cmd/compile: invalid literal causes errors in subsequent lines of valid code

[0xmohit]

$ go version
go version go1.6.2 linux/amd64

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

Tried to compile the following (https://play.golang.org/p/hbj73GGYMU):

$ cat -n t.go 
     1  package main
     2
     3  func main() {
     4          _ = append([]byte("hello "), byte('world')) // invalid
     5          _ = append([]byte("hello "), byte('a'))     // ok
     6  }
     7

$  go tool compile t.go 
t.go:4: missing '
t.go:4: syntax error: unexpected orld, expecting comma or )
t.go:4: missing '
t.go:5: syntax error: unexpected ), expecting }
t.go:6: syntax error: non-declaration statement outside function body

No errors should have been reported for lines 5 and 6. missing ' probably should have been reported just once.

[bradfitz]

This was fine in Go 1.4 and Go 1.5 and regressed in Go 1.6, not this cycle, so we can probably live with it for another cycle.

$ GOROOT=$HOME/go1.4 go build x.go
# command-line-arguments
./x.go:4: missing '
./x.go:4: syntax error: unexpected name, expecting )
./x.go:4: missing '

$ GOROOT=$HOME/go1.5 go build x.go
# command-line-arguments
./x.go:4: missing '
./x.go:4: syntax error: unexpected name, expecting )
./x.go:4: missing '

$ GOROOT=$HOME/go1.6 go build x.go
# command-line-arguments
./x.go:4: missing '
./x.go:4: syntax error: unexpected orld, expecting comma or )
./x.go:4: missing '
./x.go:5: syntax error: unexpected ), expecting }
./x.go:6: syntax error: non-declaration statement outside function body

$ GOROOT=$HOME/gotip go build x.go
# command-line-arguments
./x.go:4: missing '
./x.go:4: syntax error: unexpected orld, expecting comma or )
./x.go:4: missing '
./x.go:5: syntax error: unexpected ), expecting }
./x.go:6: syntax error: non-declaration statement outside function body

/cc @griesemer @mdempsky

[mdempsky]

Worth looking into for 1.8 for any way we can improve this, but in general recovering from syntax errors is always going to be best effort.

The reason you get two missing ' errors is because after lexing 'w, we expect a ' to end the rune literal. When we don't see one, we emit the error, and then pretend we did get one. Moving along, we then see ') and think that's supposed to be another rune literal, but it's missing a ' too.

[griesemer]

For the reference, gotype simply reports for the example above:

x.go:4:43: illegal rune literal

which is probably the best error message in this case.

The reason is that the go/scanner accepts rune literals with multiple characters and complains if there's more than one. This seems to work better in this specific case.

[gopherbot]

CL https://golang.org/cl/37138 mentions this issue.