Avoid ICE when suggesting as_ref for ill-typed closure receivers
Fixesrust-lang/rust#156299
When building mismatch suggestions, `can_use_as_ref` may inspect the receiver of a method call that is itself an ill-typed closure expression. In that recovery path, the receiver may not have a recorded type in `TypeckResults`.
Use `expr_ty_opt` instead of `expr_ty` so the optional `as_ref()` suggestion is skipped when the receiver type is unavailable.
Avoid misleading return-type note for foreign `Fn` callees
Fixesrust-lang/rust#155727.
The issue occurred because the code that emitted the `Fn`/`FnMut` suggestion only avoided the return-type fallback when it could point at a local callee argument. The local case from rust-lang/rust#125325 was fixed in rust-lang/rust#126226, but nested cross-crate cases could still suggest changing the enclosing function's return type even though the relevant `Fn` requirement came from the callee argument.
This is also the broader diagnostic shape discussed in rust-lang/rust#119985.
This PR checks the instantiated callee predicates for an exact `Fn` bound
on the closure argument, and extends `wrong-closure-arg-suggestion-125325` with cross-crate function and method cases for that path.
Provide more context on type errors in const context
- On `const` and `static` point at the type (like we do for let bindings)
- On fn calls, point at const parameter in fn definition
- On type, point at const parameter in type definition
- On array type lengths, explain that array length is always `usize`
- On enum variant discriminant, mention `repr`
- On default field value using type parameter, provide more context (Fixrust-lang/rust#147748)
- On `const` and `static` point at the type (like we do for let bindings)
- On fn calls, point at const parameter in fn definition
- On type, point at const parameter in type definition
- On array type lengths, explain that array length is always `usize`
- On enum variant discriminant, mention `repr`
Make operational semantics of pattern matching independent of crate and module
The question of "when does matching an enum against a pattern of one of its variants read its discriminant" is currently an underspecified part of the language, causing weird behavior around borrowck, drop order, and UB.
Of course, in the common cases, the discriminant must be read to distinguish the variant of the enum, but currently the following exceptions are implemented:
1. If the enum has only one variant, we currently skip the discriminant read.
- This has the advantage that single-variant enums behave the same way as structs in this regard.
- However, it means that if the discriminant exists in the layout, we can't say that this discriminant being invalid is UB. This makes me particularly uneasy in its interactions with niches – consider the following example ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=5904a6155cbdd39af4a2e7b1d32a9b1a)), where miri currently doesn't detect any UB (because the semantics don't specify any):
<details><summary>Example 1</summary>
```rust
#![allow(dead_code)]
use core::mem::{size_of, transmute};
#[repr(u8)]
enum Inner {
X(u8),
}
enum Outer {
A(Inner),
B(u8),
}
fn f(x: &Inner) {
match x {
Inner::X(v) => {
println!("{v}");
}
}
}
fn main() {
assert_eq!(size_of::<Inner>(), 2);
assert_eq!(size_of::<Outer>(), 2);
let x = Outer::B(42);
let y = &x;
f(unsafe { transmute(y) });
}
```
</details>
2. For the purpose of the above, enums with marked with `#[non_exhaustive]` are always considered to have multiple variants when observed from foreign crates, but the actual number of variants is considered in the current crate.
- This means that whether code has UB can depend on which crate it is in: https://github.com/rust-lang/rust/issues/147722
- In another case of `#[non_exhaustive]` affecting the runtime semantics, its presence or absence can change what gets captured by a closure, and by extension, the drop order: https://github.com/rust-lang/rust/issues/147722#issuecomment-3674554872
- Also at the above link, there is an example where removing `#[non_exhaustive]` can cause borrowck to suddenly start failing in another crate.
3. Moreover, we currently make a more specific check: we only read the discriminant if there is more than one *inhabited* variant in the enum.
- This means that the semantics can differ between `foo<!>`, and a copy of `foo` where `T` was manually replaced with `!`: rust-lang/rust#146803
- Moreover, due to the privacy rules for inhabitedness, it means that the semantics of code can depend on the *module* in which it is located.
- Additionally, this inhabitedness rule is even uglier due to the fact that closure capture analysis needs to happen before we can determine whether types are uninhabited, which means that whether the discriminant read happens has a different answer specifically for capture analysis.
- For the two above points, see the following example ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=a07d8a3ec0b31953942e96e2130476d9)):
<details><summary>Example 2</summary>
```rust
#![allow(unused)]
mod foo {
enum Never {}
struct PrivatelyUninhabited(Never);
pub enum A {
V(String, String),
Y(PrivatelyUninhabited),
}
fn works(mut x: A) {
let a = match x {
A::V(ref mut a, _) => a,
_ => unreachable!(),
};
let b = match x {
A::V(_, ref mut b) => b,
_ => unreachable!(),
};
a.len(); b.len();
}
fn fails(mut x: A) {
let mut f = || match x {
A::V(ref mut a, _) => (),
_ => unreachable!(),
};
let mut g = || match x {
A::V(_, ref mut b) => (),
_ => unreachable!(),
};
f(); g();
}
}
use foo::A;
fn fails(mut x: A) {
let a = match x {
A::V(ref mut a, _) => a,
_ => unreachable!(),
};
let b = match x {
A::V(_, ref mut b) => b,
_ => unreachable!(),
};
a.len(); b.len();
}
fn fails2(mut x: A) {
let mut f = || match x {
A::V(ref mut a, _) => (),
_ => unreachable!(),
};
let mut g = || match x {
A::V(_, ref mut b) => (),
_ => unreachable!(),
};
f(); g();
}
```
</details>
In light of the above, and following the discussion at rust-lang/rust#138961 and rust-lang/rust#147722, this PR ~~makes it so that, operationally, matching on an enum *always* reads its discriminant.~~ introduces the following changes to this behavior:
- matching on a `#[non_exhaustive]` enum will always introduce a discriminant read, regardless of whether the enum is from an external crate
- uninhabited variants now count just like normal ones, and don't get skipped in the checks
As per the discussion below, the resolution for point (1) above is that it should land as part of a separate PR, so that the subtler decision can be more carefully considered.
Note that this is a breaking change, due to the aforementioned changes in borrow checking behavior, new UB (or at least UB newly detected by miri), as well as drop order around closure captures. However, it seems to me that the combination of this PR with rust-lang/rust#138961 should have smaller real-world impact than rust-lang/rust#138961 by itself.
Fixesrust-lang/rust#142394Fixesrust-lang/rust#146590Fixesrust-lang/rust#146803 (though already marked as duplicate)
Fixes parts of rust-lang/rust#147722Fixesrust-lang/miri#4778
r? @Nadrieril @RalfJung
@rustbot label +A-closures +A-patterns +T-opsem +T-lang
If late lifetime resolution fails for whatever reason, forward to RBV
the guarantee that an error was emitted - thereby eliminating the need
for a "hack" to suppress subsequent/superfluous error diagnostics.
Provide more context on trait bounds being unmet due to imperfect derive
When encountering a value that has a borrow checker error where the type was previously moved, when suggesting cloning verify that it is not already being derived. If it is, explain why the `derive(Clone)` doesn't apply:
```
note: if `TypedAddress<T>` implemented `Clone`, you could clone the value
--> $DIR/derive-clone-implicit-bound.rs:6:1
|
LL | #[derive(Clone, Copy)]
| ----- derived `Clone` adds implicit bounds on type parameters
LL | pub struct TypedAddress<T>{
| ^^^^^^^^^^^^^^^^^^^^^^^^-^
| | |
| | introduces an implicit `T: Clone` bound
| consider manually implementing `Clone` for this type
...
LL | let old = self.return_value(offset);
| ------ you could clone this value
```
When encountering a bound coming from a derive macro, suggest manual impl of the trait.
Use the span for the specific param when adding bounds in builtin derive macros, so the diagnostic will point at them as well as the derive macro itself.
```
note: required for `Id<SomeNode>` to implement `PartialEq`
--> $DIR/derive-implicit-bound.rs:5:10
|
LL | #[derive(PartialEq, Eq)]
| ^^^^^^^^^
LL | pub struct Id<T>(PhantomData<T>);
| - unsatisfied trait bound introduced in this `derive` macro
= help: consider manually implementing `PartialEq` to avoid undesired bounds
```
Mention that the trait could be manually implemented in E0599.
Fixrust-lang/rust#108894. Address rust-lang/rust#143714. Address #rust-lang/rust#146515 (but ideally would also suggest constraining the fn bound correctly as well).
Preliminary match/capture test cleanup for PR 150681
Review for rust-lang/rust#150681 requested that this cleanup gets extracted to a separate PR.
r? @Zalathar
When a closure has an inferred parameter type like `|ch|` and the
expected type differs in borrowing (e.g., `char` vs `&char`), the
suggestion code would incorrectly suggest `|char|` instead of the
valid `|ch: char|`.
This happened because the code couldn't walk explicit `&` references
in the HIR when the type is inferred, and fell back to replacing the
entire parameter span with the expected type name.
Fix by only emitting the suggestion when we can properly identify the
`&` syntax to remove.
add comment to closure-move-use-after-move-diagnostic.rs
add comment to missing-operator-after-float.rs
add comment to closure-array-break-length.rs
add comment to box-lifetime-argument-not-allowed.rs
add comment to const-return-outside-fn.rs
add comment to drop-conflicting-impls.rs
add comment to unbalanced-doublequote-2.rs
add comment to borrow-immutable-deref-box.rs
add comment to for-in-const-eval.rs
add comment to borrowck-annotated-static-lifetime.rs
cleaned up cast-rfc0401.rs
add comment to nll-anon-to-static.rs
add comment to cast-to-dyn-any.rs
add comment to missing-associated-items.rs
add comment to enum-discriminant-missing-variant.rs
Haoran Wang