internal/lsp: fix go.mod parse error parsing to show diagnostics

This change adds support for showing go.mod parse errors as diagnostics.
There was previously a bug in parsing these error messages, and the
diagnostic reports were also not being published if the IWL failed.

Change-Id: I56197b91c49fb68e35d8886175435d5c38bfb8c2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/272089
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

Author: stamblerre

gopls/internal/regtest/modfile_test.go

@@ -630,3 +630,23 @@ func main() {
 		)
 	})
 }
+
+func TestModTypoDiagnostic(t *testing.T) {
+	const mod = `
+-- go.mod --
+module mod.com
+
+go 1.12
+-- main.go --
+package main
+
+func main() {}
+`
+	run(t, mod, func(t *testing.T, env *Env) {
+		env.OpenFile("go.mod")
+		env.RegexpReplace("go.mod", "module", "modul")
+		env.Await(
+			env.DiagnosticAtRegexp("go.mod", "modul"),
+		)
+	})
+}

internal/lsp/cache/mod.go

@@ -142,7 +142,7 @@ func extractModParseErrors(uri span.URI, m *protocol.ColumnMapper, parseErr erro
 	// The error returned from the modfile package only returns a line number,
 	// so we assume that the diagnostic should be for the entire line.
 	endOfLine := len(lines[line-1])
-	sOffset, err := m.Converter.ToOffset(line, 0)
+	sOffset, err := m.Converter.ToOffset(line, 1)
 	if err != nil {
 		return nil, err
 	}

internal/lsp/diagnostics.go

@@ -194,12 +194,10 @@ func (s *Server) diagnose(ctx context.Context, snapshot source.Snapshot, alwaysA
 	if err != nil {
 		// Some error messages can also be displayed as diagnostics.
 		if errList := (*source.ErrorList)(nil); errors.As(err, &errList) {
-			if err := errorsToDiagnostic(ctx, snapshot, *errList, reports); err == nil {
-				return reports, nil
-			}
+			_ = errorsToDiagnostic(ctx, snapshot, *errList, reports)
 		}
 		event.Error(ctx, "errors diagnosing workspace", err, tag.Snapshot.Of(snapshot.ID()), tag.Directory.Of(snapshot.View().Folder()))
-		return nil, nil
+		return reports, nil
 	}
 
 	var (