internal/lsp/analysis: add simplify-range pass from "gofmt -s"

This is one of the simplifications that "gofmt -s" applies.
https://golang.org/cmd/gofmt/#hdr-The_simplify_command

A range of the form:
	for x, _ = range v {...}
will be simplified to:
	for x = range v {...}

A range of the form:
	for _ = range v {...}
will be simplified to:
	for range v {...}

Updates golang/go#37221

Change-Id: Ic6babbd0b8ab961ebb4d0d6df72df52d9acde6e7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/223661
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

gopls/doc/analyzers.md

@@ -321,6 +321,32 @@ errors is discouraged.
 
 Default value: `true`.
 
+### **simplifyrange**
+
+check for range statement simplifications
+
+A range of the form:
+```go
+for x, _ = range v {...}
+```
+will be simplified to:
+```go
+for x = range v {...}
+```
+
+A range of the form:
+```go
+for _ = range v {...}
+```
+will be simplified to:
+```go
+for range v {...}
+```
+
+This is one of the simplifications that "gofmt -s" applies.
+
+Default value: `true`.
+
 ### **sortslice**
 
 check the argument type of sort.Slice

internal/lsp/analysis/simplifyrange/simplifyrange.go

@@ -0,0 +1,114 @@
+// 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 simplifyrange defines an Analyzer that simplifies range statements.
+// https://golang.org/cmd/gofmt/#hdr-The_simplify_command
+// https://github.com/golang/go/blob/master/src/cmd/gofmt/simplify.go
+package simplifyrange
+
+import (
+	"bytes"
+	"go/ast"
+	"go/printer"
+	"go/token"
+
+	"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 range statement simplifications
+
+A range of the form:
+	for x, _ = range v {...}
+will be simplified to:
+	for x = range v {...}
+
+A range of the form:
+	for _ = range v {...}
+will be simplified to:
+	for range v {...}
+
+This is one of the simplifications that "gofmt -s" applies.`
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "simplifyrange",
+	Doc:      Doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	nodeFilter := []ast.Node{
+		(*ast.RangeStmt)(nil),
+	}
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		stmt := n.(*ast.RangeStmt)
+		end := newlineIndex(pass.Fset, stmt)
+		var old ast.Expr
+		// Range statements of the form: for i, _ := range x {}
+		if isBlank(stmt.Value) {
+			old = stmt.Value
+			defer func() {
+				stmt.Value = old
+			}()
+			stmt.Value = nil
+		}
+		// Range statements of the form: for _ := range x {}
+		if isBlank(stmt.Key) && stmt.Value == nil {
+			old = stmt.Key
+			defer func() {
+				stmt.Key = old
+			}()
+			stmt.Key = nil
+		}
+		// Return early if neither if condition is met.
+		if old == nil {
+			return
+		}
+		pass.Report(analysis.Diagnostic{
+			Pos:            old.Pos(),
+			End:            old.End(),
+			Message:        "simplify range expression",
+			SuggestedFixes: suggestedFixes(pass.Fset, stmt, end),
+		})
+	})
+	return nil, nil
+}
+
+func suggestedFixes(fset *token.FileSet, rng *ast.RangeStmt, end token.Pos) []analysis.SuggestedFix {
+	var b bytes.Buffer
+	printer.Fprint(&b, fset, rng)
+	stmt := b.Bytes()
+	index := bytes.Index(stmt, []byte("\n"))
+	// If there is a new line character, then don't replace the body.
+	if index != -1 {
+		stmt = stmt[:index]
+	}
+	return []analysis.SuggestedFix{{
+		Message: "Remove empty value",
+		TextEdits: []analysis.TextEdit{{
+			Pos:     rng.Pos(),
+			End:     end,
+			NewText: stmt[:index],
+		}},
+	}}
+}
+
+func newlineIndex(fset *token.FileSet, rng *ast.RangeStmt) token.Pos {
+	var b bytes.Buffer
+	printer.Fprint(&b, fset, rng)
+	contents := b.Bytes()
+	index := bytes.Index(contents, []byte("\n"))
+	if index == -1 {
+		return rng.End()
+	}
+	return rng.Pos() + token.Pos(index)
+}
+
+func isBlank(x ast.Expr) bool {
+	ident, ok := x.(*ast.Ident)
+	return ok && ident.Name == "_"
+}

internal/lsp/analysis/simplifyrange/simplifyrange_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 simplifyrange_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+	"golang.org/x/tools/internal/lsp/analysis/simplifyrange"
+)
+
+func Test(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.RunWithSuggestedFixes(t, testdata, simplifyrange.Analyzer, "a")
+}

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

@@ -0,0 +1,16 @@
+// 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 testdata
+
+import "log"
+
+func m() {
+	maps := make(map[string]string)
+	for k, _ := range maps { // want "simplify range expression"
+		log.Println(k)
+	}
+	for _ = range maps { // want "simplify range expression"
+	}
+}

internal/lsp/analysis/simplifyrange/testdata/src/a/a.go.golden

@@ -0,0 +1,16 @@
+// 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 testdata
+
+import "log"
+
+func m() {
+	maps := make(map[string]string)
+	for k := range maps { // want "simplify range expression"
+		log.Println(k)
+	}
+	for range maps { // want "simplify range expression"
+	}
+}

internal/lsp/source/options.go

@@ -36,6 +36,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/unreachable"
 	"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/debug/tag"
 	"golang.org/x/tools/internal/lsp/diff"
 	"golang.org/x/tools/internal/lsp/diff/myers"
@@ -511,5 +512,6 @@ func defaultAnalyzers() map[string]Analyzer {
 		testinggoroutine.Analyzer.Name: {Analyzer: testinggoroutine.Analyzer, Enabled: true},
 
 		// gofmt -s suite:
+		simplifyrange.Analyzer.Name: {Analyzer: simplifyrange.Analyzer, Enabled: true, HighConfidence: true},
 	}
 }