cmd/compile/internal/types: use Type.LinkString in TypeHash

Now that Type.LinkString always returns a fully unique string ID, we
can use it in TypeHash to avoid collisions between instantiations of
the same generic type.

Updates #51734.

Change-Id: I38cb396c88259be7afa44bd4333124ca98666c3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/393716
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

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

@@ -72,7 +72,6 @@ const (
 	fmtDebug
 	fmtTypeID
 	fmtTypeIDName
-	fmtTypeIDHash
 )
 
 // Sym
@@ -144,18 +143,6 @@ func symfmt(b *bytes.Buffer, s *Sym, verb rune, mode fmtMode) {
 	if q := pkgqual(s.Pkg, verb, mode); q != "" {
 		b.WriteString(q)
 		b.WriteByte('.')
-		switch mode {
-		case fmtTypeIDHash:
-			// If name is a generic instantiation, don't hash the instantiating types.
-			// This isn't great, but it is safe. If we hash the instantiating types, then
-			// we need to make sure they have just the package name. At this point, they
-			// either have "", or the whole package path, and it is hard to reconcile
-			// the two without depending on -p (which we might do someday).
-			// See issue 51250.
-			if i := strings.Index(name, "["); i >= 0 {
-				name = name[:i]
-			}
-		}
 	}
 	b.WriteString(name)
 }
@@ -183,7 +170,7 @@ func pkgqual(pkg *Pkg, verb rune, mode fmtMode) string {
 		case fmtDebug:
 			return pkg.Name
 
-		case fmtTypeIDName, fmtTypeIDHash:
+		case fmtTypeIDName:
 			// dcommontype, typehash
 			return pkg.Name
 
@@ -329,7 +316,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 	if t == AnyType || t == ByteType || t == RuneType {
 		// in %-T mode collapse predeclared aliases with their originals.
 		switch mode {
-		case fmtTypeIDName, fmtTypeIDHash, fmtTypeID:
+		case fmtTypeIDName, fmtTypeID:
 			t = Types[t.Kind()]
 		default:
 			sconv2(b, t.Sym(), 'S', mode)
@@ -420,7 +407,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 	case TPTR:
 		b.WriteByte('*')
 		switch mode {
-		case fmtTypeID, fmtTypeIDName, fmtTypeIDHash:
+		case fmtTypeID, fmtTypeIDName:
 			if verb == 'S' {
 				tconv2(b, t.Elem(), 'S', mode, visited)
 				return
@@ -482,7 +469,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 			case IsExported(f.Sym.Name):
 				sconv2(b, f.Sym, 'S', mode)
 			default:
-				if mode != fmtTypeIDName && mode != fmtTypeIDHash {
+				if mode != fmtTypeIDName {
 					mode = fmtTypeID
 				}
 				sconv2(b, f.Sym, 'v', mode)
@@ -552,7 +539,7 @@ func tconv2(b *bytes.Buffer, t *Type, verb rune, mode fmtMode, visited map[*Type
 			b.WriteByte(byte(open))
 			fieldVerb := 'v'
 			switch mode {
-			case fmtTypeID, fmtTypeIDName, fmtTypeIDHash, fmtGo:
+			case fmtTypeID, fmtTypeIDName, fmtGo:
 				// no argument names on function signature, and no "noescape"/"nosplit" tags
 				fieldVerb = 'S'
 			}
@@ -686,7 +673,7 @@ func fldconv(b *bytes.Buffer, f *Field, verb rune, mode fmtMode, visited map[*Ty
 				if name == ".F" {
 					name = "F" // Hack for toolstash -cmp.
 				}
-				if !IsExported(name) && mode != fmtTypeIDName && mode != fmtTypeIDHash {
+				if !IsExported(name) && mode != fmtTypeIDName {
 					name = sconv(s, 0, mode) // qualify non-exported names (used on structs, not on funarg)
 				}
 			} else {
@@ -754,7 +741,7 @@ func FmtConst(v constant.Value, sharp bool) string {
 
 // TypeHash computes a hash value for type t to use in type switch statements.
 func TypeHash(t *Type) uint32 {
-	p := tconv(t, 0, fmtTypeIDHash)
+	p := t.LinkString()
 
 	// Using SHA256 is overkill, but reduces accidental collisions.
 	h := notsha256.Sum256([]byte(p))