gopls/internal/settings: enable "unusedwrite" analyzer

This analyzer shows a very high rate of true positives,
indicating that code is (at best) unnecessarily complex
and confusing but, often, not working as intended.

+ release note

Fixes golang/go#44461

Change-Id: I1c86e7305a8d512d7e4d1bbec3b2b44ce3a9c9ab
Reviewed-on: https://go-review.googlesource.com/c/tools/+/575061
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

go/analysis/passes/unusedwrite/main.go

@@ -0,0 +1,16 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+// The unusedwrite command runs the unusedwrite analyzer
+// on the specified packages.
+package main
+
+import (
+	"golang.org/x/tools/go/analysis/passes/unusedwrite"
+	"golang.org/x/tools/go/analysis/singlechecker"
+)
+
+func main() { singlechecker.Main(unusedwrite.Analyzer) }

gopls/doc/analyzers.md

@@ -1022,7 +1022,7 @@ Another example is about non-pointer receiver:
 
 [Full documentation](https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/unusedwrite)
 
-**Disabled by default. Enable it by setting `"analyses": {"unusedwrite": true}`.**
+**Enabled by default.**
 
 ## **useany**
 

gopls/doc/release/v0.16.0.md

@@ -24,6 +24,36 @@ cause your editor to navigate to the declaration.
 TODO: demo of source linking.
 TODO: appeal to VS Code users to upvote https://github.com/microsoft/vscode/issues/208093?
 
+### `unusedwrite` analyzer
+
+The new
+[unusedwrite](https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/unusedwrite)
+analyzer reports assignments, often to fields of structs, that have no
+effect because, for example, the struct is never used again:
+
+```go
+func scheme(host string) string {
+	u := &url.URL{
+		Host:   host, // "unused write to field Host" (no need to construct a URL)
+		Scheme: "https:",
+	}
+	return u.Scheme
+}
+```
+
+This is at best an indication that the code is unnecessarily complex
+(for instance, some dead code could be removed), but often indicates a
+bug, as in this example:
+
+```go
+type S struct { x int }
+
+func (s S) set(x int) {
+	s.x = x // "unused write to field x" (s should be a *S pointer)
+}
+```
+
+
 ### Hover shows size/offset info
 
 Hovering over the identifier that declares a type or struct field now
@@ -36,3 +66,6 @@ TODO: example hover image.
 ## Bugs fixed
 
 ## Thank you to our contributors!
+
+@guodongli-google for the `unusedwrite` analyzer.
+TODO: they're a xoogler; is there a more current GH account?

gopls/internal/settings/api_json.go

@@ -1436,6 +1436,9 @@ func analyzers() map[string]*Analyzer {
 		unusedresult.Analyzer.Name:  {Analyzer: unusedresult.Analyzer, Enabled: true},
 
 		// Non-vet analyzers:
+		// - some (nilness, unusedwrite) use go/ssa;
+		// - some (unusedwrite) report bad code but not always a bug,
+		//   so are not suitable for vet.
 		atomicalign.Analyzer.Name:      {Analyzer: atomicalign.Analyzer, Enabled: true},
 		deepequalerrors.Analyzer.Name:  {Analyzer: deepequalerrors.Analyzer, Enabled: true},
 		fieldalignment.Analyzer.Name:   {Analyzer: fieldalignment.Analyzer, Enabled: false},
@@ -1444,7 +1447,7 @@ func analyzers() map[string]*Analyzer {
 		sortslice.Analyzer.Name:        {Analyzer: sortslice.Analyzer, Enabled: true},
 		testinggoroutine.Analyzer.Name: {Analyzer: testinggoroutine.Analyzer, Enabled: true},
 		unusedparams.Analyzer.Name:     {Analyzer: unusedparams.Analyzer, Enabled: true},
-		unusedwrite.Analyzer.Name:      {Analyzer: unusedwrite.Analyzer, Enabled: false},
+		unusedwrite.Analyzer.Name:      {Analyzer: unusedwrite.Analyzer, Enabled: true},
 		useany.Analyzer.Name:           {Analyzer: useany.Analyzer, Enabled: false},
 		infertypeargs.Analyzer.Name: {
 			Analyzer: infertypeargs.Analyzer,

gopls/internal/settings/settings.go

@@ -50,6 +50,11 @@ func _(ptr *int) {
 	}
 }
 
+// unusedwrite
+func _(s struct{x int}) {
+	s.x = 1  //@diag("x", re"unused write to field x")
+}
+
 -- bad_test_go121.go --
 //go:build go1.21
 

gopls/internal/test/marker/testdata/diagnostics/analyzers.txt

@@ -55,10 +55,11 @@ import "mod.test/b"
 
 func _() {
 	x := b.B{
-		F: 42, //@refs("F", FDef, "F")
+		F: 42, //@refs("F", FDef, "F", Fuse)
 	}
 	x.G = "hi" //@refs("G", GDef, "G")
 	_ = x.E //@refs("E", EDef, "E")
+	_ = x.F //@loc(Fuse, "F")
 }
 
 func _(y b.BI) {