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
Add `-Zprecise-enum-drop-elaboration`
Its purpose is to assist in debugging #77382 and #74551. Passing `-Zprecise-enum-drop-elaboration=no` will turn off the added precision that seems to be causing issues on some platforms. This assumes that we can reproduce #77382 on the latest master. I should have done this earlier. Oh well.
cc @cuviper
r? @pnkfelix
Overhaul const-checking diagnostics
The primary purpose of this PR was to remove `NonConstOp::STOPS_CONST_CHECKING`, which causes any additional errors found by the const-checker to be silenced. I used this flag to preserve diagnostic parity with `qualify_min_const_fn.rs`, which has since been removed.
However, simply removing the flag caused a deluge of errors in some cases, since an error would be emitted any time a local or temporary had a wrong type. To remedy this, I added an alternative system (`DiagnosticImportance`) to silence additional error messages that were likely to distract the user from the underlying issue. When an error of the highest importance occurs, all less important errors are silenced. When no error of the highest importance occurs, all less important errors are emitted after checking is complete. Following the suggestions from the important error is usually enough to fix the less important errors, so this should lead to better UX most of the time.
There's also some unrelated diagnostics improvements in this PR isolated in their own commits. Splitting them out would be possible, but a bit of a pain. This isn't as tidy as some of my other PRs, but it should *only* affect diagnostics, never whether or not something passes const-checking. Note that there are a few trivial exceptions to this, like banning `Yield` in all const-contexts, not just `const fn`.
As always, meant to be reviewed commit-by-commit.
r? `@oli-obk`
adt_destructor by default also validates the Drop impl using
dropck::check_drop_impl, which contains an expect_local(). This
leads to ICE in check_const_item_mutation if the const's type is
not a local type.
thread 'rustc' panicked at 'DefId::expect_local: `DefId(5:4805 ~ alloc[d7e9]::vec::{impl#50})` isn't local', compiler/rustc_span/src/def_id.rs:174:43
stack backtrace:
0: rust_begin_unwind
1: rustc_span::def_id::DefId::expect_local::{{closure}}
2: rustc_typeck::check::dropck::check_drop_impl
3: rustc_middle::ty::util::<impl rustc_middle::ty::context::TyCtxt>::calculate_dtor::{{closure}}
4: rustc_middle::ty::trait_def::<impl rustc_middle::ty::context::TyCtxt>::for_each_relevant_impl
5: rustc_middle::ty::util::<impl rustc_middle::ty::context::TyCtxt>::calculate_dtor
6: rustc_typeck::check::adt_destructor
7: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::adt_destructor>::compute
8: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
9: rustc_query_system::query::plumbing::get_query_impl
10: rustc_mir::transform::check_const_item_mutation::ConstMutationChecker::is_const_item_without_destructor
This helper function was meant to reduce code duplication between
const-checking pre- and post-drop-elaboration. Most of the functionality
is only relevant for the pre-drop-elaboration pass.
Rollup of 7 pull requests
Successful merges:
- #76454 (UI to unit test for those using Cell/RefCell/UnsafeCell)
- #76474 (Add option to pass a custom codegen backend from a driver)
- #76711 (diag: improve closure/generic parameter mismatch)
- #77170 (Remove `#[rustc_allow_const_fn_ptr]` and add `#![feature(const_fn_fn_ptr_basics)]`)
- #77194 (Add doc alias for iterator fold)
- #77288 (fix building libstd for Miri on macOS)
- #77295 (Update unstable-book: Fix ABNF in inline assembly docs)
Failed merges:
r? `@ghost`
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
[mir-opt] Introduce a new flag to enable experimental/unsound mir opts
This implements part of https://github.com/rust-lang/compiler-team/issues/319. The exact name of this flag was not decided as part of that MCP and some people expressed that it should include "unsound" in some way.
I've chosen to use `enable-experimental-unsound-mir-opts` as the name. While long, I don't think that matters too much as really it will only be used by some mir-opt tests. If you object or have a better name, please leave a comment!
r? `@oli-obk`
cc `@rust-lang/wg-mir-opt` `@RalfJung`
Replace `discriminant_switch_effect` with more general version
#68528 added a new edge-specific effect for `SwitchInt` terminators, `discriminant_switch_effect`, to the dataflow framework. While this accomplished the short-term goal of making drop elaboration more precise, it wasn't really useful in other contexts: It only supported `SwitchInt`s on the discriminant of an `enum` and did not allow effects to be applied along the "otherwise" branch. In const-propagation, for example, arbitrary edge-specific effects for the targets of a `SwitchInt` can be used to remember the value a `match` scrutinee must have in each arm.
This PR replaces `discriminant_switch_effect` with a more general `switch_int_edge_effects` method. The new method has a slightly different interface from the other edge-specific effect methods (e.g. `call_return_effect`). This divergence is explained in the new method's documentation, and reading the changes to the various dataflow impls as well as `direction.rs` should further clarify things. This PR should not change behavior.
This 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 the
bespoke attribute.
