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>

diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index 8d777c399ea57d97649e64085a031d491f8f79ae..55773641ed13ab0fe9f23d9b7a3ba35522eeafa2 100644 (file)
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/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

diff --git a/test/cannotassign.go b/test/cannotassign.go
new file mode 100644 (file)
index 0000000..0de04ec
--- /dev/null
+++ b/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\{\}$"
+
+       var x int = 7
+       x + 1 = 7 // ERROR "cannot assign to x \+ 1$"
+}