x/tools/gopls: unexpected line/character calculations in certain diagnostics

[mattmassicotte]

What did you do?

I've run into a situation where gopls was consistently returning a diagnostic with range data that was inconsistent with the editor state. Critically, it was producing a range with a character that was past the end of the line.

While I uncovered the issue interacting with gopls via the LSP interface, it appears to exhibit the behavior on the command line as well. Here's the code that reproduces the issue for me:

package main

func testFunc() error {
	releases := []int{1, 2, 3}

	for _, _ := range releases {
		if ;
	}

	return nil
}

func main() {
	testFunc()
}

What did you expect to see?

The line at issue, which has an intentional error, is the if ;. When running gopls from the command line, I get the following (expected) output:

file.go:8:2: expected operand, found '}'

However if I exchange that offending semi-colon for an r character, I get the following unexpected, and possibly incorrect, output:

file.m:7:7: expected operand, found '}'

Not only is the line change surprising, the character offset is past the end of the line.

What did you see instead?

I would have expected the line/character output to be unchanged, or changed, but still consistent with the length of the line in question.

Build info

golang.org/x/tools/gopls 0.4.3
    golang.org/x/tools/gopls@v0.4.3 h1:irz7Q+XdHNECamFKbNWKvMV2Ak6zBbwdwbZndG4545I=
    github.com/BurntSushi/toml@v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
    github.com/sergi/go-diff@v1.1.0 h1:we8PVUC3FE2uYfodKH/nBHMSetSfHDR6scGdBi+erh0=
    golang.org/x/mod@v0.2.0 h1:KU7oHjnv3XNWfa5COkzUifxZmxp1TyI7ImMXqFxLwvQ=
    golang.org/x/sync@v0.0.0-20190911185100-cd5d95a43a6e h1:vcxGaoTs7kV8m5Np9uUNQin4BrLOthgV7252N8V+FwY=
    golang.org/x/tools@v0.0.0-20200708181441-6004c8539734 h1:Vc0Vx98oU/O3+qPQ36fnTT5UduS55KLh2uSGbL7mqEo=
    golang.org/x/xerrors@v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
    honnef.co/go/tools@v0.0.1-2020.1.4 h1:UoveltGrhghAA7ePc+e+QYDHXrBps2PqFZiHkGR/xK8=
    mvdan.cc/xurls/v2@v2.2.0 h1:NSZPykBXJFCetGZykLAxaL6SIpvbVy/UFEniIfHAa8A=

Go info

go version go1.13.8 darwin/amd64

[stamblerre]

Thanks for the report! Confirmed that this is a gopls-specific issue by testing out the error messages coming from go/parser and go/types: https://play.golang.org/p/9RFsdfY3haH.