cmd/compile: implement slice-to-array conversions

The conversion T(x) is implemented as *(*T)(x). Accordingly, runtime
panic messages for (*T)(x) are made more general.

Fixes #46505.

Change-Id: I76317c0878b6a5908299506d392eed50d7ef6523
Reviewed-on: https://go-review.googlesource.com/c/go/+/430415
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/walk/convert.go b/src/cmd/compile/internal/walk/convert.go
index 57f28e9800..c67a29fc09 100644
--- a/src/cmd/compile/internal/walk/convert.go
+++ b/src/cmd/compile/internal/walk/convert.go
@@ -503,3 +503,21 @@ func walkCheckPtrArithmetic(n *ir.ConvExpr, init *ir.Nodes) ir.Node {
 
 	return cheap
 }
+
+// walkSliceToArray walks an OSLICE2ARR expression.
+func walkSliceToArray(n *ir.ConvExpr, init *ir.Nodes) ir.Node {
+	// Replace T(x) with *(*T)(x).
+	conv := typecheck.Expr(ir.NewConvExpr(base.Pos, ir.OCONV, types.NewPtr(n.Type()), n.X)).(*ir.ConvExpr)
+	deref := typecheck.Expr(ir.NewStarExpr(base.Pos, conv)).(*ir.StarExpr)
+
+	// The OSLICE2ARRPTR conversion handles checking the slice length,
+	// so the dereference can't fail.
+	//
+	// However, this is more than just an optimization: if T is a
+	// zero-length array, then x (and thus (*T)(x)) can be nil, but T(x)
+	// should *not* panic. So suppressing the nil check here is
+	// necessary for correctness in that case.
+	deref.SetBounded(true)
+
+	return walkExpr(deref, init)
+}