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Auto merge of #75585 - RalfJung:demotion, r=oli-obk

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Do not promote &mut of a non-ZST ever

Since ~pre-1.0~ 1.36, we have accepted code like this:
```rust
static mut TEST: &'static mut [i32] = {
    let x = &mut [1,2,3];
    x
};
```
I tracked it back to https://github.com/rust-lang/rust/pull/21744, but unfortunately could not find any discussion or RFC that would explain why we thought this was a good idea. And it's not, it breaks all sorts of things -- see https://github.com/rust-lang/rust/issues/75556.

To fix https://github.com/rust-lang/rust/issues/75556, we have to stop promoting non-ZST mutable references no matter the context, which is what this PR does. It's a breaking change.

Notice that this still works, since it does not rely on promotion:
```rust
static mut TEST: &'static mut [i32] = &mut [0,1,2];
```

Cc `@rust-lang/wg-const-eval`
This commit is contained in:
bors 2020-09-08 05:13:42 +00:00
commit e82584a77d
4 changed files with 52 additions and 16 deletions
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18
compiler/rustc_mir/src/transform/promote_consts.rs
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@ -364,15 +364,7 @@ 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.
if self.const_kind
== Some(hir::ConstContext::Static(hir::Mutability::Mut))
{
// Inside a `static mut`, &mut [...] is also allowed.
match ty.kind() {
ty::Array(..) | ty::Slice(_) => {}
_ => return Err(Unpromotable),
}
} else if let ty::Array(_, len) = ty.kind() {
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) {
@ -673,13 +665,7 @@ 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.
if self.const_kind == Some(hir::ConstContext::Static(hir::Mutability::Mut)) {
// Inside a `static mut`, &mut [...] is also allowed.
match ty.kind() {
ty::Array(..) | ty::Slice(_) => {}
_ => return Err(Unpromotable),
}
} else if let ty::Array(_, len) = ty.kind() {
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) {