cmd/compile/internal: support new(expr)

This CL adds compiler support for new(expr),
a feature of go1.26 that allows the user to specify
the initial value of the variable instead of its
type.

Also, a basic test of dynamic behavior.

See CL 704737 for spec change and CL 704935 for
type-checker changes.

For #45624

Change-Id: I65d27de1ee3aabb819b57cce8ea77f3073447757
Reviewed-on: https://go-review.googlesource.com/c/go/+/705157
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

4 files changed, 23 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/ir/node.go b/src/cmd/compile/internal/ir/node.go
index 003ec15de1..8c61bb6ed5 100644
--- a/src/cmd/compile/internal/ir/node.go
+++ b/src/cmd/compile/internal/ir/node.go
@@ -215,7 +215,7 @@ const (
 	ORSH              // X >> Y
 	OAND              // X & Y
 	OANDNOT           // X &^ Y
-	ONEW              // new(X); corresponds to calls to new in source code
+	ONEW              // new(X); corresponds to calls to new(T) in source code
 	ONOT              // !X
 	OBITNOT           // ^X
 	OPLUS             // +X
diff --git a/src/cmd/compile/internal/ir/type.go b/src/cmd/compile/internal/ir/type.go
index 6daca856a6..0f44cf8d04 100644
--- a/src/cmd/compile/internal/ir/type.go
+++ b/src/cmd/compile/internal/ir/type.go
@@ -42,6 +42,10 @@ func TypeNode(t *types.Type) Node {
 
 // A DynamicType represents a type expression whose exact type must be
 // computed dynamically.
+//
+// TODO(adonovan): I think "dynamic" is a misnomer here; it's really a
+// type with free type parameters that needs to be instantiated to obtain
+// a ground type for which an rtype can exist.
 type DynamicType struct {
 	miniExpr
 
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 45e2bfd727..ca7c6bf151 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -2431,8 +2431,16 @@ func (r *reader) expr() (res ir.Node) {
 
 	case exprNew:
 		pos := r.pos()
-		typ := r.exprType()
-		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, typ))
+		if r.Bool() {
+			// new(expr) -> tmp := expr; &tmp
+			x := r.expr()
+			var init ir.Nodes
+			addr := ir.NewAddrExpr(pos, r.tempCopy(pos, x, &init))
+			addr.SetInit(init)
+			return typecheck.Expr(addr)
+		}
+		// new(T)
+		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, r.exprType()))
 
 	case exprSizeof:
 		return ir.NewUintptr(r.pos(), r.typ().Size())
@@ -3239,6 +3247,7 @@ func (r *reader) exprType() ir.Node {
 	var rtype, itab ir.Node
 
 	if r.Bool() {
+		// non-empty interface
 		typ, rtype, _, _, itab = r.itab(pos)
 		if !typ.IsInterface() {
 			rtype = nil // TODO(mdempsky): Leave set?
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 54e5f1ea5f..9c90d221c2 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -2035,10 +2035,16 @@ func (w *writer) expr(expr syntax.Expr) {
 			case "new":
 				assert(len(expr.ArgList) == 1)
 				assert(!expr.HasDots)
+				arg := expr.ArgList[0]
 
 				w.Code(exprNew)
 				w.pos(expr)
-				w.exprType(nil, expr.ArgList[0])
+				tv := w.p.typeAndValue(arg)
+				if w.Bool(!tv.IsType()) {
+					w.expr(arg) // new(expr), go1.26
+				} else {
+					w.exprType(nil, arg) // new(T)
+				}
 				return
 
 			case "Sizeof":