[tools] go/analysis/passes/modernize: rangeint: result vars are implicit uses

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Alan Donovan (Gerrit)

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Dec 18, 2025, 1:34:22 PM (23 hours ago) Dec 18
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go/analysis/passes/modernize: rangeint: result vars are implicit uses

This CL fixes a bug in rangeint that caused it to make unsound
fixes because it failed to consider that result variables can be
implicit uses of a loop index variable after the loop, and thus
the fix to golang/go#71952 was insufficient.

+ test

Fixes golang/go#76880
Change-Id: I9c4d2082d93204023f50afb07318037d9134de23

Change diff

diff --git a/go/analysis/passes/modernize/rangeint.go b/go/analysis/passes/modernize/rangeint.go
index c42ec58..edffcee 100644
--- a/go/analysis/passes/modernize/rangeint.go
+++ b/go/analysis/passes/modernize/rangeint.go
@@ -18,6 +18,7 @@
 	"golang.org/x/tools/internal/analysis/analyzerutil"
 	typeindexanalyzer "golang.org/x/tools/internal/analysis/typeindex"
 	"golang.org/x/tools/internal/astutil"
+	"golang.org/x/tools/internal/moreiters"
 	"golang.org/x/tools/internal/typesinternal"
 	"golang.org/x/tools/internal/typesinternal/typeindex"
 	"golang.org/x/tools/internal/versions"
@@ -133,6 +134,14 @@
 						if typesinternal.IsPackageLevel(v) {
 							continue nextLoop
 						}
+
+						// If v is a named result, it is implicitly
+						// used after the loop (go.dev/issue/76880).
+						// TODO(adonovan): use go1.25 v.Kind() == types.ResultVar.
+						if moreiters.Contains(enclosingSignature(curLoop, info).Results().Variables(), v) {
+							continue nextLoop
+						}
+
 						used := false
 						for curId := range curLoop.Child(loop.Body).Preorder((*ast.Ident)(nil)) {
 							id := curId.Node().(*ast.Ident)
@@ -320,3 +329,24 @@
 	}
 	return false
 }
+
+// enclosingSignature returns the signature of the innermost
+// function enclosing the syntax node denoted by cur
+// or nil if the node is not within a function.
+//
+// TODO(adonovan): factor with gopls/internal/util/typesutil.EnclosingSignature.
+func enclosingSignature(cur inspector.Cursor, info *types.Info) *types.Signature {
+	if c, ok := enclosingFunc(cur); ok {
+		switch n := c.Node().(type) {
+		case *ast.FuncDecl:
+			if f, ok := info.Defs[n.Name]; ok {
+				return f.Type().(*types.Signature)
+			}
+		case *ast.FuncLit:
+			if f, ok := info.Types[n]; ok {
+				return f.Type.(*types.Signature)
+			}
+		}
+	}
+	return nil
+}
diff --git a/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go b/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
index 2a3de6b..11ddc69 100644
--- a/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
+++ b/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
@@ -262,6 +262,13 @@
 	}()
 }
 
+// See go.dev/issue/76880.
+func _() (i int) {
+	for i = 0; i < 3; i++ { // nope: i is implicitly accessed after the loop
+	}
+	return
+}
+
 func issue74687() {
 	for i := a.ID(0); i < 10; i++ { // want "for loop can be modernized using range over int"
 		println(i)
diff --git a/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden b/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
index b8d9d7c..2b52145 100644
--- a/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
+++ b/go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
@@ -261,6 +261,13 @@
 	}()
 }
 
+// See go.dev/issue/76880.
+func _() (i int) {
+	for i = 0; i < 3; i++ { // nope: i is implicitly accessed after the loop
+	}
+	return
+}
+
 func issue74687() {
 	for i := range a.ID(10) { // want "for loop can be modernized using range over int"
 		println(i)

Change information

Files:
  • M go/analysis/passes/modernize/rangeint.go
  • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
  • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
Change size: S
Delta: 3 files changed, 44 insertions(+), 0 deletions(-)
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Alan Donovan (Gerrit)

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Kirill Kolyshkin (Gerrit)

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Kirill Kolyshkin . resolved

Tested this on real code from math/rand and it appears to solve the issue.

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Kirill Kolyshkin (Gerrit)

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Kirill Kolyshkin . resolved

I've also updated CL 730960 (re-generated with changes from this CL included) for testing purposes, it passes the tests now.

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    Tested this on real code from math/rand and it appears to solve the issue.

    Alan Donovan

    Thanks!

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      Change information

      Commit message:
      go/analysis/passes/modernize: rangeint: result vars are implicit uses

      This CL fixes a bug in rangeint that caused it to make unsound
      fixes because it failed to consider that result variables can be
      implicit uses of a loop index variable after the loop, and thus
      the fix to golang/go#71952 was insufficient.

      + test

      Fixes golang/go#76880
      Change-Id: I9c4d2082d93204023f50afb07318037d9134de23
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      Files:
      • M go/analysis/passes/modernize/rangeint.go
      • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
      • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
      Change size: S
      Delta: 3 files changed, 44 insertions(+), 0 deletions(-)
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      Alan Donovan has uploaded the change for review

      Commit message

      [internal-branch.go1.26-vendor] go/analysis/passes/modernize: rangeint: result vars are implicit uses


      This CL fixes a bug in rangeint that caused it to make unsound
      fixes because it failed to consider that result variables can be
      implicit uses of a loop index variable after the loop, and thus
      the fix to golang/go#71952 was insufficient.

      + test

      Fixes golang/go#76880

      Change-Id: I9c4d2082d93204023f50afb07318037d9134de23
      Reviewed-on: https://go-review.googlesource.com/c/tools/+/731220
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      (cherry picked from commit 95246c4aa04977c8425289eb81fd0054c9fcd48a)

      Change information

      Files:
      • M go/analysis/passes/modernize/rangeint.go
      • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go
      • M go/analysis/passes/modernize/testdata/src/rangeint/rangeint.go.golden
      Change size: S
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