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Auto merge of #76411 - RalfJung:promote-in-const-fn, r=ecstatic-morse

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Some promotion cleanup

Based on top of both https://github.com/rust-lang/rust/pull/75502 and https://github.com/rust-lang/rust/pull/75585, this does some cleanup of the promotion code. The last 2 commits are new.

* Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes https://github.com/rust-lang/rust/issues/75586.
* ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~
* Promote `&mut []` everywhere, not just in non-`const` functions, for consistency.
* Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~

This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from `@rust-lang/wg-const-eval` .
This commit is contained in:
bors 2020-09-20 06:20:16 +00:00
commit 10b3595ba6
6 changed files with 90 additions and 41 deletions
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61
compiler/rustc_mir/src/transform/promote_consts.rs
View file

@ -297,6 +297,17 @@ impl std::ops::Deref for Validator<'a, 'tcx> {
struct Unpromotable;
impl<'tcx> Validator<'_, 'tcx> {
/// Determines if this code could be executed at runtime and thus is subject to codegen.
/// That means even unused constants need to be evaluated.
///
/// `const_kind` should not be used in this file other than through this method!
fn maybe_runtime(&self) -> bool {
match self.const_kind {
None | Some(hir::ConstContext::ConstFn) => true,
Some(hir::ConstContext::Static(_) | hir::ConstContext::Const) => false,
}
}
fn validate_candidate(&self, candidate: Candidate) -> Result<(), Unpromotable> {
match candidate {
Candidate::Ref(loc) => {
@ -363,12 +374,10 @@ impl<'tcx> Validator<'_, 'tcx> {
// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now, and only in functions.
// is allowed right now.
if let ty::Array(_, len) = ty.kind() {
// FIXME(eddyb) the `self.is_non_const_fn` condition
// seems unnecessary, given that this is merely a ZST.
match len.try_eval_usize(self.tcx, self.param_env) {
Some(0) if self.const_kind.is_none() => {}
Some(0) => {}
_ => return Err(Unpromotable),
}
} else {
@ -495,9 +504,10 @@ impl<'tcx> Validator<'_, 'tcx> {
match place {
PlaceRef { local, projection: [] } => self.validate_local(local),
PlaceRef { local, projection: [proj_base @ .., elem] } => {
// Validate topmost projection, then recurse.
match *elem {
ProjectionElem::Deref => {
let mut not_promotable = true;
let mut promotable = false;
// This is a special treatment for cases like *&STATIC where STATIC is a
// global static variable.
// This pattern is generated only when global static variables are directly
@ -512,6 +522,9 @@ impl<'tcx> Validator<'_, 'tcx> {
}) = def_stmt
{
if let Some(did) = c.check_static_ptr(self.tcx) {
// Evaluating a promoted may not read statics except if it got
// promoted from a static (this is a CTFE check). So we
// can only promote static accesses inside statics.
if let Some(hir::ConstContext::Static(..)) = self.const_kind {
// The `is_empty` predicate is introduced to exclude the case
// where the projection operations are [ .field, * ].
@ -524,13 +537,13 @@ impl<'tcx> Validator<'_, 'tcx> {
if proj_base.is_empty()
&& !self.tcx.is_thread_local_static(did)
{
not_promotable = false;
promotable = true;
}
}
}
}
}
if not_promotable {
if !promotable {
return Err(Unpromotable);
}
}
@ -545,7 +558,7 @@ impl<'tcx> Validator<'_, 'tcx> {
}
ProjectionElem::Field(..) => {
if self.const_kind.is_none() {
if self.maybe_runtime() {
let base_ty =
Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
if let Some(def) = base_ty.ty_adt_def() {
@ -573,6 +586,10 @@ impl<'tcx> Validator<'_, 'tcx> {
if let Some(def_id) = c.check_static_ptr(self.tcx) {
// Only allow statics (not consts) to refer to other statics.
// FIXME(eddyb) does this matter at all for promotion?
// FIXME(RalfJung) it makes little sense to not promote this in `fn`/`const fn`,
// and in `const` this cannot occur anyway. The only concern is that we might
// promote even `let x = &STATIC` which would be useless, but this applies to
// promotion inside statics as well.
let is_static = matches!(self.const_kind, Some(hir::ConstContext::Static(_)));
if !is_static {
return Err(Unpromotable);
@ -591,20 +608,20 @@ impl<'tcx> Validator<'_, 'tcx> {
fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
match *rvalue {
Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if self.const_kind.is_none() => {
Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
let operand_ty = operand.ty(self.body, self.tcx);
let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
match (cast_in, cast_out) {
(CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
// in normal functions, mark such casts as not promotable
// ptr-to-int casts are not possible in consts and thus not promotable
return Err(Unpromotable);
}
_ => {}
}
}
Rvalue::BinaryOp(op, ref lhs, _) if self.const_kind.is_none() => {
Rvalue::BinaryOp(op, ref lhs, _) => {
if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind() {
assert!(
op == BinOp::Eq
@ -616,13 +633,14 @@ impl<'tcx> Validator<'_, 'tcx> {
|| op == BinOp::Offset
);
// raw pointer operations are not allowed inside promoteds
// raw pointer operations are not allowed inside consts and thus not promotable
return Err(Unpromotable);
}
}
Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),
// FIXME(RalfJung): the rest is *implicitly considered promotable*... that seems dangerous.
_ => {}
}
@ -644,8 +662,8 @@ impl<'tcx> Validator<'_, 'tcx> {
}
Rvalue::AddressOf(_, place) => {
// Raw reborrows can come from reference to pointer coercions,
// so are allowed.
// We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
// no problem, only using it is.
if let [proj_base @ .., ProjectionElem::Deref] = place.projection.as_ref() {
let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
if let ty::Ref(..) = base_ty.kind() {
@ -664,12 +682,10 @@ impl<'tcx> Validator<'_, 'tcx> {
// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now, and only in functions.
// is allowed right now.
if let ty::Array(_, len) = ty.kind() {
// FIXME(eddyb): We only return `Unpromotable` for `&mut []` inside a
// const context which seems unnecessary given that this is merely a ZST.
match len.try_eval_usize(self.tcx, self.param_env) {
Some(0) if self.const_kind.is_none() => {}
Some(0) => {}
_ => return Err(Unpromotable),
}
} else {
@ -734,14 +750,7 @@ impl<'tcx> Validator<'_, 'tcx> {
) -> Result<(), Unpromotable> {
let fn_ty = callee.ty(self.body, self.tcx);
// `const` and `static` use the explicit rules for promotion regardless of the `Candidate`,
// meaning calls to `const fn` can be promoted.
let context_uses_explicit_promotion_rules = matches!(
self.const_kind,
Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
);
if !self.explicit && !context_uses_explicit_promotion_rules {
if !self.explicit && self.maybe_runtime() {
if let ty::FnDef(def_id, _) = *fn_ty.kind() {
// Never promote runtime `const fn` calls of
// functions without `#[rustc_promotable]`.