go/types: remove structuralString in favor of inlined code

This is a clean port of CL 363154 from types2 to go/types.

Change-Id: I26c18767041db096390e84ba9200ec69b66778d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/364234
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/builtins.go

@@ -339,7 +339,26 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		if y.mode == invalid {
 			return
 		}
-		src, _ := structuralString(y.typ).(*Slice)
+		// src, _ := structuralType(y.typ).(*Slice); but also accepts strings
+		var src *Slice
+		var elem Type // == src.elem if valid
+		if underIs(y.typ, func(u Type) bool {
+			switch u := u.(type) {
+			case *Basic:
+				if isString(u) && (elem == nil || Identical(elem, universeByte)) {
+					elem = universeByte
+					return true
+				}
+			case *Slice:
+				if elem == nil || Identical(elem, u.elem) {
+					elem = u.elem
+					return true
+				}
+			}
+			return false
+		}) {
+			src = NewSlice(elem)
+		}
 
 		if dst == nil || src == nil {
 			check.invalidArg(x, _InvalidCopy, "copy expects slice arguments; found %s and %s", x, &y)

src/go/types/type.go

@@ -80,30 +80,6 @@ func structuralType(typ Type) Type {
 	return nil
 }
 
-// structuralString is like structuralType but also considers []byte
-// and string as "identical". In this case, if successful, the result
-// is always []byte.
-func structuralString(typ Type) Type {
-	var su Type
-	if underIs(typ, func(u Type) bool {
-		if isString(u) {
-			u = NewSlice(universeByte)
-		}
-		if su != nil {
-			u = match(su, u)
-			if u == nil {
-				return false
-			}
-		}
-		// su == nil || match(su, u) != nil
-		su = u
-		return true
-	}) {
-		return su
-	}
-	return nil
-}
-
 // If t is a defined type, asNamed returns that type (possibly after resolving it), otherwise it returns nil.
 func asNamed(t Type) *Named {
 	e, _ := t.(*Named)