Steal MIR for CTFE when possible.

Author: cjgillot

compiler/rustc_mir_transform/src/lib.rs

@@ -338,7 +338,14 @@ fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: LocalDefId) -> Body<'_> {
         return shim::build_adt_ctor(tcx, def.to_def_id());
     }
 
-    let body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
+    let body = tcx.mir_drops_elaborated_and_const_checked(def);
+    let body = match tcx.hir().body_const_context(def) {
+        // consts and statics do not have `optimized_mir`, so we can steal the body instead of
+        // cloning it.
+        Some(hir::ConstContext::Const | hir::ConstContext::Static(_)) => body.steal(),
+        Some(hir::ConstContext::ConstFn) => body.borrow().clone(),
+        None => bug!("`mir_for_ctfe` called on non-const {def:?}"),
+    };
 
     let mut body = remap_mir_for_const_eval_select(tcx, body, hir::Constness::Const);
     pm::run_passes(tcx, &mut body, &[&ctfe_limit::CtfeLimit], None);