diff --git a/internal/lsp/cache/file.go b/internal/lsp/cache/file.go
index d0683ebe856..f71a15e570e 100644
--- a/internal/lsp/cache/file.go
+++ b/internal/lsp/cache/file.go
@@ -6,7 +6,6 @@ package cache
 
 import (
 	"context"
-	"go/ast"
 	"go/token"
 	"path/filepath"
 	"strings"
@@ -18,6 +17,7 @@ import (
 // viewFile extends source.File with helper methods for the view package.
 type viewFile interface {
 	source.File
+
 	filename() string
 	addURI(uri span.URI) int
 }
@@ -33,16 +33,6 @@ type fileBase struct {
 	token *token.File
 }
 
-// goFile holds all the information we know about a go file.
-type goFile struct {
-	fileBase
-
-	ast     *ast.File
-	pkg     *pkg
-	meta    *metadata
-	imports []*ast.ImportSpec
-}
-
 func basename(filename string) string {
 	return strings.ToLower(filepath.Base(filename))
 }
@@ -72,50 +62,7 @@ func (f *fileBase) FileSet() *token.FileSet {
 	return f.view.Session().Cache().FileSet()
 }
 
-func (f *goFile) GetToken(ctx context.Context) *token.File {
-	f.view.mu.Lock()
-	defer f.view.mu.Unlock()
-	if f.token == nil || len(f.view.contentChanges) > 0 {
-		if _, err := f.view.parse(ctx, f); err != nil {
-			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
-			return nil
-		}
-	}
-	return f.token
-}
-
-func (f *goFile) GetAST(ctx context.Context) *ast.File {
-	f.view.mu.Lock()
-	defer f.view.mu.Unlock()
-
-	if f.ast == nil || len(f.view.contentChanges) > 0 {
-		if _, err := f.view.parse(ctx, f); err != nil {
-			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
-			return nil
-		}
-	}
-	return f.ast
-}
-
-func (f *goFile) GetPackage(ctx context.Context) source.Package {
-	f.view.mu.Lock()
-	defer f.view.mu.Unlock()
-
-	if f.pkg == nil || len(f.view.contentChanges) > 0 {
-		if errs, err := f.view.parse(ctx, f); err != nil {
-			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
-
-			// Create diagnostics for errors if we are able to.
-			if len(errs) > 0 {
-				return &pkg{errors: errs}
-			}
-			return nil
-		}
-	}
-	return f.pkg
-}
-
-// read is the internal part of Content. It assumes that the caller is
+// read is the internal part of GetContent. It assumes that the caller is
 // holding the mutex of the file's view.
 func (f *fileBase) read(ctx context.Context) {
 	if err := ctx.Err(); err != nil {
@@ -144,53 +91,3 @@ func (f *fileBase) read(ctx context.Context) {
 func (f *goFile) isPopulated() bool {
 	return f.ast != nil && f.token != nil && f.pkg != nil && f.meta != nil && f.imports != nil
 }
-
-func (f *goFile) GetActiveReverseDeps(ctx context.Context) []source.GoFile {
-	pkg := f.GetPackage(ctx)
-	if pkg == nil {
-		return nil
-	}
-
-	f.view.mu.Lock()
-	defer f.view.mu.Unlock()
-
-	f.view.mcache.mu.Lock()
-	defer f.view.mcache.mu.Unlock()
-
-	seen := make(map[string]struct{}) // visited packages
-	results := make(map[*goFile]struct{})
-	f.view.reverseDeps(ctx, seen, results, pkg.PkgPath())
-
-	var files []source.GoFile
-	for rd := range results {
-		if rd == nil {
-			continue
-		}
-		// Don't return any of the active files in this package.
-		if rd.pkg != nil && rd.pkg == pkg {
-			continue
-		}
-		files = append(files, rd)
-	}
-	return files
-}
-
-func (v *view) reverseDeps(ctx context.Context, seen map[string]struct{}, results map[*goFile]struct{}, pkgPath string) {
-	if _, ok := seen[pkgPath]; ok {
-		return
-	}
-	seen[pkgPath] = struct{}{}
-	m, ok := v.mcache.packages[pkgPath]
-	if !ok {
-		return
-	}
-	for _, filename := range m.files {
-		uri := span.FileURI(filename)
-		if f, err := v.getFile(uri); err == nil && v.session.IsOpen(uri) {
-			results[f.(*goFile)] = struct{}{}
-		}
-	}
-	for parentPkgPath := range m.parents {
-		v.reverseDeps(ctx, seen, results, parentPkgPath)
-	}
-}
diff --git a/internal/lsp/cache/gofile.go b/internal/lsp/cache/gofile.go
new file mode 100644
index 00000000000..aab3d829005
--- /dev/null
+++ b/internal/lsp/cache/gofile.go
@@ -0,0 +1,113 @@
+package cache
+
+import (
+	"context"
+	"go/ast"
+	"go/token"
+
+	"golang.org/x/tools/internal/lsp/source"
+	"golang.org/x/tools/internal/span"
+)
+
+// goFile holds all of the information we know about a Go file.
+type goFile struct {
+	fileBase
+
+	ast     *ast.File
+	pkg     *pkg
+	meta    *metadata
+	imports []*ast.ImportSpec
+}
+
+func (f *goFile) GetToken(ctx context.Context) *token.File {
+	f.view.mu.Lock()
+	defer f.view.mu.Unlock()
+	if f.token == nil || len(f.view.contentChanges) > 0 {
+		if _, err := f.view.parse(ctx, f); err != nil {
+			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
+			return nil
+		}
+	}
+	return f.token
+}
+
+func (f *goFile) GetAST(ctx context.Context) *ast.File {
+	f.view.mu.Lock()
+	defer f.view.mu.Unlock()
+
+	if f.ast == nil || len(f.view.contentChanges) > 0 {
+		if _, err := f.view.parse(ctx, f); err != nil {
+			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
+			return nil
+		}
+	}
+	return f.ast
+}
+
+func (f *goFile) GetPackage(ctx context.Context) source.Package {
+	f.view.mu.Lock()
+	defer f.view.mu.Unlock()
+
+	if f.pkg == nil || len(f.view.contentChanges) > 0 {
+		if errs, err := f.view.parse(ctx, f); err != nil {
+			f.View().Session().Logger().Errorf(ctx, "unable to check package for %s: %v", f.URI(), err)
+
+			// Create diagnostics for errors if we are able to.
+			if len(errs) > 0 {
+				return &pkg{errors: errs}
+			}
+			return nil
+		}
+	}
+	return f.pkg
+}
+
+func (f *goFile) GetActiveReverseDeps(ctx context.Context) []source.GoFile {
+	pkg := f.GetPackage(ctx)
+	if pkg == nil {
+		return nil
+	}
+
+	f.view.mu.Lock()
+	defer f.view.mu.Unlock()
+
+	f.view.mcache.mu.Lock()
+	defer f.view.mcache.mu.Unlock()
+
+	seen := make(map[string]struct{}) // visited packages
+	results := make(map[*goFile]struct{})
+	f.view.reverseDeps(ctx, seen, results, pkg.PkgPath())
+
+	var files []source.GoFile
+	for rd := range results {
+		if rd == nil {
+			continue
+		}
+		// Don't return any of the active files in this package.
+		if rd.pkg != nil && rd.pkg == pkg {
+			continue
+		}
+		files = append(files, rd)
+	}
+	return files
+}
+
+func (v *view) reverseDeps(ctx context.Context, seen map[string]struct{}, results map[*goFile]struct{}, pkgPath string) {
+	if _, ok := seen[pkgPath]; ok {
+		return
+	}
+	seen[pkgPath] = struct{}{}
+	m, ok := v.mcache.packages[pkgPath]
+	if !ok {
+		return
+	}
+	for _, filename := range m.files {
+		uri := span.FileURI(filename)
+		if f, err := v.getFile(uri); err == nil && v.session.IsOpen(uri) {
+			results[f.(*goFile)] = struct{}{}
+		}
+	}
+	for parentPkgPath := range m.parents {
+		v.reverseDeps(ctx, seen, results, parentPkgPath)
+	}
+}
diff --git a/internal/lsp/cache/modfile.go b/internal/lsp/cache/modfile.go
new file mode 100644
index 00000000000..b912a6cc241
--- /dev/null
+++ b/internal/lsp/cache/modfile.go
@@ -0,0 +1,16 @@
+package cache
+
+import (
+	"context"
+	"go/token"
+)
+
+// modFile holds all of the information we know about a mod file.
+type modFile struct {
+	fileBase
+}
+
+func (*modFile) GetToken(context.Context) *token.File { return nil }
+func (*modFile) setContent(content []byte)            {}
+func (*modFile) filename() string                     { return "" }
+func (*modFile) isActive() bool                       { return false }
diff --git a/internal/lsp/cache/sumfile.go b/internal/lsp/cache/sumfile.go
new file mode 100644
index 00000000000..03c11a00a6e
--- /dev/null
+++ b/internal/lsp/cache/sumfile.go
@@ -0,0 +1,16 @@
+package cache
+
+import (
+	"context"
+	"go/token"
+)
+
+// sumFile holds all of the information we know about a sum file.
+type sumFile struct {
+	fileBase
+}
+
+func (*sumFile) GetToken(context.Context) *token.File { return nil }
+func (*sumFile) setContent(content []byte)            {}
+func (*sumFile) filename() string                     { return "" }
+func (*sumFile) isActive() bool                       { return false }
diff --git a/internal/lsp/cache/view.go b/internal/lsp/cache/view.go
index 7908f0b6250..96983b1cebd 100644
--- a/internal/lsp/cache/view.go
+++ b/internal/lsp/cache/view.go
@@ -159,6 +159,13 @@ func (v *view) shutdown(context.Context) {
 	}
 }
 
+// Ignore checks if the given URI is a URI we ignore.
+// As of right now, we only ignore files in the "builtin" package.
+func (v *view) Ignore(uri span.URI) bool {
+	_, ok := v.ignoredURIs[uri]
+	return ok
+}
+
 func (v *view) BackgroundContext() context.Context {
 	v.mu.Lock()
 	defer v.mu.Unlock()
@@ -306,7 +313,7 @@ func (v *view) getFile(uri span.URI) (viewFile, error) {
 	if err != nil {
 		return nil, err
 	}
-	var f *goFile
+	var f viewFile
 	switch ext := filepath.Ext(filename); ext {
 	case ".go":
 		f = &goFile{
@@ -315,25 +322,30 @@ func (v *view) getFile(uri span.URI) (viewFile, error) {
 				fname: filename,
 			},
 		}
+		v.session.filesWatchMap.Watch(uri, func() {
+			f.(*goFile).invalidate()
+		})
 	case ".mod":
-		return nil, fmt.Errorf("mod files are not yet supported")
+		f = &modFile{
+			fileBase: fileBase{
+				view:  v,
+				fname: filename,
+			},
+		}
+	case ".sum":
+		f = &sumFile{
+			fileBase: fileBase{
+				view:  v,
+				fname: filename,
+			},
+		}
 	default:
 		return nil, fmt.Errorf("unsupported file extension: %s", ext)
 	}
-	v.session.filesWatchMap.Watch(uri, func() {
-		f.invalidate()
-	})
 	v.mapFile(uri, f)
 	return f, nil
 }
 
-// Ignore checks if the given URI is a URI we ignore.
-// As of right now, we only ignore files in the "builtin" package.
-func (v *view) Ignore(uri span.URI) bool {
-	_, ok := v.ignoredURIs[uri]
-	return ok
-}
-
 // findFile checks the cache for any file matching the given uri.
 //
 // An error is only returned for an irreparable failure, for example, if the
diff --git a/internal/lsp/diagnostics.go b/internal/lsp/diagnostics.go
index 4e4f7fc34ef..ffc7c82babf 100644
--- a/internal/lsp/diagnostics.go
+++ b/internal/lsp/diagnostics.go
@@ -13,14 +13,24 @@ import (
 	"golang.org/x/tools/internal/span"
 )
 
-func (s *Server) Diagnostics(ctx context.Context, view source.View, uri span.URI) {
+func (s *Server) Diagnostics(ctx context.Context, v source.View, uri span.URI) {
 	if ctx.Err() != nil {
 		s.session.Logger().Errorf(ctx, "canceling diagnostics for %s: %v", uri, ctx.Err())
 		return
 	}
-	reports, err := source.Diagnostics(ctx, view, uri)
+	f, err := v.GetFile(ctx, uri)
 	if err != nil {
-		s.session.Logger().Errorf(ctx, "failed to compute diagnostics for %s: %v", uri, err)
+		s.session.Logger().Errorf(ctx, "no file for %s: %v", uri, err)
+		return
+	}
+	// For non-Go files, don't return any diagnostics.
+	gof, ok := f.(source.GoFile)
+	if !ok {
+		return
+	}
+	reports, err := source.Diagnostics(ctx, v, gof)
+	if err != nil {
+		s.session.Logger().Errorf(ctx, "failed to compute diagnostics for %s: %v", gof.URI(), err)
 		return
 	}
 
@@ -28,7 +38,7 @@ func (s *Server) Diagnostics(ctx context.Context, view source.View, uri span.URI
 	defer s.undeliveredMu.Unlock()
 
 	for uri, diagnostics := range reports {
-		if err := s.publishDiagnostics(ctx, view, uri, diagnostics); err != nil {
+		if err := s.publishDiagnostics(ctx, v, uri, diagnostics); err != nil {
 			if s.undelivered == nil {
 				s.undelivered = make(map[span.URI][]source.Diagnostic)
 			}
@@ -41,7 +51,7 @@ func (s *Server) Diagnostics(ctx context.Context, view source.View, uri span.URI
 	// Anytime we compute diagnostics, make sure to also send along any
 	// undelivered ones (only for remaining URIs).
 	for uri, diagnostics := range s.undelivered {
-		err := s.publishDiagnostics(ctx, view, uri, diagnostics)
+		err := s.publishDiagnostics(ctx, v, uri, diagnostics)
 		if err != nil {
 			s.session.Logger().Errorf(ctx, "failed to deliver diagnostic for %s: %v", uri, err)
 		}
diff --git a/internal/lsp/lsp_test.go b/internal/lsp/lsp_test.go
index 7d20a5d300b..61156f25bfc 100644
--- a/internal/lsp/lsp_test.go
+++ b/internal/lsp/lsp_test.go
@@ -61,7 +61,15 @@ func testLSP(t *testing.T, exporter packagestest.Exporter) {
 func (r *runner) Diagnostics(t *testing.T, data tests.Diagnostics) {
 	v := r.server.session.View(viewName)
 	for uri, want := range data {
-		results, err := source.Diagnostics(context.Background(), v, uri)
+		f, err := v.GetFile(context.Background(), uri)
+		if err != nil {
+			t.Fatalf("no file for %s: %v", f, err)
+		}
+		gof, ok := f.(source.GoFile)
+		if !ok {
+			t.Fatalf("%s is not a Go file: %v", uri, err)
+		}
+		results, err := source.Diagnostics(context.Background(), v, gof)
 		if err != nil {
 			t.Fatal(err)
 		}
diff --git a/internal/lsp/source/analysis.go b/internal/lsp/source/analysis.go
index f7021b20b14..46e64b41228 100644
--- a/internal/lsp/source/analysis.go
+++ b/internal/lsp/source/analysis.go
@@ -80,7 +80,7 @@ type packageFactKey struct {
 }
 
 func (act *Action) String() string {
-	return fmt.Sprintf("%s@%s", act.Analyzer, act.Pkg)
+	return fmt.Sprintf("%s@%s", act.Analyzer, act.Pkg.PkgPath())
 }
 
 func execAll(ctx context.Context, fset *token.FileSet, actions []*Action) error {
@@ -112,7 +112,7 @@ func (act *Action) execOnce(ctx context.Context, fset *token.FileSet) error {
 	var failed []string
 	for _, dep := range act.Deps {
 		if dep.err != nil {
-			failed = append(failed, dep.String())
+			failed = append(failed, fmt.Sprintf("%s: %v", dep.String(), dep.err))
 		}
 	}
 	if failed != nil {
@@ -159,7 +159,7 @@ func (act *Action) execOnce(ctx context.Context, fset *token.FileSet) error {
 	act.pass = pass
 
 	if len(act.Pkg.GetErrors()) > 0 && !pass.Analyzer.RunDespiteErrors {
-		act.err = fmt.Errorf("analysis skipped due to errors in package")
+		act.err = fmt.Errorf("analysis skipped due to errors in package: %v", act.Pkg.GetErrors())
 	} else {
 		act.result, act.err = pass.Analyzer.Run(pass)
 		if act.err == nil {
diff --git a/internal/lsp/source/diagnostics.go b/internal/lsp/source/diagnostics.go
index b3d78689afe..922840f1940 100644
--- a/internal/lsp/source/diagnostics.go
+++ b/internal/lsp/source/diagnostics.go
@@ -51,19 +51,10 @@ const (
 	SeverityError
 )
 
-func Diagnostics(ctx context.Context, v View, uri span.URI) (map[span.URI][]Diagnostic, error) {
-	f, err := v.GetFile(ctx, uri)
-	if err != nil {
-		return singleDiagnostic(uri, "no file found for %s", uri), nil
-	}
-	// For non-Go files, don't return any diagnostics.
-	gof, ok := f.(GoFile)
-	if !ok {
-		return nil, nil
-	}
-	pkg := gof.GetPackage(ctx)
+func Diagnostics(ctx context.Context, v View, f GoFile) (map[span.URI][]Diagnostic, error) {
+	pkg := f.GetPackage(ctx)
 	if pkg == nil {
-		return singleDiagnostic(uri, "%s is not part of a package", uri), nil
+		return singleDiagnostic(f.URI(), "%s is not part of a package", f.URI()), nil
 	}
 	// Prepare the reports we will send for this package.
 	reports := make(map[span.URI][]Diagnostic)
@@ -84,11 +75,11 @@ func Diagnostics(ctx context.Context, v View, uri span.URI) (map[span.URI][]Diag
 	if !diagnostics(ctx, v, pkg, reports) {
 		// If we don't have any list, parse, or type errors, run analyses.
 		if err := analyses(ctx, v, pkg, reports); err != nil {
-			return singleDiagnostic(uri, "failed to run analyses for %s: %v", uri, err), nil
+			v.Session().Logger().Errorf(ctx, "failed to run analyses for %s: %v", f.URI(), err)
 		}
 	}
 	// Updates to the diagnostics for this package may need to be propagated.
-	for _, f := range gof.GetActiveReverseDeps(ctx) {
+	for _, f := range f.GetActiveReverseDeps(ctx) {
 		pkg := f.GetPackage(ctx)
 		if pkg == nil {
 			continue
@@ -184,7 +175,6 @@ func packageErrorSpan(pkgErr packages.Error) span.Span {
 	if pkgErr.Pos == "" {
 		return parseDiagnosticMessage(pkgErr.Msg)
 	}
-
 	return span.Parse(pkgErr.Pos)
 }
 
diff --git a/internal/lsp/source/source_test.go b/internal/lsp/source/source_test.go
index f89ec0c63e7..fa2209188e0 100644
--- a/internal/lsp/source/source_test.go
+++ b/internal/lsp/source/source_test.go
@@ -52,7 +52,11 @@ func testSource(t *testing.T, exporter packagestest.Exporter) {
 
 func (r *runner) Diagnostics(t *testing.T, data tests.Diagnostics) {
 	for uri, want := range data {
-		results, err := source.Diagnostics(context.Background(), r.view, uri)
+		f, err := r.view.GetFile(context.Background(), uri)
+		if err != nil {
+			t.Fatal(err)
+		}
+		results, err := source.Diagnostics(context.Background(), r.view, f.(source.GoFile))
 		if err != nil {
 			t.Fatal(err)
 		}
diff --git a/internal/lsp/source/view.go b/internal/lsp/source/view.go
index b4380317210..e00cff87aa5 100644
--- a/internal/lsp/source/view.go
+++ b/internal/lsp/source/view.go
@@ -156,6 +156,14 @@ type GoFile interface {
 	GetActiveReverseDeps(ctx context.Context) []GoFile
 }
 
+type ModFile interface {
+	File
+}
+
+type SumFile interface {
+	File
+}
+
 // Package represents a Go package that has been type-checked. It maintains
 // only the relevant fields of a *go/packages.Package.
 type Package interface {