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Rollup merge of #77170 - ecstatic-morse:const-fn-ptr, r=oli-obk

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Remove `#[rustc_allow_const_fn_ptr]` and add `#![feature(const_fn_fn_ptr_basics)]`

`rustc_allow_const_fn_ptr` was a hack to work around the lack of an escape hatch for the "min `const fn`" checks in const-stable functions. Now that we have co-opted `allow_internal_unstable` for this purpose, we no longer need a bespoke attribute.

Now this functionality is gated under `const_fn_fn_ptr_basics` (how concise!), and `#[allow_internal_unstable(const_fn_fn_ptr_basics)]` replaces `#[rustc_allow_const_fn_ptr]`. `const_fn_fn_ptr_basics` allows function pointer types to appear in the arguments and locals of a `const fn` as well as function pointer casts to be performed inside a `const fn`. Both of these were allowed in constants and statics already. Notably, this does **not** allow users to invoke function pointers in a const context. Presumably, we will use a nicer name for that (`const_fn_ptr`?).

r? @oli-obk
This commit is contained in:
Ralf Jung
2020-09-28 18:39:44 +02:00
committed by GitHub
parent 88ae20d8aa 807260be9f
commit 85a59d40f1
39 changed files with 146 additions and 135 deletions
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28
compiler/rustc_mir/src/transform/check_consts/ops.rs
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@@ -213,11 +213,21 @@ impl NonConstOp for FnPtrCast {
const STOPS_CONST_CHECKING: bool = true;
fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
mcf_status_in_item(ccx)
if ccx.const_kind() != hir::ConstContext::ConstFn {
Status::Allowed
} else {
Status::Unstable(sym::const_fn_fn_ptr_basics)
}
}
fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
mcf_emit_error(ccx, span, "function pointer casts are not allowed in const fn");
feature_err(
&ccx.tcx.sess.parse_sess,
sym::const_fn_fn_ptr_basics,
span,
&format!("function pointer casts are not allowed in {}s", ccx.const_kind()),
)
.emit()
}
}
@@ -596,17 +606,21 @@ pub mod ty {
const STOPS_CONST_CHECKING: bool = true;
fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
// FIXME: This attribute a hack to allow the specialization of the `futures` API. See
// #59739. We should have a proper feature gate for this.
if ccx.tcx.has_attr(ccx.def_id.to_def_id(), sym::rustc_allow_const_fn_ptr) {
if ccx.const_kind() != hir::ConstContext::ConstFn {
Status::Allowed
} else {
mcf_status_in_item(ccx)
Status::Unstable(sym::const_fn_fn_ptr_basics)
}
}
fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
mcf_emit_error(ccx, span, "function pointers in const fn are unstable");
feature_err(
&ccx.tcx.sess.parse_sess,
sym::const_fn_fn_ptr_basics,
span,
&format!("function pointers cannot appear in {}s", ccx.const_kind()),
)
.emit()
}
}