cmd/compile: use new for loop semantics for Go 1.22+ compilations

This includes version-dependent support for GOEXPERIMENT and
-d=loopvar, -d=loopvarhash, to allow testing/porting of old code.

Includes tests of downgrade (1.22 -> 1.21) and upgrade (1.21 -> 1.22)
based on //go:build lines (while running a 1.22 build/compiler).

Change-Id: Idd3be61a2b46acec33c7e7edac0924158cc726b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/508819
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

3 files changed, 23 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/noder/irgen.go b/src/cmd/compile/internal/noder/irgen.go
index df5de63620..6019c5986c 100644
--- a/src/cmd/compile/internal/noder/irgen.go
+++ b/src/cmd/compile/internal/noder/irgen.go
@@ -64,6 +64,7 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		Implicits:          make(map[syntax.Node]types2.Object),
 		Scopes:             make(map[syntax.Node]*types2.Scope),
 		Instances:          make(map[*syntax.Name]types2.Instance),
+		FileVersions:       make(map[*syntax.PosBase]types2.Version),
 		// expand as needed
 	}
 
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 610d02c07c..6dec060c8c 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1897,10 +1897,10 @@ func (r *reader) forStmt(label *types.Sym) ir.Node {
 	cond := r.optExpr()
 	post := r.stmt()
 	body := r.blockStmt()
-	dv := r.Bool()
+	perLoopVars := r.Bool()
 	r.closeAnotherScope()
 
-	stmt := ir.NewForStmt(pos, init, cond, post, body, dv)
+	stmt := ir.NewForStmt(pos, init, cond, post, body, perLoopVars)
 	stmt.Label = label
 	return stmt
 }
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index afe452bc9c..1d8c0bf933 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -1456,10 +1456,29 @@ func (w *writer) forStmt(stmt *syntax.ForStmt) {
 	}
 
 	w.blockStmt(stmt.Body)
-	w.Bool(base.Debug.LoopVar > 0)
+	w.Bool(w.distinctVars(stmt))
 	w.closeAnotherScope()
 }
 
+func (w *writer) distinctVars(stmt *syntax.ForStmt) bool {
+	lv := base.Debug.LoopVar
+	v := w.p.info.FileVersions[stmt.Pos().Base()]
+	is122 := v.Major == 0 && v.Minor == 0 || v.Major == 1 && v.Minor >= 22
+
+	// Turning off loopvar for 1.22 is only possible with loopvarhash=qn
+	//
+	// Debug.LoopVar values to be preserved for 1.21 compatibility are 1 and 2,
+	// which are also set (=1) by GOEXPERIMENT=loopvar.  The knobs for turning on
+	// the new, unshared, loopvar behavior apply to versions less than 1.21 because
+	// (1) 1.21 also did that and (2) this is believed to be the likely use case;
+	// anyone checking to see if it affects their code will just run the GOEXPERIMENT
+	// but will not also update all their go.mod files to 1.21.
+	//
+	// -gcflags=-d=loopvar=3 enables logging for 1.22 but does not turn loopvar on for <= 1.21.
+
+	return is122 || lv > 0 && lv != 3
+}
+
 // rangeTypes returns the types of values produced by ranging over
 // expr.
 func (pw *pkgWriter) rangeTypes(expr syntax.Expr) (key, value types2.Type) {