gopls/internal/analysis/modernize: improve rangeint transformation

  for i := 0; i < len(slice); i++ {}

is currently reduced to

  for i := range len(slice) {}

but this CL delivers the better style of:

  for i := range slice {}

Fixes golang/go#71725

Change-Id: Idb025315047c3be992267f6c1783757798e0c840
Reviewed-on: https://go-review.googlesource.com/c/tools/+/649356
Reviewed-by: Jonathan Amsterdam <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/analysis/modernize/rangeint.go

@@ -8,10 +8,12 @@ import (
 	"fmt"
 	"go/ast"
 	"go/token"
+	"go/types"
 
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/inspect"
 	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/go/types/typeutil"
 	"golang.org/x/tools/internal/analysisinternal"
 	"golang.org/x/tools/internal/astutil/cursor"
 	"golang.org/x/tools/internal/astutil/edge"
@@ -98,6 +100,14 @@ func rangeint(pass *analysis.Pass) {
 							})
 						}
 
+						// If limit is len(slice),
+						// simplify "range len(slice)" to "range slice".
+						if call, ok := limit.(*ast.CallExpr); ok &&
+							typeutil.Callee(info, call) == builtinLen &&
+							is[*types.Slice](info.TypeOf(call.Args[0]).Underlying()) {
+							limit = call.Args[0]
+						}
+
 						pass.Report(analysis.Diagnostic{
 							Pos:      init.Pos(),
 							End:      inc.End(),

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go

@@ -1,6 +1,6 @@
 package rangeint
 
-func _(i int, s struct{ i int }) {
+func _(i int, s struct{ i int }, slice []int) {
 	for i := 0; i < 10; i++ { // want "for loop can be modernized using range over int"
 		println(i)
 	}
@@ -9,6 +9,9 @@ func _(i int, s struct{ i int }) {
 	for i := 0; i < 10; i++ { // want "for loop can be modernized using range over int"
 		// i unused within loop
 	}
+	for i := 0; i < len(slice); i++ { // want "for loop can be modernized using range over int"
+		println(slice[i])
+	}
 
 	// nope
 	for i := 0; i < 10; { // nope: missing increment

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go.golden

@@ -1,6 +1,6 @@
 package rangeint
 
-func _(i int, s struct{ i int }) {
+func _(i int, s struct{ i int }, slice []int) {
 	for i := range 10 { // want "for loop can be modernized using range over int"
 		println(i)
 	}
@@ -9,6 +9,9 @@ func _(i int, s struct{ i int }) {
 	for range 10 { // want "for loop can be modernized using range over int"
 		// i unused within loop
 	}
+	for i := range slice { // want "for loop can be modernized using range over int"
+		println(slice[i])
+	}
 
 	// nope
 	for i := 0; i < 10; { // nope: missing increment