internal/lsp/analysis: add pass for unused parameters

This change adds a pass that checks for unused parameters inside of a function. It is disabled by default.

Updates golang/go#36602

Change-Id: I9e8de3368f16f27e7816ec4ddb16935e1a05584e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/222817
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

gopls/doc/analyzers.md

@@ -373,4 +373,19 @@ func TestFoo(t *testing.T) {
 }
 ```
 
-Default value: `true`.
+Default value: `true`.
+
+### **unusedparams**
+
+check for unused parameters of functions
+
+The unusedparams analyzer checks functions to see if there are
+any parameters that are not being used.
+
+To reduce false positives it ignores:
+- methods
+- parameters that do not have a name or are underscored
+- functions in test files
+- functions with empty bodies or those with just a return stmt
+
+Default value: `false`.

internal/lsp/analysis/unusedparams/testdata/src/a/a.go

@@ -0,0 +1,55 @@
+// Copyright 2020 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.
+
+package a
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+)
+
+type parent interface {
+	n(f bool)
+}
+
+type yuh struct {
+	a int
+}
+
+func (y *yuh) n(f bool) {
+	for i := 0; i < 10; i++ {
+		fmt.Println(i)
+	}
+}
+
+func a(i1 int, i2 int, i3 int) int { // want "potentially unused parameter: 'i2'"
+	i3 += i1
+	_ = func(z int) int { // want "potentially unused parameter: 'z'"
+		_ = 1
+		return 1
+	}
+	return i3
+}
+
+func b(c bytes.Buffer) { // want "potentially unused parameter: 'c'"
+	_ = 1
+}
+
+func z(h http.ResponseWriter, _ *http.Request) { // want "potentially unused parameter: 'h'"
+	fmt.Println("Before")
+}
+
+func l(h http.Handler) http.Handler {
+	return http.HandlerFunc(z)
+}
+
+func mult(a, b int) int { // want "potentially unused parameter: 'b'"
+	a += 1
+	return a
+}
+
+func y(a int) {
+	panic("yo")
+}

internal/lsp/analysis/unusedparams/unusedparams.go

@@ -0,0 +1,146 @@
+// Copyright 2020 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.
+
+// Package unusedparams defines an analyzer that checks for unused
+// parameters of functions.
+package unusedparams
+
+import (
+	"fmt"
+	"go/ast"
+	"go/types"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+)
+
+const Doc = `check for unused parameters of functions
+
+The unusedparams analyzer checks functions to see if there are
+any parameters that are not being used.
+
+To reduce false positives it ignores:
+- methods
+- parameters that do not have a name or are underscored
+- functions in test files
+- functions with empty bodies or those with just a return stmt`
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "unusedparams",
+	Doc:      Doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+type paramData struct {
+	field  *ast.Field
+	ident  *ast.Ident
+	typObj types.Object
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	nodeFilter := []ast.Node{
+		(*ast.FuncDecl)(nil),
+		(*ast.FuncLit)(nil),
+	}
+
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		var fieldList *ast.FieldList
+		var body *ast.BlockStmt
+
+		// Get the fieldList and body from the function node.
+		switch f := n.(type) {
+		case *ast.FuncDecl:
+			fieldList, body = f.Type.Params, f.Body
+			// TODO(golang/go#36602): add better handling for methods, if we enable methods
+			// we will get false positives if a struct is potentially implementing
+			// an interface.
+			if f.Recv != nil {
+				return
+			}
+			// Ignore functions in _test.go files to reduce false positives.
+			if file := pass.Fset.File(n.Pos()); file != nil && strings.HasSuffix(file.Name(), "_test.go") {
+				return
+			}
+		case *ast.FuncLit:
+			fieldList, body = f.Type.Params, f.Body
+		}
+		// If there are no arguments or the function is empty, then return.
+		if fieldList.NumFields() == 0 || len(body.List) == 0 {
+			return
+		}
+
+		switch expr := body.List[0].(type) {
+		case *ast.ReturnStmt:
+			// Ignore functions that only contain a return statement to reduce false positives.
+			return
+		case *ast.ExprStmt:
+			callExpr, ok := expr.X.(*ast.CallExpr)
+			if !ok || len(body.List) > 1 {
+				break
+			}
+			// Ignore functions that only contain a panic statement to reduce false positives.
+			if fun, ok := callExpr.Fun.(*ast.Ident); ok && fun.Name == "panic" {
+				return
+			}
+		}
+
+		// Get the useful data from each field.
+		params := make(map[string]*paramData)
+		unused := make(map[*paramData]bool)
+		for _, f := range fieldList.List {
+			for _, i := range f.Names {
+				if i.Name == "_" {
+					continue
+				}
+				params[i.Name] = &paramData{
+					field:  f,
+					ident:  i,
+					typObj: pass.TypesInfo.ObjectOf(i),
+				}
+				unused[params[i.Name]] = true
+			}
+		}
+
+		// Traverse through the body of the function and
+		// check to see which parameters are unused.
+		ast.Inspect(body, func(node ast.Node) bool {
+			n, ok := node.(*ast.Ident)
+			if !ok {
+				return true
+			}
+			param, ok := params[n.Name]
+			if !ok {
+				return false
+			}
+			if nObj := pass.TypesInfo.ObjectOf(n); nObj != param.typObj {
+				return false
+			}
+			if _, ok := unused[param]; ok {
+				delete(unused, param)
+			}
+			return false
+		})
+
+		// Create the reports for the unused parameters.
+		for u, _ := range unused {
+			start, end := u.field.Pos(), u.field.End()
+			if len(u.field.Names) > 1 {
+				start, end = u.ident.Pos(), u.ident.End()
+			}
+			// TODO(golang/go#36602): add suggested fixes to automatically remove the unused parameter,
+			// to start, just remove it from the function declaration,
+			// later, remove from every use of this function
+			pass.Report(analysis.Diagnostic{
+				Pos:     start,
+				End:     end,
+				Message: fmt.Sprintf("potentially unused parameter: '%s'", u.ident.Name),
+			})
+		}
+	})
+	return nil, nil
+}

internal/lsp/analysis/unusedparams/unusedparams_test.go

@@ -0,0 +1,17 @@
+// Copyright 2020 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.
+
+package unusedparams_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+	"golang.org/x/tools/internal/lsp/analysis/unusedparams"
+)
+
+func Test(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, unusedparams.Analyzer, "a")
+}

internal/lsp/source/options.go

@@ -37,6 +37,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/unsafeptr"
 	"golang.org/x/tools/go/analysis/passes/unusedresult"
 	"golang.org/x/tools/internal/lsp/analysis/simplifyrange"
+	"golang.org/x/tools/internal/lsp/analysis/unusedparams"
 	"golang.org/x/tools/internal/lsp/debug/tag"
 	"golang.org/x/tools/internal/lsp/diff"
 	"golang.org/x/tools/internal/lsp/diff/myers"
@@ -510,6 +511,7 @@ func defaultAnalyzers() map[string]Analyzer {
 		deepequalerrors.Analyzer.Name:  {Analyzer: deepequalerrors.Analyzer, Enabled: true},
 		sortslice.Analyzer.Name:        {Analyzer: sortslice.Analyzer, Enabled: true},
 		testinggoroutine.Analyzer.Name: {Analyzer: testinggoroutine.Analyzer, Enabled: true},
+		unusedparams.Analyzer.Name:     {Analyzer: unusedparams.Analyzer, Enabled: false},
 
 		// gofmt -s suite:
 		simplifyrange.Analyzer.Name: {Analyzer: simplifyrange.Analyzer, Enabled: true, HighConfidence: true},