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 8d777c3..5577364 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/src/cmd/compile/internal/gc/typecheck.go
@@ -3135,9 +3135,14 @@
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
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\{\}$"
+
+ var x int = 7
+ x + 1 = 7 // ERROR "cannot assign to x \+ 1$"
+}
