Stabilize casts and coercions to &[T] in const fn

[oli-obk]

Part of #64992

There was never a reason to not stabilize this, we just accidentally prevented them when we implemented the min_const_fn feature that gave us const fn on stable. This PR stabilizes these casts (which are already stable in const outside const fn), while keeping all other unsizing casts (so T -> dyn Trait) unstable within const fn.
These casts have no forward compatibility concerns with any future features for const eval and users were able to use them under the const_fn feature gate already since at least the miri merger, possibly longer.

r? @rust-lang/lang

[oli-obk]

cc @rust-lang/wg-const-eval

[nikomatsakis]

@rfcbot fcp merge

I move that we merge this proposal. It permits const functions to coerce &[T; N] to &[T]. It's hard to imagine not wanting to allow this eventually.

[rfcbot]

Team member @nikomatsakis has proposed to merge this. The next step is review by the rest of the tagged team members:

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

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Diff in /checkout/src/librustc_mir/transform/qualify_min_const_fn.rs at line 191:
             _,
             _,
         ) => Err((span, "function pointer casts are not allowed in const fn".into())),
-        Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => if cast_ty.is_slice() {
-            check_operand(tcx, op, span, def_id, body)?;
-            // Casting/coercing things to slices is fine.
-        } else {
-            // We just can't allow trait objects until we have figured out trait method calls.
-            // We just can't allow trait objects until we have figured out trait method calls.
-            Err((span, "unsizing casts are not allowed in const fn".into()))
+        Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
+            if cast_ty.is_slice() {
+                check_operand(tcx, op, span, def_id, body)?;
+                // Casting/coercing things to slices is fine.
+            } else {
+                // We just can't allow trait objects until we have figured out trait method calls.
+                // We just can't allow trait objects until we have figured out trait method calls.
+                Err((span, "unsizing casts are not allowed in const fn".into()))
         }
         // binops are fine on integers
         // binops are fine on integers
         Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/src/librustc_mir/transform/qualify_min_const_fn.rs"` failed.
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== clock drift check ==
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[RalfJung]

There was never a reason to not stabilize this, we just accidentally prevented them when we implemented the min_const_fn feature that gave us const fn on stable. This PR stabilizes these casts, while keeping all other unsizing casts (so T -> dyn Trait) unstable within const fn.

So this was also stable already in const outside const fn?

I have no objections.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[oli-obk]

So this was also stable already in const outside const fn?

yes, proof: const FOO: &[u8] = &[1, 2, 4, 5]; works.

[RalfJung]

Okay; added that to the OP as it seems very relevant. :)

[rylev]

FWIW this would have made the implementation of const-sha1 more ergonomic. It is possible to work around this, but being able to coerce to &[T] would have helped.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[oli-obk]

@rust-lang/wg-const-eval want to review this?

[RalfJung]

LGTM. @bors r+

[bors]

📌 Commit 7e682d3 has been approved by RalfJung