x/tools/gopls: fix handling of end of file in internal/span

[dandua98]

Package span currently treats end of file as start of a new line (https://github.com/golang/tools/blob/c024452afbcdebb4a0fbe1bb0eaea0d2dbff835b/internal/span/token.go#L117), which is inaccurate for files with end of file not being a newline. This affects completion on end of file line and we're currently considering a temporary fix there (https://go-review.googlesource.com/c/tools/+/249706). A better fix should come from span itself and the package should check if a final new line is present before converting end of file to start of new line. Just removing this check breaks diff tests and is not a viable fix currently.

[stamblerre]

I think that, rather than changing the span package, we should do our best to work-around this in the cases where it comes up (so far, completion, and perhaps other features that insert edits like code actions). The work-around added in CL 249706 is not ideal because it is untested, so once we have support for completion in package declarations, we should see if we can improve and test it.

/cc @findleyr since we discussed this, and it's relevant to #37702.

[stamblerre]

Given that we have a reasonable work-around for this in completion, removing from the 1.0 milestone.

[findleyr]

I don't understand how this isn't just a bug. Presumably the limitation is driven by the behavior of token.File, which IIRC ignores a trailing newline.

[gopherbot]

Change https://go.dev/cl/459736 mentions this issue: gopls/internal/lsp/safetoken: funnel more calls through this package