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Merged
merged 3 commits into from
Dec 21, 2020
+68 −37
Merged

Acknowledge that [CONST; N] is stable #79270

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54a3ed3
use exhaustive match for checking Rvalue::Repeat
RalfJung Nov 21, 2020
f8d4883
add test that repeating non-Copy constants works
RalfJung Nov 21, 2020
7f3e18c
make sure [CONST; N] drops N times
RalfJung Nov 28, 2020
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78 changes: 41 additions & 37 deletions compiler/rustc_mir/src/borrow_check/type_check/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1988,44 +1988,48 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
// If the length is larger than 1, the repeat expression will need to copy the
// element, so we require the `Copy` trait.
if len.try_eval_usize(tcx, self.param_env).map_or(true, |len| len > 1) {
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) {
let ccx = ConstCx::new_with_param_env(tcx, body, self.param_env);
// To determine if `const_in_array_repeat_expressions` feature gate should
// be mentioned, need to check if the rvalue is promotable.
let should_suggest =
should_suggest_const_in_array_repeat_expressions_attribute(
&ccx, operand,
);
debug!("check_rvalue: should_suggest={:?}", should_suggest);

let def_id = body.source.def_id().expect_local();
self.infcx.report_selection_error(
&traits::Obligation::new(
ObligationCause::new(
span,
self.tcx().hir().local_def_id_to_hir_id(def_id),
traits::ObligationCauseCode::RepeatVec(should_suggest),
),
self.param_env,
ty::Binder::bind(ty::TraitRef::new(
self.tcx().require_lang_item(
LangItem::Copy,
Some(self.last_span),
match operand {
Operand::Copy(..) | Operand::Constant(..) => {
// These are always okay: direct use of a const, or a value that can evidently be copied.
}
Operand::Move(_) => {
// 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) {
let ccx = ConstCx::new_with_param_env(tcx, body, self.param_env);
// To determine if `const_in_array_repeat_expressions` feature gate should
// be mentioned, need to check if the rvalue is promotable.
let should_suggest =
should_suggest_const_in_array_repeat_expressions_attribute(
&ccx, operand,
);
debug!("check_rvalue: should_suggest={:?}", should_suggest);

let def_id = body.source.def_id().expect_local();
self.infcx.report_selection_error(
&traits::Obligation::new(
ObligationCause::new(
span,
self.tcx().hir().local_def_id_to_hir_id(def_id),
traits::ObligationCauseCode::RepeatVec(should_suggest),
),
tcx.mk_substs_trait(ty, &[]),
))
.without_const()
.to_predicate(self.tcx()),
),
&traits::SelectionError::Unimplemented,
false,
false,
);
self.param_env,
ty::Binder::bind(ty::TraitRef::new(
self.tcx().require_lang_item(
LangItem::Copy,
Some(self.last_span),
),
tcx.mk_substs_trait(ty, &[]),
))
.without_const()
.to_predicate(self.tcx()),
),
&traits::SelectionError::Unimplemented,
false,
false,
);
}
}
}
}
Expand Down
27 changes: 27 additions & 0 deletions src/test/ui/consts/rfc-2203-const-array-repeat-exprs/const-repeat.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
// run-pass

// Repeating a *constant* of non-Copy type (not just a constant expression) is already stable.

const EMPTY: Vec<i32> = Vec::new();

pub fn bar() -> [Vec<i32>; 2] {
[EMPTY; 2]
}

struct Bomb;

impl Drop for Bomb {
fn drop(&mut self) {
panic!("BOOM!");
}
}

const BOOM: Bomb = Bomb;

fn main() {
let _x = bar();

// Make sure the destructor does not get called for empty arrays. `[CONST; N]` should
// instantiate (and then later drop) the const exactly `N` times.
let _x = [BOOM; 0];
}