cmd/compile, runtime: fix pedantic int->string conversions

Previously, cmd/compile rejected constant int->string conversions if
the integer value did not fit into an "int" value. Also, runtime
incorrectly truncated 64-bit values to 32-bit before checking if
they're a valid Unicode code point. According to the Go spec, both of
these cases should instead yield "\uFFFD".

Fixes #15039.

Change-Id: I3c8a3ad9a0780c0a8dc1911386a523800fec9764
Reviewed-on: https://go-review.googlesource.com/21344Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -468,12 +468,12 @@ func overflow(v Val, t *Type) {
 func tostr(v Val) Val {
 	switch v.Ctype() {
 	case CTINT, CTRUNE:
-		if v.U.(*Mpint).Cmp(Minintval[TINT]) < 0 || v.U.(*Mpint).Cmp(Maxintval[TINT]) > 0 {
-			Yyerror("overflow in int -> string")
+		var i int64 = 0xFFFD
+		if u := v.U.(*Mpint); u.Cmp(Minintval[TUINT32]) >= 0 && u.Cmp(Maxintval[TUINT32]) <= 0 {
+			i = u.Int64()
 		}
-		r := uint(v.U.(*Mpint).Int64())
 		v = Val{}
-		v.U = string(r)
+		v.U = string(i)
 
 	case CTFLT:
 		Yyerror("no float -> string")

src/runtime/string.go

@@ -236,6 +236,9 @@ func intstring(buf *[4]byte, v int64) string {
 	} else {
 		s, b = rawstring(4)
 	}
+	if int64(rune(v)) != v {
+		v = runeerror
+	}
 	n := runetochar(b, rune(v))
 	return s[:n]
 }

test/fixedbugs/issue15039.go

+// run
+
+// Copyright 2016 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
+
+func main() {
+	const fffd = "\uFFFD"
+
+	// runtime.intstring used to convert int64 to rune without checking
+	// for truncation.
+	u := uint64(0x10001f4a9)
+	big := string(u)
+	if big != fffd {
+		panic("big != bad")
+	}
+
+	// cmd/compile used to require integer constants to fit into an "int".
+	const huge = string(1<<100)
+	if huge != fffd {
+		panic("huge != bad")
+	}
+}