Consts and unit structs in patterns can be confusing if they are mistaken for new bindings. We already provide some context for unit structs and consts that come from the current crate, we now also point at those from foreign crates, and we properly skip cases where the pattern has type parameters which can't be confused with a new binding. Make new binding suggestion verbose.
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
Clean up or resolve cfg-related instances of `FIXME(f16_f128)`
* Replace target-specific config that has a `FIXME` with `cfg(target_has_reliable_f*)`
* Take care of trivial intrinsic-related FIXMEs
* Split `FIXME(f16_f128)` into `FIXME(f16)`, `FIXME(f128)`, or `FIXME(f16,f128)` to more clearly identify what they block
The individual commit messages have more details.
There are a number of instances of `FIXME(f16_f128)` related to target
configuration; either these could use `target_has_reliable_f128`, or the
FIXME is describing such a cfg and is thus redundant (since any
`cfg(target_has_reliable_f*)` needs to be removed before stabilization
anyway).
Switch to using `target_has_reliable_*` where applicable and remove the
redundant FIXMEs.
Currently all core and std macros are automatically added to the prelude
via #[macro_use]. However a situation arose where we want to add a new macro
`assert_matches` but don't want to pull it into the standard prelude for
compatibility reasons. By explicitly exporting the macros found in the core and
std crates we get to decide on a per macro basis and can later add them via
the rust_20xx preludes.
Suggest struct pattern when destructuring Range with .. syntax
implemented a new diagnostic in rustc_resolve to detect invalid range destructuring attempts (e.g., let start..end = range). The fix identifies when resolution fails for identifiers acting as range bounds specifically handling cases where bounds are parsed as expressions and suggests the correct struct pattern syntax (std::ops::Range { start, end }). This replaces confusing "cannot find value" errors with actionable help, verified by a new UI test covering various identifier names.
Fixesrust-lang/rust#149777
Tidying up tests/ui/issues 14 tests [5/N]
> [!NOTE]
> Intermediate commits are intended to help review, but will be squashed add comment commit prior to merge.
part of rust-lang/rust#133895
move `tests/ui/inherent-impls-overlap-check` to `tests/ui/duplicate/inherent-impls-overlap-check`.
r? Kivooeo
Tidying up tests/ui/issues tests [3/N]
> [!NOTE]
> Intermediate commits are intended to help review, but will be squashed add comment commit prior to merge.
part of rust-lang/rust#133895
r? Kivooeo
Merged tests/ui/typeck/non-function-call-error-2 with
tests/ui/typeck/non-function-call-error
Add comment to
tests/ui/traits/normalize-associated-type-in-where-clause.rs
Merged tests/ui/privacy/private-item-simple-2.rs with
tests/ui/privacy/private-item-simple.rs
Merged tests/ui/str/str-add-operator-2.rs with
tests/ui/str/str-add-operator.rs
Add comment to tests/ui/imports/duplicate-empty-imports.rs
Add comment to tests/ui/for-loop-while/nested-loop-break-unit.rs
Add comment to tests/ui/match/match-ref-option-pattern.rs
Add comment to tests/ui/closures/simple-capture-and-call.rs
Add comment to tests/ui/type/never-type-inference-fail.rs
Add comment to tests/ui/match/match-stack-overflow-72933.rs
Tidying up `tests/ui/issues` tests [1/N]
> [!NOTE]
> Intermediate commits are intended to help review, but will be squashed add comment commit prior to merge.
part of rust-lang/rust#133895
merged tests/ui/issues/issue-2951.rs with
tests/ui/type/type-parameter-names.rs
Merged
tests/ui/for-loop-while/break-continue-in-loop-while-contiditoin-1.rs
with
tests/ui/for-loop-while/break-continue-in-loop-while-contiditoin-2.rs
Removed tests/ui/issues/issue-2383.rs
duplicated of library\alloc\src\collections\vec_deque\tests.rs
Removed tests/ui/issues/issue-20714.rs
duplicated of tests/ui/empty/empty-struct-unit-expr.rs
Added comment to tests/ui/match/match-option-result-mismatch.rs, tests/ui/numeric/ref-int.rs,
tests/ui/box/self-assignment.rs
reword error for invalid range patterns
For half-open ranges, specifies that the upper bound cannot be the minimum.
Also specify that this only applies to range patterns and not also expressions.
Fixesrust-lang/rust#149165
For half-open ranges, specifies that the upper bound cannot be the minimum.
Also specify that this only applies to range patterns and not also expressions.
Rehome 30 `tests/ui/issues/` tests to other subdirectories under `tests/ui/` [#2 of Batch #2]
Part of rust-lang/rust#133895
Methodology:
1. Refer to the previously written `tests/ui/SUMMARY.md`
2. Find an appropriate category for the test, using the original issue thread and the test contents.
3. Add the issue URL at the bottom (not at the top, as that would mess up stderr line numbers)
4. Rename the tests to make their purpose clearer
Inspired by the methodology that `@Kivooeo` was using.
r? `@jieyouxu`
`tests/ui/issues/`: The Issues Strike Back [3/N]
Some `tests/ui/issues/` housekeeping, to trim down number of tests directly under `tests/ui/issues/`. Part of https://github.com/rust-lang/rust/issues/133895.
r? ```@jieyouxu```
Rehome 33 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`
rust-lang/rust#143902 divided into smaller, easier to review chunks.
Part of rust-lang/rust#133895
Methodology:
1. Refer to the previously written `tests/ui/SUMMARY.md`
2. Find an appropriate category for the test, using the original issue thread and the test contents.
3. Add the issue URL at the bottom (not at the top, as that would mess up stderr line numbers)
4. Rename the tests to make their purpose clearer
Inspired by the methodology that ``@Kivooeo`` was using.
r? ``@jieyouxu``
`tests/ui/issues/`: The Issues Strike Back [2/N]
Some `tests/ui/issues/` housekeeping, to trim down number of tests directly under `tests/ui/issues/`. Part of https://github.com/rust-lang/rust/issues/133895.
r? ``@jieyouxu``
Use `splice` to avoid shifting the other items twice.
Put `extern crate std;` first so it's already resolved when we resolve `::std::prelude::rust_20XX`.
Esteban Küber