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

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

A slice expression of the form:
	s[a:len(s)]
will be simplified to:
	s[a:]

Updates golang/go#37221

Change-Id: Ibd4dedaadc9b129d5d39524f0c1ccc8a95bf7e0d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/223659
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

gopls/doc/analyzers.md

@@ -364,6 +364,23 @@ This is one of the simplifications that "gofmt -s" applies.
 
 Default value: `true`.
 
+### **simplifyslice**
+
+check for slice simplifications
+
+A slice expression of the form:
+```go
+s[a:len(s)]
+```
+will be simplified to:
+```go
+s[a:]
+```
+
+This is one of the simplifications that "gofmt -s" applies.
+
+Default value: `true`.
+
 ### **sortslice**
 
 check the argument type of sort.Slice

internal/lsp/analysis/simplifyslice/simplifyslice.go

@@ -0,0 +1,96 @@
+// 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 simplifyslice defines an Analyzer that simplifies slice statements.
+// https://github.com/golang/go/blob/master/src/cmd/gofmt/simplify.go
+// https://golang.org/cmd/gofmt/#hdr-The_simplify_command
+package simplifyslice
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/printer"
+
+	"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 slice simplifications
+
+A slice expression of the form:
+	s[a:len(s)]
+will be simplified to:
+	s[a:]
+
+This is one of the simplifications that "gofmt -s" applies.`
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "simplifyslice",
+	Doc:      Doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+// Note: We could also simplify slice expressions of the form s[0:b] to s[:b]
+//       but we leave them as is since sometimes we want to be very explicit
+//       about the lower bound.
+// An example where the 0 helps:
+//       x, y, z := b[0:2], b[2:4], b[4:6]
+// An example where it does not:
+//       x, y := b[:n], b[n:]
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	nodeFilter := []ast.Node{
+		(*ast.SliceExpr)(nil),
+	}
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		expr := n.(*ast.SliceExpr)
+		// - 3-index slices always require the 2nd and 3rd index
+		if expr.Max != nil {
+			return
+		}
+		s, ok := expr.X.(*ast.Ident)
+		// the array/slice object is a single, resolved identifier
+		if !ok || s.Obj == nil {
+			return
+		}
+		call, ok := expr.High.(*ast.CallExpr)
+		// the high expression is a function call with a single argument
+		if !ok || len(call.Args) != 1 || call.Ellipsis.IsValid() {
+			return
+		}
+		fun, ok := call.Fun.(*ast.Ident)
+		// the function called is "len" and it is not locally defined; and
+		// because we don't have dot imports, it must be the predefined len()
+		if !ok || fun.Name != "len" || fun.Obj != nil {
+			return
+		}
+		arg, ok := call.Args[0].(*ast.Ident)
+		// the len argument is the array/slice object
+		if !ok || arg.Obj != s.Obj {
+			return
+		}
+		old := expr.High
+		var b bytes.Buffer
+		printer.Fprint(&b, pass.Fset, old)
+		pass.Report(analysis.Diagnostic{
+			Pos:     old.Pos(),
+			End:     old.End(),
+			Message: fmt.Sprintf("unneeded: %s", b.String()),
+			SuggestedFixes: []analysis.SuggestedFix{{
+				Message: fmt.Sprintf("Remove '%s'", b.String()),
+				TextEdits: []analysis.TextEdit{{
+					Pos:     old.Pos(),
+					End:     old.End(),
+					NewText: []byte{},
+				}},
+			}},
+		})
+		expr.High = old
+	})
+	return nil, nil
+}

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

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

@@ -0,0 +1,70 @@
+// 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
+
+var (
+	a [10]byte
+	b [20]float32
+	s []int
+	t struct {
+		s []byte
+	}
+
+	_ = a[0:]
+	_ = a[1:10]
+	_ = a[2:len(a)] // want "unneeded: len\\(a\\)"
+	_ = a[3:(len(a))]
+	_ = a[len(a)-1 : len(a)] // want "unneeded: len\\(a\\)"
+	_ = a[2:len(a):len(a)]
+
+	_ = a[:]
+	_ = a[:10]
+	_ = a[:len(a)] // want "unneeded: len\\(a\\)"
+	_ = a[:(len(a))]
+	_ = a[:len(a)-1]
+	_ = a[:len(a):len(a)]
+
+	_ = s[0:]
+	_ = s[1:10]
+	_ = s[2:len(s)] // want "unneeded: len\\(s\\)"
+	_ = s[3:(len(s))]
+	_ = s[len(a) : len(s)-1]
+	_ = s[0:len(b)]
+	_ = s[2:len(s):len(s)]
+
+	_ = s[:]
+	_ = s[:10]
+	_ = s[:len(s)] // want "unneeded: len\\(s\\)"
+	_ = s[:(len(s))]
+	_ = s[:len(s)-1]
+	_ = s[:len(b)]
+	_ = s[:len(s):len(s)]
+
+	_ = t.s[0:]
+	_ = t.s[1:10]
+	_ = t.s[2:len(t.s)]
+	_ = t.s[3:(len(t.s))]
+	_ = t.s[len(a) : len(t.s)-1]
+	_ = t.s[0:len(b)]
+	_ = t.s[2:len(t.s):len(t.s)]
+
+	_ = t.s[:]
+	_ = t.s[:10]
+	_ = t.s[:len(t.s)]
+	_ = t.s[:(len(t.s))]
+	_ = t.s[:len(t.s)-1]
+	_ = t.s[:len(b)]
+	_ = t.s[:len(t.s):len(t.s)]
+)
+
+func _() {
+	s := s[0:len(s)] // want "unneeded: len\\(s\\)"
+	_ = s
+}
+
+func m() {
+	maps := []int{}
+	_ = maps[1:len(maps)] // want "unneeded: len\\(maps\\)"
+}

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

@@ -0,0 +1,70 @@
+// 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
+
+var (
+	a [10]byte
+	b [20]float32
+	s []int
+	t struct {
+		s []byte
+	}
+
+	_ = a[0:]
+	_ = a[1:10]
+	_ = a[2:] // want "unneeded: len\\(a\\)"
+	_ = a[3:(len(a))]
+	_ = a[len(a)-1:] // want "unneeded: len\\(a\\)"
+	_ = a[2:len(a):len(a)]
+
+	_ = a[:]
+	_ = a[:10]
+	_ = a[:] // want "unneeded: len\\(a\\)"
+	_ = a[:(len(a))]
+	_ = a[:len(a)-1]
+	_ = a[:len(a):len(a)]
+
+	_ = s[0:]
+	_ = s[1:10]
+	_ = s[2:] // want "unneeded: len\\(s\\)"
+	_ = s[3:(len(s))]
+	_ = s[len(a) : len(s)-1]
+	_ = s[0:len(b)]
+	_ = s[2:len(s):len(s)]
+
+	_ = s[:]
+	_ = s[:10]
+	_ = s[:] // want "unneeded: len\\(s\\)"
+	_ = s[:(len(s))]
+	_ = s[:len(s)-1]
+	_ = s[:len(b)]
+	_ = s[:len(s):len(s)]
+
+	_ = t.s[0:]
+	_ = t.s[1:10]
+	_ = t.s[2:len(t.s)]
+	_ = t.s[3:(len(t.s))]
+	_ = t.s[len(a) : len(t.s)-1]
+	_ = t.s[0:len(b)]
+	_ = t.s[2:len(t.s):len(t.s)]
+
+	_ = t.s[:]
+	_ = t.s[:10]
+	_ = t.s[:len(t.s)]
+	_ = t.s[:(len(t.s))]
+	_ = t.s[:len(t.s)-1]
+	_ = t.s[:len(b)]
+	_ = t.s[:len(t.s):len(t.s)]
+)
+
+func _() {
+	s := s[0:] // want "unneeded: len\\(s\\)"
+	_ = s
+}
+
+func m() {
+	maps := []int{}
+	_ = maps[1:] // want "unneeded: len\\(maps\\)"
+}

internal/lsp/source/options.go

@@ -38,6 +38,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/unusedresult"
 	"golang.org/x/tools/internal/lsp/analysis/simplifycompositelit"
 	"golang.org/x/tools/internal/lsp/analysis/simplifyrange"
+	"golang.org/x/tools/internal/lsp/analysis/simplifyslice"
 	"golang.org/x/tools/internal/lsp/analysis/unusedparams"
 	"golang.org/x/tools/internal/lsp/debug/tag"
 	"golang.org/x/tools/internal/lsp/diff"
@@ -517,5 +518,6 @@ func defaultAnalyzers() map[string]Analyzer {
 		// gofmt -s suite:
 		simplifycompositelit.Analyzer.Name: {Analyzer: simplifycompositelit.Analyzer, Enabled: true, HighConfidence: true},
 		simplifyrange.Analyzer.Name:        {Analyzer: simplifyrange.Analyzer, Enabled: true, HighConfidence: true},
+		simplifyslice.Analyzer.Name:        {Analyzer: simplifyslice.Analyzer, Enabled: true, HighConfidence: true},
 	}
 }