTest();
-    try comptime S.doTheTest();
-}
-
 test "packed struct with undefined initializers" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
@@ -2183,3 +2105,20 @@ test "extern struct @FieldType" {
     comptime assert(@FieldType(S, "b") == f64);
     comptime assert(@FieldType(S, "c") == *S);
 }
+
+test "anonymous struct equivalence" {
+    const S = struct {
+        fn anonStructType(comptime x: anytype) type {
+            const val = .{ .a = "hello", .b = x };
+            return @TypeOf(val);
+        }
+    };
+
+    const A = S.anonStructType(123);
+    const B = S.anonStructType(123);
+    const C = S.anonStructType(456);
+
+    comptime assert(A == B);
+    comptime assert(A != C);
+    comptime assert(B != C);
+}