cmd/compile/internal/syntax: better error msg for some 'if' statements

R=go1.11

A common error is to write '=' instead of '==' inside the condition
of a simple 'if' statement:

	if x = 0 { ... }

Highlight the fact that we have an assignment in the error message
to prevent further confusion.

Fixes #23385.

Change-Id: I1552050fd6da927bd12a1be0977bd2e98eca5885
Reviewed-on: https://go-review.googlesource.com/87316
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index 6b52950b94e1061e9931e12eda93cf083030d3b5..d01ad6056e4bfdeacc0a4cea831a3c95453bfb3a 100644 (file)
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -170,9 +170,9 @@ func (p *parser) syntax_error_at(pos src.Pos, msg string) {
        switch {
        case msg == "":
                // nothing to do
-       case strings.HasPrefix(msg, "in"), strings.HasPrefix(msg, "at"), strings.HasPrefix(msg, "after"):
+       case strings.HasPrefix(msg, "in "), strings.HasPrefix(msg, "at "), strings.HasPrefix(msg, "after "):
                msg = " " + msg
-       case strings.HasPrefix(msg, "expecting"):
+       case strings.HasPrefix(msg, "expecting "):
                msg = ", " + msg
        default:
                // plain error - we don't care about current token
@@ -1844,7 +1844,15 @@ done:
        case *ExprStmt:
                cond = s.X
        default:
-               p.syntax_error(fmt.Sprintf("%s used as value", String(s)))
+               // A common syntax error is to write '=' instead of '==',
+               // which turns an expression into an assignment. Provide
+               // a more explicit error message in that case to prevent
+               // further confusion.
+               str := String(s)
+               if as, ok := s.(*AssignStmt); ok && as.Op == 0 {
+                       str = "assignment " + str
+               }
+               p.syntax_error(fmt.Sprintf("%s used as value", str))
        }
 
        p.xnest = outer