Rollup merge of #75517 - RalfJung:promo-intern-comments, r=oli-obk

Promotion and const interning comments

I understood some things today which I felt should be put into comments.

Cc @rust-lang/wg-const-eval

3 files changed, 9 insertions, 2 deletions

diff --git a/src/librustc_mir/const_eval/eval_queries.rs b/src/librustc_mir/const_eval/eval_queries.rs
index 7fbe5c409d3..4101f70b820 100644
--- a/src/librustc_mir/const_eval/eval_queries.rs
+++ b/src/librustc_mir/const_eval/eval_queries.rs
@@ -57,6 +57,12 @@ fn eval_body_using_ecx<'mir, 'tcx>(
     ecx.run()?;
 
     // Intern the result
+    // FIXME: since the DefId of a promoted is the DefId of its owner, this
+    // means that promoteds in statics are actually interned like statics!
+    // However, this is also currently crucial because we promote mutable
+    // non-empty slices in statics to extend their lifetime, and this
+    // ensures that they are put into a mutable allocation.
+    // For other kinds of promoteds in statics (like array initializers), this is rather silly.
     let intern_kind = match tcx.static_mutability(cid.instance.def_id()) {
         Some(m) => InternKind::Static(m),
         None if cid.promoted.is_some() => InternKind::Promoted,
diff --git a/src/librustc_mir/interpret/intern.rs b/src/librustc_mir/interpret/intern.rs
index dffbc969c21..6c8ee72bc66 100644
--- a/src/librustc_mir/interpret/intern.rs
+++ b/src/librustc_mir/interpret/intern.rs
@@ -312,7 +312,8 @@ pub fn intern_const_alloc_recursive<M: CompileTimeMachine<'mir, 'tcx>>(
     let tcx = ecx.tcx;
     let base_intern_mode = match intern_kind {
         InternKind::Static(mutbl) => InternMode::Static(mutbl),
-        // FIXME: what about array lengths, array initializers?
+        // `Constant` includes array lengths.
+        // `Promoted` includes non-`Copy` array initializers and `rustc_args_required_const` arguments.
         InternKind::Constant | InternKind::Promoted => InternMode::ConstBase,
     };
 
diff --git a/src/librustc_mir/transform/promote_consts.rs b/src/librustc_mir/transform/promote_consts.rs
index c0564105701..94637bae44a 100644
--- a/src/librustc_mir/transform/promote_consts.rs
+++ b/src/librustc_mir/transform/promote_consts.rs
@@ -101,7 +101,7 @@ impl TempState {
 /// of a larger candidate.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum Candidate {
-    /// Borrow of a constant temporary.
+    /// Borrow of a constant temporary, candidate for lifetime extension.
     Ref(Location),
 
     /// Promotion of the `x` in `[x; 32]`.