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Rollup merge of #77251 - dtolnay:drop, r=Aaron1011

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Bypass const_item_mutation if const's type has Drop impl

Follow-up to #75573. This PR disables the const_item_mutation lint in cases that the const has a Drop impl which observes the mutation.

```rust
struct Log { msg: &'static str }
const LOG: Log = Log { msg: "" };
impl Drop for Log {
    fn drop(&mut self) { println!("{}", self.msg); }
}

LOG.msg = "wow";  // prints "wow"
```

r? @Aaron1011
This commit is contained in:
Jonas Schievink 2020-10-03 00:31:12 +02:00 • committed by GitHub
commit 6522868664
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3 changed files with 102 additions and 14 deletions
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31
compiler/rustc_mir/src/transform/check_const_item_mutation.rs
View file

@ -31,6 +31,35 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
None
}
}
fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> {
let def_id = self.is_const_item(local)?;
let mut any_dtor = |_tcx, _def_id| Ok(());
// We avoid linting mutation of a const item if the const's type has a
// Drop impl. The Drop logic observes the mutation which was performed.
//
// pub struct Log { msg: &'static str }
// pub const LOG: Log = Log { msg: "" };
// impl Drop for Log {
// fn drop(&mut self) { println!("{}", self.msg); }
// }
//
// LOG.msg = "wow"; // prints "wow"
//
// FIXME(https://github.com/rust-lang/rust/issues/77425):
// Drop this exception once there is a stable attribute to suppress the
// const item mutation lint for a single specific const only. Something
// equivalent to:
//
// #[const_mutation_allowed]
// pub const LOG: Log = Log { msg: "" };
match self.tcx.calculate_dtor(def_id, &mut any_dtor) {
Some(_) => None,
None => Some(def_id),
}
}
fn lint_const_item_usage(
&self,
const_item: DefId,
@ -59,7 +88,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
// Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
// so emitting a lint would be redundant.
if !lhs.projection.is_empty() {
if let Some(def_id) = self.is_const_item(lhs.local) {
if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) {
// Don't lint on writes through a pointer
// (e.g. `unsafe { *FOO = 0; *BAR.field = 1; }`)
if !matches!(lhs.projection.last(), Some(PlaceElem::Deref)) {