Auto merge of #125916 - tesuji:mir-opt-const-array-locals, r=<try>

[WIP] mir-opt: promoting const read-only arrays

Modified from a copy of PromoteTemps. It's kind of a hack so nothing fancy or easy to follow and review.
I'll  to reuse structures from PromoteTemps when there is [consensus for this pass][zulip].

Compiler is doing more work now with this opt. So I don't think this pass improves compiler performance.
But anyway, for statistics, can I get a perf run?

cc #73825

r? ghost

### Current status
- [ ] Waiting for [consensus][zulip].
- [ ] Maybe rewrite to [use GVN with mentor from oli][mentor]
- [x] ~ICE on unstable feature:  tests/assembly/simd-intrinsic-mask-load.rs#x86-avx512.~
  In particular `Simd([literal array])` now transformed to `Simd(array_var)`. Maybe I should ignore array in constructor.
- [x] *~Fail test on nested arrays~*

[zulip]: https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Could.20const.20read-only.20arrays.20be.20const.20promoted.3F
[mentor]: #125916 (comment)

compiler/rustc_mir_transform/src/lib.rs

@@ -91,6 +91,7 @@ mod normalize_array_len;
 mod nrvo;
 mod prettify;
 mod promote_consts;
+mod promote_consts_local_arrays;
 mod ref_prop;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
@@ -342,14 +343,22 @@ fn mir_promoted(
 
     // What we need to run borrowck etc.
     let promote_pass = promote_consts::PromoteTemps::default();
+    let promote_array = promote_consts_local_arrays::PromoteArraysOpt::default();
     pm::run_passes(
         tcx,
         &mut body,
-        &[&promote_pass, &simplify::SimplifyCfg::PromoteConsts, &coverage::InstrumentCoverage],
+        &[
+            &promote_pass,
+            &promote_array,
+            &simplify::SimplifyCfg::PromoteConsts,
+            &coverage::InstrumentCoverage,
+        ],
         Some(MirPhase::Analysis(AnalysisPhase::Initial)),
     );
 
-    let promoted = promote_pass.promoted_fragments.into_inner();
+    let mut promoted = promote_pass.promoted_fragments.into_inner();
+    let array_promoted = promote_array.promoted_fragments.into_inner();
+    promoted.extend(array_promoted);
     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 }
 

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -98,33 +98,36 @@ struct Collector<'a, 'tcx> {
 }
 
 impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
+    #[instrument(level = "debug", skip(self))]
     fn visit_local(&mut self, index: Local, context: PlaceContext, location: Location) {
-        debug!("visit_local: index={:?} context={:?} location={:?}", index, context, location);
         // We're only interested in temporaries and the return place
         match self.ccx.body.local_kind(index) {
             LocalKind::Arg => return,
-            LocalKind::Temp if self.ccx.body.local_decls[index].is_user_variable() => return,
+            LocalKind::Temp
+                if {
+                    let is_user_variable = self.ccx.body.local_decls[index].is_user_variable();
+                    debug!(?is_user_variable);
+                    is_user_variable
+                } =>
+            {
+                return;
+            }
             LocalKind::ReturnPointer | LocalKind::Temp => {}
         }
 
         // Ignore drops, if the temp gets promoted,
         // then it's constant and thus drop is noop.
         // Non-uses are also irrelevant.
         if context.is_drop() || !context.is_use() {
-            debug!(
-                "visit_local: context.is_drop={:?} context.is_use={:?}",
-                context.is_drop(),
-                context.is_use(),
-            );
+            debug!(is_drop = context.is_drop(), is_use = context.is_use());
             return;
         }
 
         let temp = &mut self.temps[index];
-        debug!("visit_local: temp={:?}", temp);
+        debug!(?temp);
         *temp = match *temp {
             TempState::Undefined => match context {
-                PlaceContext::MutatingUse(MutatingUseContext::Store)
-                | PlaceContext::MutatingUse(MutatingUseContext::Call) => {
+                PlaceContext::MutatingUse(MutatingUseContext::Store | MutatingUseContext::Call) => {
                     TempState::Defined { location, uses: 0, valid: Err(()) }
                 }
                 _ => TempState::Unpromotable,
@@ -137,7 +140,7 @@ impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
                     | PlaceContext::NonMutatingUse(_) => true,
                     PlaceContext::MutatingUse(_) | PlaceContext::NonUse(_) => false,
                 };
-                debug!("visit_local: allowed_use={:?}", allowed_use);
+                debug!(?allowed_use);
                 if allowed_use {
                     *uses += 1;
                     return;
@@ -146,6 +149,7 @@ impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
             }
             TempState::Unpromotable | TempState::PromotedOut => TempState::Unpromotable,
         };
+        debug!(?temp);
     }
 
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
@@ -972,7 +976,12 @@ fn promote_candidates<'tcx>(
     candidates: Vec<Candidate>,
 ) -> IndexVec<Promoted, Body<'tcx>> {
     // Visit candidates in reverse, in case they're nested.
-    debug!("promote_candidates({:?})", candidates);
+    debug!(promote_candidates = ?candidates);
+
+    // eagerly fail fast
+    if candidates.is_empty() {
+        return IndexVec::new();
+    }
 
     let mut promotions = IndexVec::new();