Add pass for simple phase change

compiler/rustc_mir_transform/src/lib.rs

@@ -61,6 +61,7 @@ mod inline;
 mod instcombine;
 mod lower_intrinsics;
 mod lower_slice_len;
+mod marker;
 mod match_branches;
 mod multiple_return_terminators;
 mod normalize_array_len;

compiler/rustc_mir_transform/src/marker.rs

@@ -0,0 +1,20 @@
+use std::borrow::Cow;
+
+use crate::MirPass;
+use rustc_middle::mir::{Body, MirPhase};
+use rustc_middle::ty::TyCtxt;
+
+/// Changes the MIR phase without changing the MIR itself.
+pub struct PhaseChange(pub MirPhase);
+
+impl<'tcx> MirPass<'tcx> for PhaseChange {
+    fn phase_change(&self) -> Option<MirPhase> {
+        Some(self.0)
+    }
+
+    fn name(&self) -> Cow<'_, str> {
+        Cow::from(format!("PhaseChange-{:?}", self.0))
+    }
+
+    fn run_pass(&self, _: TyCtxt<'tcx>, _body: &mut Body<'tcx>) {}
+}