Auto merge of #61749 - davidtwco:rfc-2203-const-array-repeat-exprs, r=eddyb

rustc/rustc_mir: Implement RFC 2203.

This PR implements RFC 2203, allowing constants in array repeat
expressions. Part of #49147.

r? @eddyb

Author: bors

src/doc/unstable-book/src/language-features/const-in-array-repeat-expressions.md

@@ -0,0 +1,11 @@
+# `const_in_array_repeat_expressions`
+
+The tracking issue for this feature is: [#49147]
+
+[#44109]: https://github.com/rust-lang/rust/issues/49147
+
+------------------------
+
+Relaxes the rules for repeat expressions, `[x; N]` such that `x` may also be `const` (strictly
+speaking rvalue promotable), in addition to `typeof(x): Copy`. The result of `[x; N]` where `x` is
+`const` is itself also `const`.

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -35,7 +35,7 @@ use rustc::mir::*;
 use rustc::traits::query::type_op;
 use rustc::traits::query::type_op::custom::CustomTypeOp;
 use rustc::traits::query::{Fallible, NoSolution};
-use rustc::traits::{ObligationCause, PredicateObligations};
+use rustc::traits::{self, ObligationCause, PredicateObligations};
 use rustc::ty::adjustment::{PointerCast};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::subst::{Subst, SubstsRef, UnpackedKind, UserSubsts};
@@ -501,28 +501,38 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             // FIXME use place_projection.is_empty() when is available
             if let Place::Base(_) = place {
                 if let PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) = context {
-                    let tcx = self.tcx();
-                    let trait_ref = ty::TraitRef {
-                        def_id: tcx.lang_items().copy_trait().unwrap(),
-                        substs: tcx.mk_substs_trait(place_ty.ty, &[]),
+                    let is_promoted = match place {
+                        Place::Base(PlaceBase::Static(box Static {
+                            kind: StaticKind::Promoted(_),
+                            ..
+                        })) => true,
+                        _ => false,
                     };
 
-                    // In order to have a Copy operand, the type T of the
-                    // value must be Copy. Note that we prove that T: Copy,
-                    // rather than using the `is_copy_modulo_regions`
-                    // test. This is important because
-                    // `is_copy_modulo_regions` ignores the resulting region
-                    // obligations and assumes they pass. This can result in
-                    // bounds from Copy impls being unsoundly ignored (e.g.,
-                    // #29149). Note that we decide to use Copy before knowing
-                    // whether the bounds fully apply: in effect, the rule is
-                    // that if a value of some type could implement Copy, then
-                    // it must.
-                    self.cx.prove_trait_ref(
-                        trait_ref,
-                        location.to_locations(),
-                        ConstraintCategory::CopyBound,
-                    );
+                    if !is_promoted {
+                        let tcx = self.tcx();
+                        let trait_ref = ty::TraitRef {
+                            def_id: tcx.lang_items().copy_trait().unwrap(),
+                            substs: tcx.mk_substs_trait(place_ty.ty, &[]),
+                        };
+
+                        // In order to have a Copy operand, the type T of the
+                        // value must be Copy. Note that we prove that T: Copy,
+                        // rather than using the `is_copy_modulo_regions`
+                        // test. This is important because
+                        // `is_copy_modulo_regions` ignores the resulting region
+                        // obligations and assumes they pass. This can result in
+                        // bounds from Copy impls being unsoundly ignored (e.g.,
+                        // #29149). Note that we decide to use Copy before knowing
+                        // whether the bounds fully apply: in effect, the rule is
+                        // that if a value of some type could implement Copy, then
+                        // it must.
+                        self.cx.prove_trait_ref(
+                            trait_ref,
+                            location.to_locations(),
+                            ConstraintCategory::CopyBound,
+                        );
+                    }
                 }
             }
 
@@ -1953,18 +1963,32 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             }
 
             Rvalue::Repeat(operand, len) => if *len > 1 {
-                let operand_ty = operand.ty(body, tcx);
-
-                let trait_ref = ty::TraitRef {
-                    def_id: tcx.lang_items().copy_trait().unwrap(),
-                    substs: tcx.mk_substs_trait(operand_ty, &[]),
-                };
-
-                self.prove_trait_ref(
-                    trait_ref,
-                    location.to_locations(),
-                    ConstraintCategory::CopyBound,
-                );
+                if let Operand::Move(_) = operand {
+                    // While this is located in `nll::typeck` this error is not an NLL error, it's
+                    // a required check to make sure that repeated elements implement `Copy`.
+                    let span = body.source_info(location).span;
+                    let ty = operand.ty(body, tcx);
+                    if !self.infcx.type_is_copy_modulo_regions(self.param_env, ty, span) {
+                        self.infcx.report_selection_error(
+                            &traits::Obligation::new(
+                                ObligationCause::new(
+                                    span,
+                                    self.tcx().hir().def_index_to_hir_id(self.mir_def_id.index),
+                                    traits::ObligationCauseCode::RepeatVec,
+                                ),
+                                self.param_env,
+                                ty::Predicate::Trait(ty::Binder::bind(ty::TraitPredicate {
+                                    trait_ref: ty::TraitRef::new(
+                                        self.tcx().lang_items().copy_trait().unwrap(),
+                                        tcx.mk_substs_trait(ty, &[]),
+                                    ),
+                                })),
+                            ),
+                            &traits::SelectionError::Unimplemented,
+                            false,
+                        );
+                    }
+                }
             },
 
             Rvalue::NullaryOp(_, ty) => {

src/librustc_mir/transform/promote_consts.rs

@@ -60,6 +60,9 @@ pub enum Candidate {
     /// Borrow of a constant temporary.
     Ref(Location),
 
+    /// Promotion of the `x` in `[x; 32]`.
+    Repeat(Location),
+
     /// Currently applied to function calls where the callee has the unstable
     /// `#[rustc_args_required_const]` attribute as well as the SIMD shuffle
     /// intrinsic. The intrinsic requires the arguments are indeed constant and
@@ -322,6 +325,17 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                         _ => bug!()
                     }
                 }
+                Candidate::Repeat(loc) => {
+                    let ref mut statement = blocks[loc.block].statements[loc.statement_index];
+                    match statement.kind {
+                        StatementKind::Assign(_, box Rvalue::Repeat(ref mut operand, _)) => {
+                            let ty = operand.ty(local_decls, self.tcx);
+                            let span = statement.source_info.span;
+                            mem::replace(operand, Operand::Copy(promoted_place(ty, span)))
+                        }
+                        _ => bug!()
+                    }
+                },
                 Candidate::Argument { bb, index } => {
                     let terminator = blocks[bb].terminator_mut();
                     match terminator.kind {
@@ -380,6 +394,7 @@ pub fn promote_candidates<'tcx>(
 
     for candidate in candidates.into_iter().rev() {
         match candidate {
+            Candidate::Repeat(Location { block, statement_index }) |
             Candidate::Ref(Location { block, statement_index }) => {
                 match body[block].statements[statement_index].kind {
                     StatementKind::Assign(Place::Base(PlaceBase::Local(local)), _) => {