Encode MIR for const ctors in metadata and allow eval for them

This is needed for them to be usable under `min_generic_const_args` (mgca).

Whenever a const ctor appears as an expression, like in `let x = Foo;`,
it is directly lowered to a construction of an aggregate value in MIR,
instead of using the const generated for it. The anon const approach for
generic const exprs works similarly, by creating an anon const with the
aggregate as its body. However, with mgca, we want to avoid anon consts,
so we need to use the const item. They were not previously encoded in
the metadata MIR, so they could not be used cross-crate, and they were
not able to evaluated. This change fixes that.

Author: camelid

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -2,7 +2,7 @@ use std::sync::atomic::Ordering::Relaxed;
 
 use either::{Left, Right};
 use rustc_abi::{self as abi, BackendRepr};
-use rustc_hir::def::DefKind;
+use rustc_hir::def::{CtorKind, DefKind};
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo, ReportedErrorInfo};
 use rustc_middle::mir::{self, ConstAlloc, ConstValue};
 use rustc_middle::query::TyCtxtAt;
@@ -41,6 +41,7 @@ fn eval_body_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
                     | DefKind::AnonConst
                     | DefKind::InlineConst
                     | DefKind::AssocConst
+                    | DefKind::Ctor(_, CtorKind::Const)
             ),
         "Unexpected DefKind: {:?}",
         ecx.tcx.def_kind(cid.instance.def_id())

compiler/rustc_mir_transform/src/lib.rs

@@ -21,7 +21,7 @@ use rustc_const_eval::util;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_data_structures::steal::Steal;
 use rustc_hir as hir;
-use rustc_hir::def::{CtorKind, DefKind};
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::IndexVec;
 use rustc_middle::mir::{
@@ -324,7 +324,7 @@ fn mir_keys(tcx: TyCtxt<'_>, (): ()) -> FxIndexSet<LocalDefId> {
     for item in tcx.hir_crate_items(()).free_items() {
         if let DefKind::Struct | DefKind::Enum = tcx.def_kind(item.owner_id) {
             for variant in tcx.adt_def(item.owner_id).variants() {
-                if let Some((CtorKind::Fn, ctor_def_id)) = variant.ctor {
+                if let Some((_, ctor_def_id)) = variant.ctor {
                     set.insert(ctor_def_id.expect_local());
                 }
             }

tests/crashes/132985.rs

@@ -27,7 +27,7 @@ const CRATE_NAME: &str = "input";
 /// This function uses the Stable MIR APIs to get information about the test crate.
 fn test_item_kind() -> ControlFlow<()> {
     let items = stable_mir::all_local_items();
-    assert_eq!(items.len(), 4);
+    assert_eq!(items.len(), 5);
     // Constructor item.
     for item in items {
         let expected_kind = match item.name().as_str() {

tests/crashes/auxiliary/aux132985.rs

@@ -1,4 +1,7 @@
+// NOTE: This aux file inherits revisions from its parent tests.
+
 #![feature(adt_const_params)]
+#![cfg_attr(mgca, feature(min_generic_const_args), allow(incomplete_features))]
 
 use std::marker::ConstParamTy;
 

tests/ui-fulldeps/stable-mir/check_item_kind.rs

@@ -1,7 +1,9 @@
 //@ check-pass
+//@ revisions: normal mgca
 //@ aux-build:xcrate-const-ctor-a.rs
 
 #![feature(adt_const_params)]
+#![cfg_attr(mgca, feature(min_generic_const_args), allow(incomplete_features))]
 
 extern crate xcrate_const_ctor_a;
 use xcrate_const_ctor_a::Foo;

tests/ui/const-generics/auxiliary/xcrate-const-ctor-a.rs

@@ -250,6 +250,13 @@ fn closures::{closure#0}(_1: {closure@$DIR/operands.rs:47:5: 47:19}, _2: bool) -
         return;
     }
 }
+fn Ctors::Unit() -> Ctors {
+    let mut _0: Ctors;
+    bb0: {
+        _0 = Ctors::Unit;
+        return;
+    }
+}
 fn Ctors::TupLike(_1: bool) -> Ctors {
     let mut _0: Ctors;
     bb0: {