cmd/compile: label redefinition error position is wrong

[odeke-em]

I was just studying the syntax checker and parser and noticed that for label reuse errors in

go/src/cmd/compile/internal/syntax/branches.go

Lines 74 to 80 in 5c08b9e

	name := s.Label.Value
	labels := ls.labels
	if labels == nil {
	labels = make(map[string]*label)
	ls.labels = labels
	} else if alt := labels[name]; alt != nil {
	ls.err(s.Pos(), "label %s already defined at %s", name, alt.lstmt.Label.Pos().String())

while printing the error, we use a label's parsed statement position, of which that position is until the last token that delimits a label aka ":" instead of actually where the label starts from. However, for the original label, we actually use the label's start position. I believe this issue might affect people whose code editors jump to line numbers/positions on errors.

A simple exhibit below https://play.golang.org/p/Oi-4doqbEN1 or inlined below:

package main

func main() {
foo:
foo:
} 

gives the error

prog.go:4:1: label foo defined and not used
prog.go:5:4: label foo already defined at prog.go:4:1

but notice the error message
prog.go:5:4: label foo already defined at prog.go:4:1

the position for the first label is correctly reported as 4:1 but for the redefined one as 5:4 yet it should be 5:1

I am filling this issue for the purposes of asking if this is the intended behavior and if not, for purposes of tracking the bug. This issue is not a regression and exists also in Go1.10

The fix seems straightforward to me, which is to use s.Label.Pos() instead of s.Pos() aka

diff --git a/src/cmd/compile/internal/syntax/branches.go b/src/cmd/compile/internal/syntax/branches.go
index a03e2734d2..cdeec863bc 100644
--- a/src/cmd/compile/internal/syntax/branches.go
+++ b/src/cmd/compile/internal/syntax/branches.go
@@ -77,7 +77,7 @@ func (ls *labelScope) declare(b *block, s *LabeledStmt) *label {
                labels = make(map[string]*label)
                ls.labels = labels
        } else if alt := labels[name]; alt != nil {
-               ls.err(s.Pos(), "label %s already defined at %s", name, alt.lstmt.Label.Pos().String())
+               ls.err(s.Label.Pos(), "label %s already defined at %s", name, alt.lstmt.Label.Pos())
                return alt
        }
        l := &label{b, s, false}

which will then correctly print

$ go tool compile branches.go 
prog.go:4:1: label foo defined and not used
prog.go:5:1: label foo already defined at prog.go:4:1

/cc @griesemer

[mvdan]

@odeke-em why not send that patch as a CL, along with a test?

[gopherbot]

Change https://golang.org/cl/124595 mentions this issue: cmd/compile: fix label redefinition error column numbers

[gopherbot]

Change https://golang.org/cl/142720 mentions this issue: cmd/compile: simplified test case (cleanup)