cmd/compile: unique LinkString for renamed, embedded fields

Using type aliases, it's possible to create structs with embedded
fields that have no corresponding type literal notation. However, we
still need to generate a unique name for these types to use for linker
symbols. This CL introduces a new "struct{ Name = Type }" syntax for
use in LinkString formatting to represent these types.

Reattempt at CL 372914, which was rolled back due to race-y
LocalPkg.Lookup call that isn't safe for concurrency.

Fixes #50190.

Change-Id: I0b7fd81e1b0b3199a6afcffde96ade42495ad8d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/378434
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/types/fmt.go

@@ -631,6 +631,7 @@ func fldconv(b *bytes.Buffer, f *Field, verb rune, mode fmtMode, visited map[*Ty
 	}
 
 	var name string
+	nameSep := " "
 	if verb != 'S' {
 		s := f.Sym
 
@@ -639,7 +640,47 @@ func fldconv(b *bytes.Buffer, f *Field, verb rune, mode fmtMode, visited map[*Ty
 			s = OrigSym(s)
 		}
 
-		if s != nil && f.Embedded == 0 {
+		// Using type aliases and embedded fields, it's possible to
+		// construct types that can't be directly represented as a
+		// type literal. For example, given "type Int = int" (#50190),
+		// it would be incorrect to format "struct{ Int }" as either
+		// "struct{ int }" or "struct{ Int int }", because those each
+		// represent other, distinct types.
+		//
+		// So for the purpose of LinkString (i.e., fmtTypeID), we use
+		// the non-standard syntax "struct{ Int = int }" to represent
+		// embedded fields that have been renamed through the use of
+		// type aliases.
+		if f.Embedded != 0 {
+			if mode == fmtTypeID {
+				nameSep = " = "
+
+				// Compute tsym, the symbol that would normally be used as
+				// the field name when embedding f.Type.
+				// TODO(mdempsky): Check for other occurences of this logic
+				// and deduplicate.
+				typ := f.Type
+				if typ.IsPtr() {
+					base.Assertf(typ.Sym() == nil, "embedded pointer type has name: %L", typ)
+					typ = typ.Elem()
+				}
+				tsym := typ.Sym()
+
+				// If the field name matches the embedded type's name, then
+				// suppress printing of the field name. For example, format
+				// "struct{ T }" as simply that instead of "struct{ T = T }".
+				if tsym != nil && (s == tsym || IsExported(tsym.Name) && s.Name == tsym.Name) {
+					s = nil
+				}
+			} else {
+				// Suppress the field name for embedded fields for
+				// non-LinkString formats, to match historical behavior.
+				// TODO(mdempsky): Re-evaluate this.
+				s = nil
+			}
+		}
+
+		if s != nil {
 			if funarg != FunargNone {
 				name = fmt.Sprint(f.Nname)
 			} else if verb == 'L' {
@@ -658,7 +699,7 @@ func fldconv(b *bytes.Buffer, f *Field, verb rune, mode fmtMode, visited map[*Ty
 
 	if name != "" {
 		b.WriteString(name)
-		b.WriteString(" ")
+		b.WriteString(nameSep)
 	}
 
 	if f.IsDDD() {

test/fixedbugs/issue50190.go

@@ -0,0 +1,31 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type Int = int
+
+type A = struct{ int }
+type B = struct{ Int }
+
+func main() {
+	var x, y interface{} = A{}, B{}
+	if x == y {
+		panic("FAIL")
+	}
+
+	{
+		type C = int32
+		x = struct{ C }{}
+	}
+	{
+		type C = uint32
+		y = struct{ C }{}
+	}
+	if x == y {
+		panic("FAIL")
+	}
+}