Rollup merge of #65949 - ecstatic-morse:promote-only-pass, r=eddyb

Move promotion into its own pass

**edited**

This adds a `PromoteTemps` pass, which runs after the old `QualifyAndPromoteConsts` pass, that *only* does promotion (no const-checking). Everything related to promotion has been removed from `QualifyAndPromoteConstants`: it no longer even visits the body of a non-const `fn`.

As a result we no longer need to keep the `BitSet` of promotable locals that was returned by `mir_const_qualif`. Rvalue-static promotion in a `const` is now done in `promote_consts`, and it operates on a set of `Candidate`s instead. This will allow me–in a later PR–to create promoted MIR fragments for `const`s when necessary, which could resolve some shortcomings of the current approach (removing `StorageDead`).

r? @eddyb

13 files changed, 186 insertions, 197 deletions

diff --git a/src/librustc/query/mod.rs b/src/librustc/query/mod.rs
index 12ae2c32015..bd7b77b0abb 100644
--- a/src/librustc/query/mod.rs
+++ b/src/librustc/query/mod.rs
@@ -93,7 +93,7 @@ rustc_queries! {
         /// Maps DefId's that have an associated `mir::Body` to the result
         /// of the MIR qualify_consts pass. The actual meaning of
         /// the value isn't known except to the pass itself.
-        query mir_const_qualif(key: DefId) -> (u8, &'tcx BitSet<mir::Local>) {
+        query mir_const_qualif(key: DefId) -> u8 {
             desc { |tcx| "const checking `{}`", tcx.def_path_str(key) }
             cache_on_disk_if { key.is_local() }
         }
diff --git a/src/librustc/ty/query/mod.rs b/src/librustc/ty/query/mod.rs
index 9b15ad560b5..0615004125b 100644
--- a/src/librustc/ty/query/mod.rs
+++ b/src/librustc/ty/query/mod.rs
@@ -42,7 +42,6 @@ use crate::util::common::ErrorReported;
 use crate::util::profiling::ProfileCategory::*;
 
 use rustc_data_structures::svh::Svh;
-use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_data_structures::fx::{FxIndexMap, FxHashMap, FxHashSet};
 use rustc_data_structures::stable_hasher::StableVec;
diff --git a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
index 6eacfc28de2..1118162bdeb 100644
--- a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
+++ b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
@@ -32,7 +32,6 @@ use syntax::parse::parser::emit_unclosed_delims;
 use syntax::source_map::Spanned;
 use syntax::symbol::Symbol;
 use syntax_pos::{Span, FileName};
-use rustc_index::bit_set::BitSet;
 
 macro_rules! provide {
     (<$lt:tt> $tcx:ident, $def_id:ident, $other:ident, $cdata:ident,
@@ -122,9 +121,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     }
     optimized_mir => { tcx.arena.alloc(cdata.get_optimized_mir(tcx, def_id.index)) }
     promoted_mir => { tcx.arena.alloc(cdata.get_promoted_mir(tcx, def_id.index)) }
-    mir_const_qualif => {
-        (cdata.mir_const_qualif(def_id.index), tcx.arena.alloc(BitSet::new_empty(0)))
-    }
+    mir_const_qualif => { cdata.mir_const_qualif(def_id.index) }
     fn_sig => { cdata.fn_sig(def_id.index, tcx) }
     inherent_impls => { cdata.get_inherent_implementations_for_type(tcx, def_id.index) }
     is_const_fn_raw => { cdata.is_const_fn_raw(def_id.index) }
diff --git a/src/librustc_metadata/rmeta/encoder.rs b/src/librustc_metadata/rmeta/encoder.rs
index c6677ea3534..ad1ab16a410 100644
--- a/src/librustc_metadata/rmeta/encoder.rs
+++ b/src/librustc_metadata/rmeta/encoder.rs
@@ -955,7 +955,7 @@ impl EncodeContext<'tcx> {
         record!(self.per_def.kind[def_id] <- match impl_item.kind {
             ty::AssocKind::Const => {
                 if let hir::ImplItemKind::Const(_, body_id) = ast_item.kind {
-                    let mir = self.tcx.at(ast_item.span).mir_const_qualif(def_id).0;
+                    let mir = self.tcx.at(ast_item.span).mir_const_qualif(def_id);
 
                     EntryKind::AssocConst(container,
                         ConstQualif { mir },
@@ -1089,7 +1089,7 @@ impl EncodeContext<'tcx> {
             hir::ItemKind::Static(_, hir::MutMutable, _) => EntryKind::MutStatic,
             hir::ItemKind::Static(_, hir::MutImmutable, _) => EntryKind::ImmStatic,
             hir::ItemKind::Const(_, body_id) => {
-                let mir = self.tcx.at(item.span).mir_const_qualif(def_id).0;
+                let mir = self.tcx.at(item.span).mir_const_qualif(def_id);
                 EntryKind::Const(
                     ConstQualif { mir },
                     self.encode_rendered_const_for_body(body_id)
@@ -1368,7 +1368,7 @@ impl EncodeContext<'tcx> {
         let id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
         let body_id = self.tcx.hir().body_owned_by(id);
         let const_data = self.encode_rendered_const_for_body(body_id);
-        let mir = self.tcx.mir_const_qualif(def_id).0;
+        let mir = self.tcx.mir_const_qualif(def_id);
 
         record!(self.per_def.kind[def_id] <- EntryKind::Const(ConstQualif { mir }, const_data));
         record!(self.per_def.visibility[def_id] <- ty::Visibility::Public);
diff --git a/src/librustc_mir/transform/check_consts/qualifs.rs b/src/librustc_mir/transform/check_consts/qualifs.rs
index 840ad303016..496a5679067 100644
--- a/src/librustc_mir/transform/check_consts/qualifs.rs
+++ b/src/librustc_mir/transform/check_consts/qualifs.rs
@@ -123,7 +123,7 @@ pub trait Qualif {
                     if cx.tcx.trait_of_item(def_id).is_some() {
                         Self::in_any_value_of_ty(cx, constant.literal.ty)
                     } else {
-                        let (bits, _) = cx.tcx.at(constant.span).mir_const_qualif(def_id);
+                        let bits = cx.tcx.at(constant.span).mir_const_qualif(def_id);
 
                         let qualif = QualifSet(bits).contains::<Self>();
 
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index dbe6c784592..6d6d6bea2a0 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -210,13 +210,14 @@ fn mir_validated(
     }
 
     let mut body = tcx.mir_const(def_id).steal();
-    let qualify_and_promote_pass = qualify_consts::QualifyAndPromoteConstants::default();
+    let promote_pass = promote_consts::PromoteTemps::default();
     run_passes(tcx, &mut body, InstanceDef::Item(def_id), None, MirPhase::Validated, &[
         // What we need to run borrowck etc.
-        &qualify_and_promote_pass,
+        &qualify_consts::QualifyAndPromoteConstants::default(),
+        &promote_pass,
         &simplify::SimplifyCfg::new("qualify-consts"),
     ]);
-    let promoted = qualify_and_promote_pass.promoted.into_inner();
+    let promoted = promote_pass.promoted_fragments.into_inner();
     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 }
 
diff --git a/src/librustc_mir/transform/promote_consts.rs b/src/librustc_mir/transform/promote_consts.rs
index f5e49e32834..48a58f1d0ee 100644
--- a/src/librustc_mir/transform/promote_consts.rs
+++ b/src/librustc_mir/transform/promote_consts.rs
@@ -17,7 +17,7 @@ use rustc::mir::*;
 use rustc::mir::interpret::ConstValue;
 use rustc::mir::visit::{PlaceContext, MutatingUseContext, MutVisitor, Visitor};
 use rustc::mir::traversal::ReversePostorder;
-use rustc::ty::{self, List, TyCtxt};
+use rustc::ty::{self, List, TyCtxt, TypeFoldable};
 use rustc::ty::subst::InternalSubsts;
 use rustc::ty::cast::CastTy;
 use syntax::ast::LitKind;
@@ -25,12 +25,68 @@ use syntax::symbol::sym;
 use syntax_pos::{Span, DUMMY_SP};
 
 use rustc_index::vec::{IndexVec, Idx};
+use rustc_index::bit_set::HybridBitSet;
 use rustc_target::spec::abi::Abi;
 
+use std::cell::Cell;
 use std::{iter, mem, usize};
 
+use crate::transform::{MirPass, MirSource};
 use crate::transform::check_consts::{qualifs, Item, ConstKind, is_lang_panic_fn};
 
+/// A `MirPass` for promotion.
+///
+/// In this case, "promotion" entails the following:
+/// - Extract promotable temps in `fn` and `const fn` into their own MIR bodies.
+/// - Extend lifetimes in `const` and `static` by removing `Drop` and `StorageDead`.
+/// - Emit errors if the requirements of `#[rustc_args_required_const]` are not met.
+///
+/// After this pass is run, `promoted_fragments` will hold the MIR body corresponding to each
+/// newly created `StaticKind::Promoted`.
+#[derive(Default)]
+pub struct PromoteTemps<'tcx> {
+    pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
+}
+
+impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        // There's not really any point in promoting errorful MIR.
+        //
+        // This does not include MIR that failed const-checking, which we still try to promote.
+        if body.return_ty().references_error() {
+            tcx.sess.delay_span_bug(body.span, "PromoteTemps: MIR had errors");
+            return;
+        }
+
+        if src.promoted.is_some() {
+            return;
+        }
+
+        let def_id = src.def_id();
+
+        let item = Item::new(tcx, def_id, body);
+        let mut rpo = traversal::reverse_postorder(body);
+        let (temps, all_candidates) = collect_temps_and_candidates(tcx, body, &mut rpo);
+
+        let promotable_candidates = validate_candidates(tcx, body, def_id, &temps, &all_candidates);
+
+        // For now, lifetime extension is done in `const` and `static`s without creating promoted
+        // MIR fragments by removing `Drop` and `StorageDead` for each referent. However, this will
+        // not work inside loops when they are allowed in `const`s.
+        //
+        // FIXME: use promoted MIR fragments everywhere?
+        let promoted_fragments = if should_create_promoted_mir_fragments(item.const_kind) {
+            promote_candidates(def_id, body, tcx, temps, promotable_candidates)
+        } else {
+            // FIXME: promote const array initializers in consts.
+            remove_drop_and_storage_dead_on_promoted_locals(tcx, body, &promotable_candidates);
+            IndexVec::new()
+        };
+
+        self.promoted_fragments.set(promoted_fragments);
+    }
+}
+
 /// State of a temporary during collection and promotion.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum TempState {
@@ -538,7 +594,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                         // is gone - we can always promote constants even if they
                         // fail to pass const-checking, as compilation would've
                         // errored independently and promotion can't change that.
-                        let (bits, _) = self.tcx.at(constant.span).mir_const_qualif(def_id);
+                        let bits = self.tcx.at(constant.span).mir_const_qualif(def_id);
                         if bits == super::qualify_consts::QUALIF_ERROR_BIT {
                             self.tcx.sess.delay_span_bug(
                                 constant.span,
@@ -1154,3 +1210,83 @@ crate fn should_suggest_const_in_array_repeat_expressions_attribute<'tcx>(
             should_promote={:?} feature_flag={:?}", mir_def_id, should_promote, feature_flag);
     should_promote && !feature_flag
 }
+
+fn should_create_promoted_mir_fragments(const_kind: Option<ConstKind>) -> bool {
+    match const_kind {
+        Some(ConstKind::ConstFn) | None => true,
+        Some(ConstKind::Const) | Some(ConstKind::Static) | Some(ConstKind::StaticMut) => false,
+    }
+}
+
+/// In `const` and `static` everything without `StorageDead`
+/// is `'static`, we don't have to create promoted MIR fragments,
+/// just remove `Drop` and `StorageDead` on "promoted" locals.
+fn remove_drop_and_storage_dead_on_promoted_locals(
+    tcx: TyCtxt<'tcx>,
+    body: &mut Body<'tcx>,
+    promotable_candidates: &[Candidate],
+) {
+    debug!("run_pass: promotable_candidates={:?}", promotable_candidates);
+
+    // Removing `StorageDead` will cause errors for temps declared inside a loop body. For now we
+    // simply skip promotion if a loop exists, since loops are not yet allowed in a `const`.
+    //
+    // FIXME: Just create MIR fragments for `const`s instead of using this hackish approach?
+    if body.is_cfg_cyclic() {
+        tcx.sess.delay_span_bug(body.span, "Control-flow cycle detected in `const`");
+        return;
+    }
+
+    // The underlying local for promotion contexts like `&temp` and `&(temp.proj)`.
+    let mut requires_lifetime_extension = HybridBitSet::new_empty(body.local_decls.len());
+
+    promotable_candidates
+        .iter()
+        .filter_map(|c| {
+            match c {
+                Candidate::Ref(loc) => Some(loc),
+                Candidate::Repeat(_) | Candidate::Argument { .. } => None,
+            }
+        })
+        .map(|&Location { block, statement_index }| {
+            // FIXME: store the `Local` for each `Candidate` when it is created.
+            let place = match &body[block].statements[statement_index].kind {
+                StatementKind::Assign(box ( _, Rvalue::Ref(_, _, place))) => place,
+                _ => bug!("`Candidate::Ref` without corresponding assignment"),
+            };
+
+            match place.base {
+                PlaceBase::Local(local) => local,
+                PlaceBase::Static(_) => bug!("`Candidate::Ref` for a non-local"),
+            }
+        })
+        .for_each(|local| {
+            requires_lifetime_extension.insert(local);
+        });
+
+    // Remove `Drop` terminators and `StorageDead` statements for all promotable temps that require
+    // lifetime extension.
+    for block in body.basic_blocks_mut() {
+        block.statements.retain(|statement| {
+            match statement.kind {
+                StatementKind::StorageDead(index) => !requires_lifetime_extension.contains(index),
+                _ => true
+            }
+        });
+        let terminator = block.terminator_mut();
+        match &terminator.kind {
+            TerminatorKind::Drop {
+                location,
+                target,
+                ..
+            } => {
+                if let Some(index) = location.as_local() {
+                    if requires_lifetime_extension.contains(index) {
+                        terminator.kind = TerminatorKind::Goto { target: *target };
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+}
diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 6a7058f1930..5ad5363768d 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -15,7 +15,6 @@ use rustc::ty::cast::CastTy;
 use rustc::ty::query::Providers;
 use rustc::mir::*;
 use rustc::mir::interpret::ConstValue;
-use rustc::mir::traversal::ReversePostorder;
 use rustc::mir::visit::{PlaceContext, Visitor, MutatingUseContext, NonMutatingUseContext};
 use rustc::middle::lang_items;
 use rustc::session::config::nightly_options;
@@ -31,7 +30,6 @@ use std::usize;
 
 use rustc::hir::HirId;
 use crate::transform::{MirPass, MirSource};
-use super::promote_consts::{self, Candidate, TempState};
 use crate::transform::check_consts::ops::{self, NonConstOp};
 
 /// What kind of item we are in.
@@ -258,7 +256,7 @@ trait Qualif {
                     if cx.tcx.trait_of_item(def_id).is_some() {
                         Self::in_any_value_of_ty(cx, constant.literal.ty).unwrap_or(false)
                     } else {
-                        let (bits, _) = cx.tcx.at(constant.span).mir_const_qualif(def_id);
+                        let bits = cx.tcx.at(constant.span).mir_const_qualif(def_id);
 
                         let qualif = PerQualif::decode_from_bits(bits).0[Self::IDX];
 
@@ -477,10 +475,6 @@ struct Checker<'a, 'tcx> {
 
     span: Span,
     def_id: DefId,
-    rpo: ReversePostorder<'a, 'tcx>,
-
-    temp_promotion_state: IndexVec<Local, TempState>,
-    unchecked_promotion_candidates: Vec<Candidate>,
 
     /// If `true`, do not emit errors to the user, merely collect them in `errors`.
     suppress_errors: bool,
@@ -509,10 +503,6 @@ impl Deref for Checker<'a, 'tcx> {
 impl<'a, 'tcx> Checker<'a, 'tcx> {
     fn new(tcx: TyCtxt<'tcx>, def_id: DefId, body: &'a Body<'tcx>, mode: Mode) -> Self {
         assert!(def_id.is_local());
-        let mut rpo = traversal::reverse_postorder(body);
-        let (temps, unchecked_promotion_candidates) =
-            promote_consts::collect_temps_and_candidates(tcx, body, &mut rpo);
-        rpo.reset();
 
         let param_env = tcx.param_env(def_id);
 
@@ -539,9 +529,6 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
             cx,
             span: body.span,
             def_id,
-            rpo,
-            temp_promotion_state: temps,
-            unchecked_promotion_candidates,
             errors: vec![],
             suppress_errors: false,
         }
@@ -662,14 +649,6 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
         let kind = self.body.local_kind(index);
         debug!("store to {:?} {:?}", kind, index);
 
-        // Only handle promotable temps in non-const functions.
-        if self.mode == Mode::NonConstFn {
-            if kind != LocalKind::Temp ||
-               !self.temp_promotion_state[index].is_promotable() {
-                return;
-            }
-        }
-
         // this is overly restrictive, because even full assignments do not clear the qualif
         // While we could special case full assignments, this would be inconsistent with
         // aggregates where we overwrite all fields via assignments, which would not get
@@ -682,7 +661,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
     }
 
     /// Check a whole const, static initializer or const fn.
-    fn check_const(&mut self) -> (u8, &'tcx BitSet<Local>) {
+    fn check_const(&mut self) -> u8 {
         use crate::transform::check_consts as new_checker;
 
         debug!("const-checking {} {:?}", self.mode, self.def_id);
@@ -704,7 +683,6 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
 
         let mut seen_blocks = BitSet::new_empty(body.basic_blocks().len());
         let mut bb = START_BLOCK;
-        let mut has_controlflow_error = false;
         loop {
             seen_blocks.insert(bb.index());
 
@@ -745,7 +723,6 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
                     bb = target;
                 }
                 _ => {
-                    has_controlflow_error = true;
                     self.not_const(ops::Loop);
                     validator.check_op(ops::Loop);
                     break;
@@ -772,51 +749,7 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
             }
         }
 
-        // Collect all the temps we need to promote.
-        let mut promoted_temps = BitSet::new_empty(self.temp_promotion_state.len());
-
-        // HACK: if parts of the control-flow graph were skipped due to an error, don't try to
-        // promote anything, since that can cause errors in a `const` if e.g. rvalue static
-        // promotion is attempted within a loop body.
-        let unleash_miri = self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you;
-        let promotion_candidates = if has_controlflow_error && !unleash_miri {
-            self.tcx.sess.delay_span_bug(
-                body.span,
-                "check_const: expected control-flow error(s)",
-            );
-
-            vec![]
-        } else {
-            promote_consts::validate_candidates(
-                self.tcx,
-                self.body,
-                self.def_id,
-                &self.temp_promotion_state,
-                &self.unchecked_promotion_candidates,
-            )
-        };
-
-        debug!("qualify_const: promotion_candidates={:?}", promotion_candidates);
-        for candidate in promotion_candidates {
-            match candidate {
-                Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
-                    if let StatementKind::Assign(box( _, Rvalue::Ref(_, _, place)))
-                        = &self.body[bb].statements[stmt_idx].kind
-                    {
-                        if let PlaceBase::Local(local) = place.base {
-                            promoted_temps.insert(local);
-                        }
-                    }
-                }
-
-                // Only rvalue-static promotion requires extending the lifetime of the promoted
-                // local.
-                Candidate::Argument { .. } | Candidate::Repeat(_) => {}
-            }
-        }
-
-        let qualifs = self.qualifs_in_local(RETURN_PLACE);
-        (qualifs.encode_to_bits(), self.tcx.arena.alloc(promoted_temps))
+        self.qualifs_in_local(RETURN_PLACE).encode_to_bits()
     }
 }
 
@@ -1346,7 +1279,7 @@ pub fn provide(providers: &mut Providers<'_>) {
 // in `promote_consts`, see the comment in `validate_operand`.
 pub(super) const QUALIF_ERROR_BIT: u8 = 1 << 2;
 
-fn mir_const_qualif(tcx: TyCtxt<'_>, def_id: DefId) -> (u8, &BitSet<Local>) {
+fn mir_const_qualif(tcx: TyCtxt<'_>, def_id: DefId) -> u8 {
     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
     // cannot yet be stolen), because `mir_validated()`, which steals
     // from `mir_const(), forces this query to execute before
@@ -1355,7 +1288,7 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def_id: DefId) -> (u8, &BitSet<Local>) {
 
     if body.return_ty().references_error() {
         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
-        return (QUALIF_ERROR_BIT, tcx.arena.alloc(BitSet::new_empty(0)));
+        return QUALIF_ERROR_BIT;
     }
 
     Checker::new(tcx, def_id, body, Mode::Const).check_const()
@@ -1391,56 +1324,34 @@ impl<'tcx> MirPass<'tcx> for QualifyAndPromoteConstants<'tcx> {
         let mode = determine_mode(tcx, hir_id, def_id);
 
         debug!("run_pass: mode={:?}", mode);
-        if let Mode::NonConstFn | Mode::ConstFn = mode {
+        if let Mode::NonConstFn = mode {
+            // No need to const-check a non-const `fn` now that we don't do promotion here.
+            return;
+        } else if let Mode::ConstFn = mode {
             let mut checker = Checker::new(tcx, def_id, body, mode);
-            if let Mode::ConstFn = mode {
-                let use_min_const_fn_checks =
-                    !tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you &&
-                    tcx.is_min_const_fn(def_id);
-                if use_min_const_fn_checks {
-                    // Enforce `min_const_fn` for stable `const fn`s.
-                    use super::qualify_min_const_fn::is_min_const_fn;
-                    if let Err((span, err)) = is_min_const_fn(tcx, def_id, body) {
-                        error_min_const_fn_violation(tcx, span, err);
-                        return;
-                    }
-
-                    // `check_const` should not produce any errors, but better safe than sorry
-                    // FIXME(#53819)
-                    // NOTE(eddyb) `check_const` is actually needed for promotion inside
-                    // `min_const_fn` functions.
-                }
-
-                // Enforce a constant-like CFG for `const fn`.
-                checker.check_const();
-            } else {
-                while let Some((bb, data)) = checker.rpo.next() {
-                    checker.visit_basic_block_data(bb, data);
+            let use_min_const_fn_checks =
+                !tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you &&
+                tcx.is_min_const_fn(def_id);
+            if use_min_const_fn_checks {
+                // Enforce `min_const_fn` for stable `const fn`s.
+                use super::qualify_min_const_fn::is_min_const_fn;
+                if let Err((span, err)) = is_min_const_fn(tcx, def_id, body) {
+                    error_min_const_fn_violation(tcx, span, err);
+                    return;
                 }
             }
 
-            // Promote only the promotable candidates.
-            let temps = checker.temp_promotion_state;
-            let candidates = promote_consts::validate_candidates(
-                tcx,
-                body,
-                def_id,
-                &temps,
-                &checker.unchecked_promotion_candidates,
-            );
-
-            // Do the actual promotion, now that we know what's viable.
-            self.promoted.set(
-                promote_consts::promote_candidates(def_id, body, tcx, temps, candidates)
-            );
+            // `check_const` should not produce any errors, but better safe than sorry
+            // FIXME(#53819)
+            // Enforce a constant-like CFG for `const fn`.
+            checker.check_const();
         } else {
             check_short_circuiting_in_const_local(tcx, body, mode);
 
-            let promoted_temps = match mode {
-                Mode::Const => tcx.mir_const_qualif(def_id).1,
-                _ => Checker::new(tcx, def_id, body, mode).check_const().1,
+            match mode {
+                Mode::Const => tcx.mir_const_qualif(def_id),
+                _ => Checker::new(tcx, def_id, body, mode).check_const(),
             };
-            remove_drop_and_storage_dead_on_promoted_locals(body, promoted_temps);
         }
 
         if mode == Mode::Static && !tcx.has_attr(def_id, sym::thread_local) {
@@ -1501,40 +1412,6 @@ fn check_short_circuiting_in_const_local(tcx: TyCtxt<'_>, body: &mut Body<'tcx>,
     }
 }
 
-/// In `const` and `static` everything without `StorageDead`
-/// is `'static`, we don't have to create promoted MIR fragments,
-/// just remove `Drop` and `StorageDead` on "promoted" locals.
-fn remove_drop_and_storage_dead_on_promoted_locals(
-    body: &mut Body<'tcx>,
-    promoted_temps: &BitSet<Local>,
-) {
-    debug!("run_pass: promoted_temps={:?}", promoted_temps);
-
-    for block in body.basic_blocks_mut() {
-        block.statements.retain(|statement| {
-            match statement.kind {
-                StatementKind::StorageDead(index) => !promoted_temps.contains(index),
-                _ => true
-            }
-        });
-        let terminator = block.terminator_mut();
-        match &terminator.kind {
-            TerminatorKind::Drop {
-                location,
-                target,
-                ..
-            } => {
-                if let Some(index) = location.as_local() {
-                    if promoted_temps.contains(index) {
-                        terminator.kind = TerminatorKind::Goto { target: *target };
-                    }
-                }
-            }
-            _ => {}
-        }
-    }
-}
-
 fn check_static_is_sync(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, hir_id: HirId) {
     let ty = body.return_ty();
     tcx.infer_ctxt().enter(|infcx| {
diff --git a/src/test/mir-opt/match_false_edges.rs b/src/test/mir-opt/match_false_edges.rs
index b275c06e05c..648856b5523 100644
--- a/src/test/mir-opt/match_false_edges.rs
+++ b/src/test/mir-opt/match_false_edges.rs
@@ -39,7 +39,7 @@ fn main() {
 
 // END RUST SOURCE
 //
-// START rustc.full_tested_match.QualifyAndPromoteConstants.after.mir
+// START rustc.full_tested_match.PromoteTemps.after.mir
 //  bb0: {
 //      ...
 //      _2 = std::option::Option::<i32>::Some(const 42i32,);
@@ -108,9 +108,9 @@ fn main() {
 //      _0 = ();
 //      return;
 //  }
-// END rustc.full_tested_match.QualifyAndPromoteConstants.after.mir
+// END rustc.full_tested_match.PromoteTemps.after.mir
 //
-// START rustc.full_tested_match2.QualifyAndPromoteConstants.before.mir
+// START rustc.full_tested_match2.PromoteTemps.before.mir
 //  bb0: {
 //      ...
 //      _2 = std::option::Option::<i32>::Some(const 42i32,);
@@ -179,9 +179,9 @@ fn main() {
 //      _0 = ();
 //      return;
 //  }
-// END rustc.full_tested_match2.QualifyAndPromoteConstants.before.mir
+// END rustc.full_tested_match2.PromoteTemps.before.mir
 //
-// START rustc.main.QualifyAndPromoteConstants.before.mir
+// START rustc.main.PromoteTemps.before.mir
 //  bb0: {
 //     ...
 //      _2 = std::option::Option::<i32>::Some(const 1i32,);
@@ -276,4 +276,4 @@ fn main() {
 //      _0 = ();
 //      return;
 //  }
-// END rustc.main.QualifyAndPromoteConstants.before.mir
+// END rustc.main.PromoteTemps.before.mir
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn.rs b/src/test/ui/consts/min_const_fn/min_const_fn.rs
index d0f63b148ff..db68a05905a 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn.rs
+++ b/src/test/ui/consts/min_const_fn/min_const_fn.rs
@@ -136,7 +136,6 @@ const fn no_rpit() -> impl std::fmt::Debug {} //~ ERROR `impl Trait` in const fn
 const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {} //~ ERROR trait bounds other than `Sized`
 const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
 //~^ ERROR trait bounds other than `Sized`
-//~| ERROR cannot return reference to temporary value
 
 const fn no_unsafe() { unsafe {} }
 
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn.stderr b/src/test/ui/consts/min_const_fn/min_const_fn.stderr
index 3158b6284db..64b2ce83da2 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn.stderr
+++ b/src/test/ui/consts/min_const_fn/min_const_fn.stderr
@@ -286,17 +286,8 @@ LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
    = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error[E0515]: cannot return reference to temporary value
-  --> $DIR/min_const_fn.rs:137:63
-   |
-LL | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
-   |                                                               ^--
-   |                                                               ||
-   |                                                               |temporary value created here
-   |                                                               returns a reference to data owned by the current function
-
 error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
-  --> $DIR/min_const_fn.rs:143:41
+  --> $DIR/min_const_fn.rs:142:41
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -305,7 +296,7 @@ LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
 error[E0723]: function pointers in const fn are unstable
-  --> $DIR/min_const_fn.rs:146:21
+  --> $DIR/min_const_fn.rs:145:21
    |
 LL | const fn no_fn_ptrs(_x: fn()) {}
    |                     ^^
@@ -314,7 +305,7 @@ LL | const fn no_fn_ptrs(_x: fn()) {}
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
 error[E0723]: function pointers in const fn are unstable
-  --> $DIR/min_const_fn.rs:148:27
+  --> $DIR/min_const_fn.rs:147:27
    |
 LL | const fn no_fn_ptrs2() -> fn() { fn foo() {} foo }
    |                           ^^^^
@@ -322,7 +313,7 @@ LL | const fn no_fn_ptrs2() -> fn() { fn foo() {} foo }
    = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error: aborting due to 37 previous errors
+error: aborting due to 36 previous errors
 
-Some errors have detailed explanations: E0493, E0515, E0723.
+Some errors have detailed explanations: E0493, E0723.
 For more information about an error, try `rustc --explain E0493`.
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs b/src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs
index 3833510c0b3..6ca1e59b3af 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs
@@ -11,6 +11,5 @@ const fn no_inner_dyn_trait2(x: Hide) {
 }
 const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
 //~^ ERROR trait bounds other than `Sized`
-//~| ERROR temporary value dropped while borrowed
 
 fn main() {}
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr b/src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr
index 0ea950d678f..e20b4f9dcb4 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr
@@ -16,16 +16,6 @@ LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
    = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
    = help: add `#![feature(const_fn)]` to the crate attributes to enable
 
-error[E0716]: temporary value dropped while borrowed
-  --> $DIR/min_const_fn_dyn.rs:12:67
-   |
-LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
-   |                                                                  -^    - temporary value is freed at the end of this statement
-   |                                                                  ||
-   |                                                                  |creates a temporary which is freed while still in use
-   |                                                                  cast requires that borrow lasts for `'static`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0716, E0723.
-For more information about an error, try `rustc --explain E0716`.
+For more information about this error, try `rustc --explain E0723`.