instantiate higher ranked goals outside of candidate selection
This PR modifies `evaluate` to more eagerly instantiate higher-ranked goals, preventing the `leak_check` during candidate selection from detecting placeholder errors involving that binder.
For a general background regarding higher-ranked region solving and the leak check, see https://hackmd.io/qd9Wp03cQVy06yOLnro2Kg.
> The first is something called the **leak check**. You can think of it as a "quick and dirty" approximation for the region check, which will come later. The leak check detects some kinds of errors early, essentially deciding between "this set of outlives constraints are guaranteed to result in an error eventually" or "this set of outlives constraints may be solvable".
## The ideal future
We would like to end up with the following idealized design to handle universal binders:
```rust
fn enter_forall<'tcx, T, R>(
forall: Binder<'tcx, T>,
f: impl FnOnce(T) -> R,
) -> R {
let new_universe = infcx.increment_universe_index();
let value = instantiate_binder_with_placeholders_in(new_universe, forall);
let result = f(value);
eagerly_handle_higher_ranked_region_constraints_in(new_universe);
infcx.decrement_universe_index();
assert!(!result.has_placeholders_in_or_above(new_universe));
result
}
```
That is, when universally instantiating a binder, anything using the placeholders has to happen inside of a limited scope (the closure `f`). After this closure has completed, all constraints involving placeholders are known.
We then handle any *external constraints* which name these placeholders. We destructure `TypeOutlives` constraints involving placeholders and eagerly handle any region constraints involving these placeholders. We do not return anything mentioning the placeholders created inside of this function to the caller.
Being able to eagerly handle *all* region constraints involving placeholders will be difficult due to complex `TypeOutlives` constraints, involving inference variables or alias types, and higher ranked implied bounds. The exact issues and possible solutions are out of scope of this FCP.
#### How does the leak check fit into this
The `leak_check` is an underapproximation of `eagerly_handle_higher_ranked_region_constraints_in`. It detects some kinds of errors involving placeholders from `new_universe`, but not all of them.
It only looks at region outlives constraints, ignoring `TypeOutlives`, and checks whether one of the following two conditions are met for **placeholders in or above `new_universe`**, in which case it results in an error:
- `'!p1: '!p2` a placeholder `'!p2` outlives a different placeholder `'!p1`
- `'!p1: '?2` an inference variable `'?2` outlives a placeholder `'!p1` *which it cannot name*
It does not handle all higher ranked region constraints, so we still return constraints involving placeholders from `new_universe` which are then (re)checked by `lexical_region_resolve` or MIR borrowck.
As we check higher ranked constraints in the full regionck anyways, the `leak_check` is not soundness critical. It's current only purpose is to move some higher ranked region errors earlier, enabling it to guide type inference and trait solving. Adding additional uses of the `leak_check` in the future would only strengthen inference and is therefore not breaking.
## Where do we use currently use the leak check
The `leak_check` is currently used in two places:
Coherence does not use a proper regionck, only relying on the `leak_check` called [at the end of the implicit negative overlap check](8b94152af6/compiler/rustc_trait_selection/src/traits/coherence.rs (L235-L238)). During coherence all parameters are instantiated with inference variables, so the only possible region errors are higher-ranked. We currently also sometimes make guesses when destructuring `TypeOutlives` constraints which can theoretically result in incorrect errors. This could result in overlapping impls.
We also use the `leak_check` [at the end of `fn evaluation_probe`](8b94152af6/compiler/rustc_trait_selection/src/traits/select/mod.rs (L607-L610)). This function is used during candidate assembly for `Trait` goals. Most notably we use [inside of `evaluate_candidate` during winnowing](0e4243538b/compiler/rustc_trait_selection/src/traits/select/mod.rs (L491-L502)). Conceptionally, it is as if we compute each candidate in a separate `enter_forall`.
## The current use in `fn evaluation_probe` is undesirable
Because we only instantiate a higher-ranked goal once inside of `fn evaluation_probe`, errors involving placeholders from that binder can impact selection. This results in inconsistent behavior ([playground](
*[playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dac60ebdd517201788899ffa77364831)*)):
```rust
trait Leak<'a> {}
impl Leak<'_> for Box<u32> {}
impl Leak<'static> for Box<u16> {}
fn impls_leak<T: for<'a> Leak<'a>>() {}
trait IndirectLeak<'a> {}
impl<'a, T: Leak<'a>> IndirectLeak<'a> for T {}
fn impls_indirect_leak<T: for<'a> IndirectLeak<'a>>() {}
fn main() {
// ok
//
// The `Box<u16>` impls fails the leak check,
// meaning that we apply the `Box<u32>` impl.
impls_leak::<Box<_>>();
// error: type annotations needed
//
// While the `Box<u16>` impl would fail the leak check
// we have already instantiated the binder while applying
// the generic `IndirectLeak` impl, so during candidate
// selection of `Leak` we do not detect the placeholder error.
// Evaluation of `Box<_>: Leak<'!a>` is therefore ambiguous,
// resulting in `for<'a> Box<_>: Leak<'a>` also being ambiguous.
impls_indirect_leak::<Box<_>>();
}
```
We generally prefer `where`-bounds over implementations during candidate selection, both for [trait goals](11f32b73e0/compiler/rustc_trait_selection/src/traits/select/mod.rs (L1863-L1887)) and during [normalization](11f32b73e0/compiler/rustc_trait_selection/src/traits/project.rs (L184-L198)). However, we currently **do not** use the `leak_check` during candidate assembly in normalizing. This can result in inconsistent behavior:
```rust
trait Trait<'a> {
type Assoc;
}
impl<'a, T> Trait<'a> for T {
type Assoc = usize;
}
fn trait_bound<T: for<'a> Trait<'a>>() {}
fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
// A function with a trivial where-bound which is more
// restrictive than the impl.
fn function<T: Trait<'static, Assoc = usize>>() {
// ok
//
// Proving `for<'a> T: Trait<'a>` using the where-bound results
// in a leak check failure, so we use the more general impl,
// causing this to succeed.
trait_bound::<T>();
// error
//
// Proving the `Projection` goal `for<'a> T: Trait<'a, Assoc = usize>`
// does not use the leak check when trying the where-bound, causing us
// to prefer it over the impl, resulting in a placeholder error.
projection_bound::<T>();
// error
//
// Trying to normalize the type `for<'a> fn(<T as Trait<'a>>::Assoc)`
// only gets to `<T as Trait<'a>>::Assoc` once `'a` has been already
// instantiated, causing us to prefer the where-bound over the impl
// resulting in a placeholder error. Even if were were to also use the
// leak check during candidate selection for normalization, this
// case would still not compile.
let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
}
```
This is also likely to be more performant. It enables more caching in the new trait solver by simply [recursively calling the canonical query][new solver] after instantiating the higher-ranked goal.
It is also unclear how to add the leak check to normalization in the new solver. To handle https://github.com/rust-lang/trait-system-refactor-initiative/issues/1 `Projection` goals are implemented via `AliasRelate`. This again means that we instantiate the binder before ever normalizing any alias. Even if we were to avoid this, we lose the ability to [cache normalization by itself, ignoring the expected `term`](5bd5d214ef/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs (L34-L49)). We cannot replace the `term` with an inference variable before instantiating the binder, as otherwise `for<'a> T: Trait<Assoc<'a> = &'a ()>` breaks. If we only replace the term after instantiating the binder, we cannot easily evaluate the goal in a separate context, as [we'd then lose the information necessary for the leak check](11f32b73e0/compiler/rustc_next_trait_solver/src/canonicalizer.rs (L230-L232)). Adding this information to the canonical input also seems non-trivial.
## Proposed solution
I propose to instantiate the binder outside of candidate assembly, causing placeholders from higher-ranked goals to get ignored while selecting their candidate. This mostly[^1] matches the [current behavior of the new solver][new solver]. The impact of this change is therefore as follows:
```rust
trait Leak<'a> {}
impl Leak<'_> for Box<u32> {}
impl Leak<'static> for Box<u16> {}
fn impls_leak<T: for<'a> Leak<'a>>() {}
trait IndirectLeak<'a> {}
impl<'a, T: Leak<'a>> IndirectLeak<'a> for T {}
fn impls_indirect_leak<T: for<'a> IndirectLeak<'a>>() {}
fn guide_selection() {
// ok -> ambiguous
impls_leak::<Box<_>>();
// ambiguous
impls_indirect_leak::<Box<_>>();
}
trait Trait<'a> {
type Assoc;
}
impl<'a, T> Trait<'a> for T {
type Assoc = usize;
}
fn trait_bound<T: for<'a> Trait<'a>>() {}
fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
// A function which a trivial where-bound which is more
// restrictive than the impl.
fn function<T: Trait<'static, Assoc = usize>>() {
// ok -> error
trait_bound::<T>();
// error
projection_bound::<T>();
// error
let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
}
```
This does not change the behavior if candidates have higher ranked nested goals, as in this case the `leak_check` causes the nested goal to result in an error ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a74c25300b23db9022226de99d8a2fa6)):
```rust
trait LeakCheckFailure<'a> {}
impl LeakCheckFailure<'static> for () {}
trait Trait<T> {}
impl Trait<u32> for () where for<'a> (): LeakCheckFailure<'a> {}
impl Trait<u16> for () {}
fn impls_trait<T: Trait<U>, U>() {}
fn main() {
// ok
//
// It does not matter whether candidate assembly
// considers the placeholders from higher-ranked goal.
//
// Either `for<'a> (): LeakCheckFailure<'a>` has no
// applicable candidate or it has a single applicable candidate
// when then later results in an error. This allows us to
// infer `U` to `u16`.
impls_trait::<(), _>()
}
```
## Impact on existing crates
This is a **breaking change**. [A crater run](https://github.com/rust-lang/rust/pull/119820#issuecomment-1926862174) found 17 regressed crates with 7 root causes.
For a full analysis of all affected crates, see https://gist.github.com/lcnr/7c1c652f30567048ea240554a36ed95c.
---
I believe this breakage to be acceptable and would merge this change. I am confident that the new position of the leak check matches our idealized future and cannot envision any other consistent alternative. Where possible, I intend to open PRs fixing/avoiding the regressions before landing this PR.
I originally intended to remove the `coherence_leak_check` lint in the same PR. However, while I am confident in the *position* of the leak check, deciding on its exact behavior is left as future work, cc #112999. This PR therefore only moves the leak check while keeping the lint when relying on it in coherence.
[new solver]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs#L479-L484
[^1]: the new solver has a separate cause of inconsistent behavior rn https://github.com/rust-lang/trait-system-refactor-initiative/issues/53#issuecomment-1914310171
r? `@nikomatsakis`
Check `x86_64` size assertions on `aarch64`, too
(Context: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Checking.20size.20assertions.20on.20aarch64.3F)
Currently the compiler has around 30 sets of `static_assert_size!` for various size-critical data structures (e.g. various IR nodes), guarded by `#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]`.
(Presumably this cfg avoids having to maintain separate size values for 32-bit targets and unusual 64-bit targets. Apparently it may have been necessary before the i128/u128 alignment changes, too.)
This is slightly incovenient for people on aarch64 workstations (e.g. Macs), because the assertions normally aren't checked until we push to a PR. So this PR adds `aarch64` to the `#[cfg(..)]` guarding all of those assertions in the compiler.
---
Implemented with a simple find/replace. Verified by manually inspecting each `static_assert_size!` in `compiler/`, and checking that either the replacement succeeded, or adding aarch64 wouldn't have been appropriate.
Assert that args are actually compatible with their generics, rather than just their count
Right now we just check that the number of args is right, rather than actually checking the kinds. Uplift a helper fn that I wrote from trait selection to do just that. Found a couple bugs along the way.
r? `@lcnr` or `@fmease` (or anyone really lol)
Fix target name in NetBSD platform-support doc
NetBSD platform-support doc currently mentions `amd64-unknown-netbsd`, but it is not a valid target name (the correct name is `x86_64-unknown-netbsd`).
ceab6128fa/src/doc/rustc/src/platform-support/netbsd.md (L16)
```console
$ rustc --print target-list | grep netbsd
aarch64-unknown-netbsd
aarch64_be-unknown-netbsd
armv6-unknown-netbsd-eabihf
armv7-unknown-netbsd-eabihf
i586-unknown-netbsd
i686-unknown-netbsd
mipsel-unknown-netbsd
powerpc-unknown-netbsd
riscv64gc-unknown-netbsd
sparc64-unknown-netbsd
x86_64-unknown-netbsd
```
More postfix match fixes
These affect diagnostics only, as far as I can tell. I'm too lazy to come up with UI tests, but I could be convinced otherwise.
Specifically, I think changing the precedence computation actually doesn't change anything, but tweaking `contains_exterior_struct_lit` does mean that some diagnostics will begin parenthesizing `S {}.match {}`.
pattern analysis: fix union handling
Little known fact: rust supports union patterns. Exhaustiveness handles them soundly but reports nonsensical missing patterns. This PR fixes the reported patterns and documents what we're doing.
r? `@compiler-errors`
Rename `expose_addr` to `expose_provenance`
`expose_addr` is a bad name, an address is just a number and cannot be exposed. The operation is actually about the provenance of the pointer.
This PR thus changes the name of the method to `expose_provenance` without changing its return type. There is sufficient precedence for returning a useful value from an operation that does something else without the name indicating such, e.g. [`Option::insert`](https://doc.rust-lang.org/nightly/std/option/enum.Option.html#method.insert) and [`MaybeUninit::write`](https://doc.rust-lang.org/nightly/std/mem/union.MaybeUninit.html#method.write).
Returning the address is merely convenient, not a fundamental part of the operation. This is implied by the fact that integers do not have provenance since
```rust
let addr = ptr.addr();
ptr.expose_provenance();
let new = ptr::with_exposed_provenance(addr);
```
must behave exactly like
```rust
let addr = ptr.expose_provenance();
let new = ptr::with_exposed_provenance(addr);
```
as the result of `ptr.expose_provenance()` and `ptr.addr()` is the same integer. Therefore, this PR removes the `#[must_use]` annotation on the function and updates the documentation to reflect the important part.
~~An alternative name would be `expose_provenance`. I'm not at all opposed to that, but it makes a stronger implication than we might want that the provenance of the pointer returned by `ptr::with_exposed_provenance`[^1] is the same as that what was exposed, which is not yet specified as such IIUC. IMHO `expose` does not make that connection.~~
A previous version of this PR suggested `expose` as name, libs-api [decided on](https://github.com/rust-lang/rust/pull/122964#issuecomment-2033194319) `expose_provenance` to keep the symmetry with `with_exposed_provenance`.
CC `@RalfJung`
r? libs-api
[^1]: I'm using the new name for `from_exposed_addr` suggested by #122935 here.
Better reporting on generic argument mismatchs
This allows better reporting as per issue #116615 .
If you have a function:
```
fn foo(a: T, b: T) {}
```
and call it like so:
```
foo(1, 2.)
```
it'll give improved error reported similar to the following:
```
error[E0308]: mismatched types
--> generic-mismatch-reporting-issue-116615.rs:6:12
|
6 | foo(1, 2.);
| --- - ^^ expected integer, found floating-point number
| | |
| | expected argument `b` to be an integer because that argument needs to match the type of this parameter
| arguments to this function are incorrect
|
note: function defined here
--> generic-mismatch-reporting-issue-116615.rs:1:4
|
1 | fn foo<T>(a: T, b: T) {}
| ^^^ - ---- ----
| | | |
| | | this parameter needs to match the integer type of `a`
| | `b` needs to match the type of this parameter
| `a` and `b` all reference this parameter T
```
Open question, do we need to worry about error message translation into other languages? Not sure what the status of that is in Rust.
NB: Needs some checking over and some tests have altered that need sanity checking, but overall this is starting to get somewhere now. Will take out of draft PR status when this has been done, raising now to allow feedback at this stage, probably 90% ready.
Reproduce the bug from <https://github.com/rust-lang/rust/issues/123282>
that indicates this feature gate hits edition-dependent resolution paths.
Resolution changed in edition 2018, so test that as well.
Output URLs of CI artifacts to GitHub summary
I often want to download CI artifacts published from our workflows (I suspect others might do the same), but it's a bit annoying to extract their links from the CI logs currently. This PR also outputs URLs to them to the GitHub Actions summaries.
r? `@Mark-Simulacrum`
Assert `FnDef` kind
Only found one bug, where we were using the variant def id rather than its ctor def id to make the `FnDef` for a `type_of`
r? fmease
Add section to sanitizer doc for `-Zexternal-clangrt`
After spending a week looking for answers to how to do the very thing this flag lets me do, it felt appropriate to document it where I would've expected it to be.
Remove MIR unsafe check
Now that THIR unsafeck is enabled by default in stable I think we can remove MIR unsafeck entirely. This PR also removes safety information from MIR.
Rollup of 4 pull requests
Successful merges:
- #122411 ( Provide cabi_realloc on wasm32-wasip2 by default )
- #123349 (Fix capture analysis for by-move closure bodies)
- #123359 (Link against libc++abi and libunwind as well when building LLVM wrappers on AIX)
- #123388 (use a consistent style for links)
r? `@ghost`
`@rustbot` modify labels: rollup
Link against libc++abi and libunwind as well when building LLVM wrappers on AIX
Unlike `libc++.so` on Linux which is a linker script
```ld
INPUT(libc++.so.1 -lc++abi -lunwind)
```
AIX linker doesn't support such script, so `c++abi` and `unwind` have to be specified explicitly.
Fix capture analysis for by-move closure bodies
The check we were doing to figure out if a coroutine was borrowing from its parent coroutine-closure was flat-out wrong -- a misunderstanding of mine of the way that `tcx.closure_captures` represents its captures.
Fixes#123251 (the miri/ui test I added should more than cover that issue)
r? `@oli-obk` -- I recognize that this PR may be underdocumented, so please ask me what I should explain further.
Provide cabi_realloc on wasm32-wasip2 by default
This commit provides a component model intrinsic in the standard library
by default on the `wasm32-wasip2` target. This intrinsic is not
required by the component model itself but is quite common to use, for
example it's needed if a wasm module receives a string or a list.
The intention of this commit is to provide an overridable definition in
the standard library through a weak definition of this function. That
means that downstream crates can provide their own customized and more
specific versions if they'd like, but the standard library's version
should suffice for general-purpose use.
