gopls/internal/golang/completion: fix completion behavior when the source has syntax errors

gopls/internal/golang/completion/completion.go

@@ -305,6 +305,9 @@ type completionContext struct {
 
 	// packageCompletion is true if we are completing a package name.
 	packageCompletion bool
+
+	// removeBeforePeriod is true if we are removing content before the period.
+	removeBeforePeriod bool
 }
 
 // A Selection represents the cursor position and surrounding identifier.
@@ -734,6 +737,12 @@ func (c *completer) containingIdent(src []byte) *ast.Ident {
 		// is a keyword. This improves completion after an "accidental
 		// keyword", e.g. completing to "variance" in "someFunc(var<>)".
 		return fakeIdent
+	} else if c.isCompletionAllowed() && tkn == token.IDENT {
+		// Use manually extracted token even when syntax is incomplete
+		// or contains errors. This provides better developer experience.
+		// Only for certain conditions: when completion is allowed and
+		// the token is an IDENT.
+		return fakeIdent
 	}
 
 	return nil
@@ -743,6 +752,7 @@ func (c *completer) containingIdent(src []byte) *ast.Ident {
 func (c *completer) scanToken(contents []byte) (token.Pos, token.Token, string) {
 	tok := c.pkg.FileSet().File(c.pos)
 
+	var prdPos token.Pos
 	var s scanner.Scanner
 	s.Init(tok, contents, nil, 0)
 	for {
@@ -751,6 +761,14 @@ func (c *completer) scanToken(contents []byte) (token.Pos, token.Token, string)
 			return token.NoPos, token.ILLEGAL, ""
 		}
 
+		if tkn == token.PERIOD {
+			prdPos = tknPos
+		}
+		// Set removeBeforePeriod to true if cursor is:
+		// - Right after the period (e.g., "foo.")
+		// - One or more characters after the period (e.g., "foo.b", "foo.bar")
+		c.completionContext.removeBeforePeriod = tknPos == prdPos || tknPos == prdPos+1
+
 		if len(lit) > 0 && tknPos <= c.pos && c.pos <= tknPos+token.Pos(len(lit)) {
 			return tknPos, tkn, lit
 		}
@@ -3283,3 +3301,25 @@ func forEachPackageMember(content []byte, f func(tok token.Token, id *ast.Ident,
 		}
 	}
 }
+
+// isCompletionAllowed checks whether completion is allowed even
+// if the current source is incomplete or contains syntax errors.
+func (c *completer) isCompletionAllowed() bool {
+	// Examples
+	//
+	// BlockStmt:
+	//  minimum, maximum := 0, 0
+	//  minimum, max
+	// BadExpr:
+	//  math.Sqrt(,0)
+	//  math.Ab
+	// CallExpr:
+	//  value := 0
+	//  fmt.Println("value:" val)
+	switch c.path[0].(type) {
+	case *ast.BlockStmt, *ast.BadExpr, *ast.CallExpr:
+		return true
+	default:
+		return false
+	}
+}

gopls/internal/golang/completion/format.go

@@ -67,6 +67,15 @@ func (c *completer) item(ctx context.Context, cand candidate) (CompletionItem, e
 		}
 	}
 
+	// If source contains syntax errors and removeBeforePeriod is true,
+	// update the label and insert text with the content after the period.
+	if c.isCompletionAllowed() && c.completionContext.removeBeforePeriod {
+		_, afterPeriod, found := strings.Cut(insert, ".")
+		if found {
+			label = afterPeriod
+			insert = afterPeriod
+		}
+	}
 	snip.WriteText(insert)
 
 	switch obj := obj.(type) {

gopls/internal/test/integration/completion/completion_test.go

@@ -590,6 +590,7 @@ func F3[K comparable, V any](map[K]V, chan V) {}
 }
 
 func TestPackageMemberCompletionAfterSyntaxError(t *testing.T) {
+	// Update: not broken anymore, fixed.
 	// This test documents the current broken behavior due to golang/go#58833.
 	const src = `
 -- go.mod --
@@ -623,7 +624,7 @@ func main() {
 		// (In VSCode, "Abs" wrongly appears in the completion menu.)
 		// This is a consequence of poor error recovery in the parser
 		// causing "math.Ldex" to become a BadExpr.
-		want := "package main\n\nimport \"math\"\n\nfunc main() {\n\tmath.Sqrt(,0)\n\tmath.Ldexmath.Abs(${1:})\n}\n"
+		want := "package main\n\nimport \"math\"\n\nfunc main() {\n\tmath.Sqrt(,0)\n\tmath.Ldexp(${1:})\n}\n"
 		if diff := cmp.Diff(want, got); diff != "" {
 			t.Errorf("unimported completion (-want +got):\n%s", diff)
 		}
@@ -999,3 +1000,187 @@ func Join() {}
 		}
 	})
 }
+
+func TestCompletionAfterSyntaxError(t *testing.T) {
+	const files = `
+-- go.mod --
+module mod.com
+
+go 1.14
+
+-- test1.go --
+package main
+
+func test1() {
+	minimum := 0
+	maximum := 0
+	minimum, max
+}
+
+-- test2.go --
+package main
+
+import "math"
+
+func test2() {
+	math.Sqrt(0), abs
+}
+
+-- test3.go --
+package main
+
+import "math"
+
+func test3() {
+	math.Sqrt(0), math.ab
+}
+
+-- test4.go --
+package main
+
+type person struct {
+	name string
+	age  int
+}
+
+func test4() {
+	p := person{}
+	p.name, age
+}
+
+-- test5.go --
+package main
+
+type person struct {
+	name string
+	age  int
+}
+
+func test5() {
+	p := person{}
+	p.name, p.ag
+}
+
+-- test6.go --
+package main
+
+import "math"
+
+func test6() {
+	math.Sqrt(,0)
+	abs
+}
+
+-- test7.go --
+package main
+
+import "math"
+
+func test7() {
+	math.Sqrt(,0)
+	math.ab
+}
+
+-- test8.go --
+package main
+
+func test8() {
+	minimum := 0
+	fmt.Println("minimum:" min)
+}
+
+-- test9.go --
+package main
+
+type person struct {
+	name string
+	age  int
+}
+
+func test9() {
+	p := person{}
+	fmt.Println("name:" p.na)
+}
+`
+	tests := []struct {
+		name string
+		file string
+		re   string
+		want string
+	}{
+		{
+			name: "test 1 variable completion after comma",
+			file: "test1.go",
+			re:   ", max()",
+			want: "package main\n\nfunc test1() {\n\tminimum := 0\n\tmaximum := 0\n\tminimum, maximum\n}\n\n",
+		},
+		{
+			name: "test 2 package member completion after comma",
+			file: "test2.go",
+			re:   "abs()",
+			want: "package main\n\nimport \"math\"\n\nfunc test2() {\n\tmath.Sqrt(0), math.Abs(${1:})\n}\n\n",
+		},
+		{
+			name: "test 3 package member completion after comma with period",
+			file: "test3.go",
+			re:   "math.ab()",
+			want: "package main\n\nimport \"math\"\n\nfunc test3() {\n\tmath.Sqrt(0), math.Abs(${1:})\n}\n\n",
+		},
+		{
+			name: "test 4 struct field completion after comma",
+			file: "test4.go",
+			re:   ", age()",
+			want: "package main\n\ntype person struct {\n\tname string\n\tage  int\n}\n\nfunc test4() {\n\tp := person{}\n\tp.name, p.age\n}\n\n",
+		},
+		{
+			name: "test 5 struct field completion after comma with period",
+			file: "test5.go",
+			re:   "p.ag()",
+			want: "package main\n\ntype person struct {\n\tname string\n\tage  int\n}\n\nfunc test5() {\n\tp := person{}\n\tp.name, p.age\n}\n\n",
+		},
+		{
+			name: "test 6 package member completion after BadExpr",
+			file: "test6.go",
+			re:   "abs()",
+			want: "package main\n\nimport \"math\"\n\nfunc test6() {\n\tmath.Sqrt(,0)\n\tmath.Abs(${1:})\n}\n\n",
+		},
+		{
+			name: "test 7 package member completion after BadExpr with period",
+			file: "test7.go",
+			re:   "math.ab()",
+			want: "package main\n\nimport \"math\"\n\nfunc test7() {\n\tmath.Sqrt(,0)\n\tmath.Abs(${1:})\n}\n\n",
+		},
+		{
+			name: "test 8 variable completion after missing comma",
+			file: "test8.go",
+			re:   ":\" min()",
+			want: "package main\n\nfunc test8() {\n\tminimum := 0\n\tfmt.Println(\"minimum:\" minimum)\n}\n\n",
+		},
+		{
+			name: "test 9 struct field completion after missing comma with period",
+			file: "test9.go",
+			re:   "p.na()",
+			want: "package main\n\ntype person struct {\n\tname string\n\tage  int\n}\n\nfunc test9() {\n\tp := person{}\n\tfmt.Println(\"name:\" p.name)\n}\n",
+		},
+	}
+
+	Run(t, files, func(t *testing.T, env *Env) {
+		for _, test := range tests {
+			t.Run(test.name, func(t *testing.T) {
+				env.OpenFile(test.file)
+				env.Await(env.DoneWithOpen())
+				loc := env.RegexpSearch(test.file, test.re)
+				completions := env.Completion(loc)
+				if len(completions.Items) == 0 {
+					t.Fatalf("no completion items")
+				}
+				env.AcceptCompletion(loc, completions.Items[0])
+				env.Await(env.DoneWithChange())
+				got := env.BufferText(test.file)
+				if diff := cmp.Diff(test.want, got); diff != "" {
+					t.Errorf("incorrect completion (-want +got):\n%s", diff)
+				}
+			})
+		}
+	})
+}