gopls/internal/regtest: add a test that ignoring a file resolves errors

Add a test that when a user adds a "//go:build ignore" to a file, gopls
correctly invalidates diagnostics.

This used to be broken, but was fixed by CL 417576.

Fixes golang/go#54147

Change-Id: I554fcfc0a56b72f657e19b3c0ae53a66d1a99a76
Reviewed-on: https://go-review.googlesource.com/c/tools/+/420537
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>

gopls/internal/regtest/workspace/metadata_test.go

@@ -41,3 +41,63 @@ const C = 42
 		))
 	})
 }
+
+// Test that moving ignoring a file via build constraints causes diagnostics to
+// be resolved.
+func TestIgnoreFile(t *testing.T) {
+	testenv.NeedsGo1Point(t, 16) // needs native overlays
+
+	const src = `
+-- go.mod --
+module mod.test
+
+go 1.12
+-- foo.go --
+package main
+
+func main() {}
+-- bar.go --
+package main
+
+func main() {}
+	`
+
+	WithOptions(
+		// TODO(golang/go#54180): we don't run in 'experimental' mode here, because
+		// with "experimentalUseInvalidMetadata", this test fails because the
+		// orphaned bar.go is diagnosed using stale metadata, and then not
+		// re-diagnosed when new metadata arrives.
+		//
+		// We could fix this by re-running diagnostics after a load, but should
+		// consider whether that is worthwhile.
+		Modes(Default),
+	).Run(t, src, func(t *testing.T, env *Env) {
+		env.OpenFile("foo.go")
+		env.OpenFile("bar.go")
+		env.Await(
+			OnceMet(
+				env.DoneWithOpen(),
+				env.DiagnosticAtRegexp("foo.go", "func (main)"),
+				env.DiagnosticAtRegexp("bar.go", "func (main)"),
+			),
+		)
+		// Ignore bar.go. This should resolve diagnostics.
+		env.RegexpReplace("bar.go", "package main", "// +build ignore\n\npackage main")
+
+		// To make this test pass with experimentalUseInvalidMetadata, we could make
+		// an arbitrary edit that invalidates the snapshot, at which point the
+		// orphaned diagnostics will be invalidated.
+		//
+		// But of course, this should not be necessary: we should invalidate stale
+		// information when fresh metadata arrives.
+		// env.RegexpReplace("foo.go", "package main", "package main // test")
+		env.Await(
+			OnceMet(
+				env.DoneWithChange(),
+				EmptyDiagnostics("foo.go"),
+				env.DiagnosticAtRegexpWithMessage("bar.go", "package (main)", "No packages"),
+				env.NoDiagnosticAtRegexp("bar.go", "func (main)"),
+			),
+		)
+	})
+}