internal/lsp/cache: hide diagnostics for ignored dirs

The go command ignores go files in specially-named directories like
testdata. We probably still want to try to offer services like
autocomplete and jump-to-definition there, but diagnostics are less
likely to be helpful. As a compromise, just squash diagnostics.

Note that the rules we use are slightly wrong; see the comment on
ignoreFiles for details.

Fixes golang/go#39563.

Change-Id: I0bc00639e68bd71ea55d867af36e07ef4ec780a1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/237638
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/regtest/diagnostics_test.go

@@ -803,3 +803,24 @@ func TestHello(t *testing.T) {
 		)
 	})
 }
+
+func TestIgnoredFiles(t *testing.T) {
+	const ws = `
+-- go.mod --
+module mod.com
+
+go 1.15
+-- _foo/x.go --
+package x
+
+var _ = foo.Bar
+`
+	runner.Run(t, ws, func(t *testing.T, env *Env) {
+		env.OpenFile("_foo/x.go")
+		env.Await(
+			OnceMet(
+				CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidOpen), 1),
+				NoDiagnostics("_foo/x.go"),
+			))
+	})
+}

internal/lsp/source/diagnostics.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 	"go/ast"
+	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -42,6 +43,14 @@ type RelatedInformation struct {
 }
 
 func Diagnostics(ctx context.Context, snapshot Snapshot, ph PackageHandle, missingModules map[string]*modfile.Require, withAnalysis bool) (map[FileIdentity][]*Diagnostic, bool, error) {
+	onlyIgnoredFiles := true
+	for _, pgh := range ph.CompiledGoFiles() {
+		onlyIgnoredFiles = onlyIgnoredFiles && ignoreFile(pgh.File().URI().Filename())
+	}
+	if onlyIgnoredFiles {
+		return nil, false, nil
+	}
+
 	// If we are missing dependencies, it may because the user's workspace is
 	// not correctly configured. Report errors, if possible.
 	var warn bool
@@ -134,6 +143,24 @@ func Diagnostics(ctx context.Context, snapshot Snapshot, ph PackageHandle, missi
 	return reports, warn, nil
 }
 
+// ignoreFile is an approximation of go list's exclusion rules. go help list:
+// 		Directory and file names that begin with "." or "_" are ignored
+// 		by the go tool, as are directories named "testdata".
+// Those rules only apply if the directory in question is beneath GOPATH
+// or the module root, but we don't really have that information, so we just
+// hope that nobody's messing with us.
+func ignoreFile(path string) bool {
+	for _, component := range strings.Split(path, string(filepath.Separator)) {
+		if len(component) == 0 {
+			continue
+		}
+		if component[0] == '.' || component[0] == '_' || component == "testdata" {
+			return true
+		}
+	}
+	return false
+}
+
 func FileDiagnostics(ctx context.Context, snapshot Snapshot, uri span.URI) (FileIdentity, []*Diagnostic, error) {
 	fh, err := snapshot.GetFile(ctx, uri)
 	if err != nil {