cmd/compile: use FmtLeft to generate symbol name for unexported interface methods

The bug in 29612 is that there are two similar-looking anonymous interface
types in two different packages, ./p1/ssa and ./p2/ssa:

v.(interface{ foo() }).foo()

These types should be treated differently because the unexported method
makes the types different (according to the spec).

But when generating the type descriptors for those two types, they
both have the name "interface { ssa.foo() }". They thus get the same
symbol, and the linker happily unifies them. It picks an arbitrary one
for the runtime to use, but that breaks conversions from concrete types
that have a foo method from the package which had its interface type
overwritten.

We need to encode the metadata symbol for unexported methods as package
path qualified (The same as we did in CL 27791 for struct fields).

So switching from FmtUnsigned to Fmtleft by default fixes the issue.
In case of generating namedata, FmtUnsigned is used.

The benchmark result ends up in no significant change of compiled binary
compare to the immediate parent.

Fixes #29612

Change-Id: I775aff91ae4a1bb16eb18a48d55e3b606f3f3352
Reviewed-on: https://go-review.googlesource.com/c/go/+/170157Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -752,7 +752,11 @@ func typefmt(t *types.Type, flag FmtFlag, mode fmtMode, depth int) string {
 			case types.IsExported(f.Sym.Name):
 				buf = append(buf, sconv(f.Sym, FmtShort, mode)...)
 			default:
-				buf = append(buf, sconv(f.Sym, FmtUnsigned, mode)...)
+				flag1 := FmtLeft
+				if flag&FmtUnsigned != 0 {
+					flag1 = FmtUnsigned
+				}
+				buf = append(buf, sconv(f.Sym, flag1, mode)...)
 			}
 			buf = append(buf, tconv(f.Type, FmtShort, mode, depth)...)
 		}

test/fixedbugs/issue29612.dir/main.go

+// run
+
+// Copyright 2019 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.
+
+// Do not panic on conversion to anonymous interface, which
+// is similar-looking interface types in different packages.
+
+package main
+
+import (
+	ssa1 "./p1/ssa"
+	ssa2 "./p2/ssa"
+)
+
+func main() {
+	v1 := &ssa1.T{}
+	_ = v1
+
+	v2 := &ssa2.T{}
+	ssa2.Works(v2)
+	ssa2.Panics(v2) // This call must not panic
+}

test/fixedbugs/issue29612.dir/p1/ssa/ssa.go

+// Copyright 2019 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 ssa
+
+type T struct{}
+
+func (T) foo() {}
+
+type fooer interface {
+	foo()
+}
+
+func Unused(v interface{}) {
+	v.(fooer).foo()
+	v.(interface{ foo() }).foo()
+}

test/fixedbugs/issue29612.dir/p2/ssa/ssa.go

+// Copyright 2019 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 ssa
+
+type T struct{}
+
+func (T) foo() {}
+
+type fooer interface {
+	foo()
+}
+
+func Works(v interface{}) {
+	switch v.(type) {
+	case interface{}:
+		v.(fooer).foo()
+	}
+}
+
+func Panics(v interface{}) {
+	switch v.(type) {
+	case interface{}:
+		v.(fooer).foo()
+		v.(interface{ foo() }).foo()
+	}
+}