cmd/compile: fix ICE due to bad rune width

It was possible that

    var X interface{} = 'x'

could cause a compilation failure due to having not calculated rune's
width yet. typecheck.go normally calculates the width of things, but
it doesn't for implicit conversions to default type. We already
compute the width of all of the standard numeric types in universe.go,
but we failed to calculate it for the rune alias type. So we could
later crash if the code never otherwise explicitly mentioned 'rune'.

While here, explicitly compute widths for 'byte' and 'error' for
consistency.

Fixes #29350.

Change-Id: Ifedd4899527c983ee5258dcf75aaf635b6f812f8
Reviewed-on: https://go-review.googlesource.com/c/155380Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -386,6 +386,7 @@ func lexinit1() {
 	types.Errortype.Sym = s
 	types.Errortype.Orig = makeErrorInterface()
 	s.Def = asTypesNode(typenod(types.Errortype))
+	dowidth(types.Errortype)
 
 	// We create separate byte and rune types for better error messages
 	// rather than just creating type alias *types.Sym's for the uint8 and
@@ -401,6 +402,7 @@ func lexinit1() {
 	types.Bytetype.Sym = s
 	s.Def = asTypesNode(typenod(types.Bytetype))
 	asNode(s.Def).Name = new(Name)
+	dowidth(types.Bytetype)
 
 	// rune alias
 	s = builtinpkg.Lookup("rune")
@@ -408,6 +410,7 @@ func lexinit1() {
 	types.Runetype.Sym = s
 	s.Def = asTypesNode(typenod(types.Runetype))
 	asNode(s.Def).Name = new(Name)
+	dowidth(types.Runetype)
 
 	// backend-dependent builtin types (e.g. int).
 	for _, s := range typedefs {

test/fixedbugs/issue29350.go

+// compile
+
+// Copyright 2018 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 p
+
+var X interface{} = 'x'