x/tools/gopls: embed errors don't always appear

[heschik]

This program shows no error on the embed pattern:

package embedtest

import (
	_ "embed"
)

//go:embed NONEXISTANT
var foo string

When I add an import, the error appears, but only if it's not in the stdlib:

package embedtest

import (
	_ "embed"

	_ "golang.org/x/tools/imports"
)

//go:embed NONEXISTANT
var foo string

At first I thought this was because we should invalidate metadata when a //go:embed comment changes, but even when I restart gopls the IWL doesn't pick up the error. So there's something weird going on here that I didn't dig into. (We should probably still invalidate metadata on embed comment changes, or implement embed pattern checking ourselves.)

Reported by @myitcv in Slack.

[heschik]

go/src/go/build/read.go

Lines 174 to 273 in c73232d

	// findEmbed advances the input reader to the next //go:embed comment.
	// It reports whether it found a comment.
	// (Otherwise it found an error or EOF.)
	func (r *importReader) findEmbed(first bool) bool {
	// The import block scan stopped after a non-space character,
	// so the reader is not at the start of a line on the first call.
	// After that, each //go:embed extraction leaves the reader
	// at the end of a line.
	startLine := !first
	var c byte
	for r.err == nil && !r.eof {
	c = r.readByteNoBuf()
	Reswitch:
	switch c {
	default:
	startLine = false
	case '\n':
	startLine = true
	case ' ', '\t':
	// leave startLine alone
	case '"':
	startLine = false
	for r.err == nil {
	if r.eof {
	r.syntaxError()
	}
	c = r.readByteNoBuf()
	if c == '\\' {
	r.readByteNoBuf()
	if r.err != nil {
	r.syntaxError()
	return false
	}
	continue
	}
	if c == '"' {
	c = r.readByteNoBuf()
	goto Reswitch
	}
	}
	goto Reswitch
	case '`':
	startLine = false
	for r.err == nil {
	if r.eof {
	r.syntaxError()
	}
	c = r.readByteNoBuf()
	if c == '`' {
	c = r.readByteNoBuf()
	goto Reswitch
	}
	}
	case '/':
	c = r.readByteNoBuf()
	switch c {
	default:
	startLine = false
	goto Reswitch
	case '*':
	var c1 byte
	for (c != '*' || c1 != '/') && r.err == nil {
	if r.eof {
	r.syntaxError()
	}
	c, c1 = c1, r.readByteNoBuf()
	}
	startLine = false
	case '/':
	if startLine {
	// Try to read this as a //go:embed comment.
	for i := range goEmbed {
	c = r.readByteNoBuf()
	if c != goEmbed[i] {
	goto SkipSlashSlash
	}
	}
	c = r.readByteNoBuf()
	if c == ' ' || c == '\t' {
	// Found one!
	return true
	}
	}
	SkipSlashSlash:
	for c != '\n' && r.err == nil && !r.eof {
	c = r.readByteNoBuf()
	}
	startLine = true
	}
	}
	}
	return false
	}

is the code that reads embed comments if we want to track them.

[gopherbot]

Change https://golang.org/cl/295415 mentions this issue: gopls/internal/regtest: add test for bad embed rules

[gopherbot]

Change https://golang.org/cl/295412 mentions this issue: internal/lsp/cache: parse filenames from go list errors correctly

[gopherbot]

Change https://golang.org/cl/296549 mentions this issue: internal/lsp/cache: invalidate metadata on magic comment changes