cmd/compile: be more specific in cannot assign errors

"cannot assign to" compiler errors are very laconic: they never
explain why the lhs cannot be assigned to (with one exception, when
assigning to a struct field in a map).

This change makes them a little more specific, in two more cases: when
assigning to a string, or to a const; by giving a very brief reason
why the lhs cannot be assigned to.

Change-Id: I244cca7fc3c3814e00e0ccadeec62f747c293979
Reviewed-on: https://go-review.googlesource.com/c/go/+/255199
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3135,9 +3135,14 @@ func checkassign(stmt *Node, n *Node) {
 		return
 	}
 
-	if n.Op == ODOT && n.Left.Op == OINDEXMAP {
+	switch {
+	case n.Op == ODOT && n.Left.Op == OINDEXMAP:
 		yyerror("cannot assign to struct field %v in map", n)
-	} else {
+	case (n.Op == OINDEX && n.Left.Type.IsString()) || n.Op == OSLICESTR:
+		yyerror("cannot assign to %v (strings are immutable)", n)
+	case n.Op == OLITERAL && n.Sym != nil && n.isGoConst():
+		yyerror("cannot assign to %v (declared const)", n)
+	default:
 		yyerror("cannot assign to %v", n)
 	}
 	n.Type = nil

test/cannotassign.go

@@ -0,0 +1,33 @@
+// errorcheck
+
+// Copyright 2020 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.
+
+// Test "cannot assign" errors
+
+package main
+
+func main() {
+	var s string = "hello"
+	s[1:2] = "a" // ERROR "cannot assign to .* \(strings are immutable\)"
+	s[3] = "b"   // ERROR "cannot assign to .* \(strings are immutable\)"
+
+	const n int = 1
+	const cs string = "hello"
+	n = 2        // ERROR "cannot assign to .* \(declared const\)"
+	cs = "hi"    // ERROR "cannot assign to .* \(declared const\)"
+	true = false // ERROR "cannot assign to .* \(declared const\)"
+
+	var m map[int]struct{ n int }
+	m[0].n = 7 // ERROR "cannot assign to struct field .* in map$"
+
+	1 = 7         // ERROR "cannot assign to 1$"
+	"hi" = 7      // ERROR `cannot assign to "hi"$`
+	nil = 7       // ERROR "cannot assign to nil$"
+	len("") = 7   // ERROR `cannot assign to len\(""\)$`
+	[]int{} = nil // ERROR "cannot assign to \[\]int\{\}$"
+
+	var x int = 7
+	x + 1 = 7 // ERROR "cannot assign to x \+ 1$"
+}