gopls/internal/analysis/fillstruct: don't panic with invalid fields

Fixes golang/go#63921

Change-Id: Ic7c310f39cd92c93b9fd6cc78a35ab88ba6b3f7b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/549116
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/analysis/fillstruct/fillstruct.go

@@ -345,6 +345,8 @@ func indent(str, ind []byte) []byte {
 //
 // The reasoning here is that users will call fillstruct with the intention of
 // initializing the struct, in which case setting these fields to nil has no effect.
+//
+// populateValue returns nil if the value cannot be filled.
 func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 	switch u := typ.Underlying().(type) {
 	case *types.Basic:
@@ -357,6 +359,8 @@ func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 			return &ast.BasicLit{Kind: token.STRING, Value: `""`}
 		case u.Kind() == types.UnsafePointer:
 			return ast.NewIdent("nil")
+		case u.Kind() == types.Invalid:
+			return nil
 		default:
 			panic(fmt.Sprintf("unknown basic type %v", u))
 		}
@@ -478,9 +482,13 @@ func populateValue(f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
 				},
 			}
 		default:
+			x := populateValue(f, pkg, u.Elem())
+			if x == nil {
+				return nil
+			}
 			return &ast.UnaryExpr{
 				Op: token.AND,
-				X:  populateValue(f, pkg, u.Elem()),
+				X:  x,
 			}
 		}
 

gopls/internal/test/marker/testdata/codeaction/fill_struct.txt

@@ -558,3 +558,17 @@ func _[T any]() {
 +	_ = S{
 +		t: *new(T),
 +	} //@codeactionedit("}", "refactor.rewrite", typeparams5)
+-- issue63921.go --
+package fillstruct
+
+// Test for golang/go#63921: fillstruct panicked with invalid fields.
+type invalidStruct struct {
+  F int
+  Undefined
+}
+
+func _() {
+  // Note: the golden content for issue63921 is empty: fillstruct produces no
+  // edits, but does not panic.
+  invalidStruct{} //@codeactionedit("}", "refactor.rewrite", issue63921)
+}