rework winnowing to sensibly handle global where-bounds
There may be multiple ways to prove a given trait-bound. In case there are multiple such applicable candidates we need to somehow merge them or bail with ambiguity. When merging candidates we prefer some over others for multiple reasons:
- we want to guide inference during typeck, even if not strictly necessary
- to avoid ambiguity if there if there are at most lifetime differences
- old solver needs exactly one candidate
- new solver only needs to handle lifetime differences
- we disable normalization via impls if the goal is proven by using a where-bound
## The approach in this PR[^1]
- always prefer trivial builtin impls[^6]
- then prefer non-global[^global] where-bounds
- if there exists exactly one where-bound, guide inference
- if there are multiple where-bounds even if some of them are global, ambig
- then prefer alias bounds[^2] and builtin trait object candidates[^3][^2]
- merge everything ignoring global where-bounds
- if there are no other candidates, try using global where-bounds[^5]
**We disable normalization via impls when using non-global where-bounds or alias-bounds, even if we're unable to normalize by using the where-bound.**
[^1]: see the source for more details
[^2]: [we arbitrary select a single object and alias-bound candidate in case multiple apply and they don't impact inference](a4cedecc9e/compiler/rustc_trait_selection/src/traits/select/mod.rs (L1906-L1911)). This should be unnecessary in the new solver.
[^3]: Necessary for `dyn Any` and https://github.com/rust-lang/rust/issues/57893
[^global]: a where-bound is global if it is not higher-ranked and doesn't contain any generic parameters, `'static` is ok
[^5]: global where-bounds are only used if they are unsatisfiable, i.e. no impl candidate exists
[^6]: they don't constrain inference and don't add any lifetime constraints
## Why this behavior?
### inference guidance via where-bounds and alias-bounds
#### where-bounds
```rust
fn method_selection<T: Into<u64>>(x: T) -> Option<u32> {
x.into().try_into().ok()
// prove `T: Into<?0>` and then select a method `?0`,
// needs eager inference.
}
```
While the above pattern exists in the wild, I think that most inference guidance due to where-bounds is actually unintended. I believe we may want to restrict inference guidance in the future, e.g. limit it to where-bounds whose self-type is a param.
#### alias-bounds
```rust
pub trait Dyn {
type Word: Into<u64>;
fn d_tag(&self) -> Self::Word;
fn tag32(&self) -> Option<u32> {
self.d_tag().into().try_into().ok()
// prove `Self::Word: Into<?0>` and then select a method
// on `?0`, needs eager inference.
}
}
```
### Disable normalization via impls when using where-bounds
cc https://github.com/rust-lang/trait-system-refactor-initiative/issues/125
```rust
trait Trait<'a> {
type Assoc;
}
impl<T> Trait<'static> for T {
type Assoc = ();
}
// normalizing requires `'a == 'static`, the trait bound does not.
fn foo<'a, T: Trait<'a>>(_: T::Assoc) {}
```
If an impl adds constraints not required by a where-bound, using the impl may cause compilation failure avoided by treating the associated type as rigid.
This is also why we can always use trivial builtin impls, even for normalization. They are guaranteed to never add any requirements.
### Lower priority for global where-bounds
A where-bound is considered global if it does not refer to any generic parameters and is not higher-ranked. It may refer to `'static`.
This means global where-bounds are either always fully implied by an impl or unsatisfiable. We don't really care about the inference behavior of unsatisfiable where-bounds :3
If a where-bound is fully implied then using an applicable impl for normalization cannot result in additional constraints. As this is the - afaict only - reason why we disable normalization via impls in the first place, we don't have to disable normalization via impls when encountering global where-bounds.
### Consider global where-bounds at all
Given that we just use impls even if there exists a global where-bounds, you may ask why we don't just ignore these global where-bounds entirely: we use them to weaken the inference guidance from non-global where-bounds.
Without a global where-bound, we currently prefer non-global where bounds even though there would be an applicable impl as well. By adding a non-global where-bound, this *unnecessary* inference guidance is disabled, allowing the following to compile:
```rust
fn check<Color>(color: Color)
where
Vec: Into<Color> + Into<f32>,
{
let _: f32 = Vec.into();
// Without the global `Vec: Into<f32>` bound we'd
// eagerly use the non-global `Vec: Into<Color>` bound
// here, causing this to fail.
}
struct Vec;
impl From<Vec> for f32 {
fn from(_: Vec) -> Self {
loop {}
}
}
```
[There exist multiple crates which rely on this behavior](https://github.com/rust-lang/rust/pull/124592#issuecomment-2104541495).
## Design considerations
We would like to be able to normalize via impls as much as possible. Disabling normalization simply because there exists a where-bound is undesirable.
For the sake of backwards compatability I intend to mostly mirror the current inference guidance rules and then explore possible improvements once the new solver is done. I do believe that removing unnecessary inference guidance where possible is desirable however.
Whether a where-bound is global depends on whether used lifetimes are `'static`. The where-bound `u32: Trait<'static>` is either entirely implied by an impl, meaning that it does not have to disable normalization via impls, **while `u32: Trait<'a>` needs to disable normalization via impls as the impl may only hold for `'static`**. Considering all where-bounds to be non-global once they contain any region is unfortunately a breaking change.
## How does this differ from stable
The currently stable approach is order dependent:
- it prefers impls over global where-bounds: impl > global
- it prefers non-global where-bounds over impls: non-global > impl
- it treats all where-bounds equally: global = non-global
This means that whether we bail with ambiguity or simply use the non-global where bound depending on the *order of where-clauses* and *number of applicable impl candidates*. See the tests added in the first commit for more details. With this PR we now always bail with ambiguity.
I've previously tried to always use the non-global candidate, causing unnecessary inference guidance and undesirable breakage. This already went through an FCP in #124592. However, I consider the new approach to be preferable as it exclusively removes incompleteness. It also doesn't cause any crater breakage.
## How to support this in the new solver :o
**This is separately implemented in #133643 and not part of this FCP!**
To implement the global vs non-global where-bound distinction, we have to either keep `'static` in the `param_env` when canonicalizing, or eagerly distinguish global from non-global where-bounds and provide that information to the canonical query.
The old solver currently keeps `'static` only the `param_env`, replacing it with an inference variable in the `value`.
a4cedecc9e/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs (L49-L64)
I dislike that based on *vibes* and it may end up being a problem once we extend the environment inside of the solver as [we must not rely on `'static` in the `predicate` as it would get erased in MIR typeck](https://github.com/rust-lang/trait-system-refactor-initiative/issues/30).
An alternative would be to eagerly detect trivial where-bounds when constructing the `ParamEnv`. We can't entirely drop them [as explained above](https://hackmd.io/qoesqyzVTe2v9cOgFXd2SQ#Consider-true-global-where-bounds-at-all), so we'd instead replace them with a new clause kind `TraitImpliedByImpl` which gets entirely ignored except when checking whether we should eagerly guide inference via a where-bound. This approach can be extended to where-bounds which are currently not considered global to stop disabling normalization for them as well.
Keeping `'static` in the `param_env` is the simpler solution here and we should be able to move to the second approach without any breakage. I therefore propose to keep `'static` in the environment for now.
---
r? `@compiler-errors`
Remove queries from the driver interface
All uses of driver queries in the public api of rustc_driver have been removed in https://github.com/rust-lang/rust/pull/134130 already. This removes driver queries from rustc_interface and does a couple of cleanups around TyCtxt construction and entering enabled by this removal.
Finishes the removal of driver queries started with https://github.com/rust-lang/rust/pull/126834.
Rollup of 9 pull requests
Successful merges:
- #134124 (CI: use free runners for x86_64-gnu-llvm jobs)
- #134197 (rustc_mir_build: Clarify that 'mirrored' does not mean 'flipped' or 'reversed')
- #134260 (Correctly handle comments in attributes in doctests source code)
- #134277 (rustdoc-search: handle `impl Into<X>` better)
- #134284 (Keep track of patterns that could have introduced a binding, but didn't)
- #134337 (reject unsound toggling of RISCV target features)
- #134371 (Check for array lengths that aren't actually `usize`)
- #134385 (tests/ui/asm: Remove uses of rustc_attrs, lang_items, and decl_macro features by using minicore)
- #134386 (Some trait method vs impl method signature difference diagnostic cleanups)
r? `@ghost`
`@rustbot` modify labels: rollup
Some trait method vs impl method signature difference diagnostic cleanups
Just some things I noticed while debugging a weird diagnostic in https://github.com/rust-lang/rust/pull/134353
best reviewed commit by commit
Check for array lengths that aren't actually `usize`
I wish typeck wouldn't give us `ty::Array`s that have this problem in the first place, but we can check for it.
Fixes#134352
cc ``@matthiaskrgr``
reject unsound toggling of RISCV target features
~~Stacked on top of https://github.com/rust-lang/rust/pull/133417, only the last commit is new.~~
Works towards https://github.com/rust-lang/rust/issues/132618 (but more [remains to be done](https://github.com/rust-lang/rust/pull/134337#issuecomment-2544228958))
Part of https://github.com/rust-lang/rust/issues/116344
Cc ``@beetrees`` I hope I got everything. I didn't do anything about "The f and zfinx features are incompatible" and that's not an ABI thing (right?) and I am not sure how to handle it with these ABI checks.
r? ``@workingjubilee``
Ideally we'd also reject target specs that disable the `f` feature but set an ABI that requires `f`... but I don't want to duplicate this logic. I have some ideas for how maybe the entire float ABI check logic should be different, now that we have some examples of what these ABI checks look like, but that will be a future PR.
Keep track of patterns that could have introduced a binding, but didn't
When we recover from a pattern parse error, or a pattern uses `..`, we keep track of that and affect resolution error for missing bindings that could have been provided by that pattern. We differentiate between `..` and parse recovery. We silence resolution errors likely caused by the pattern parse error.
```
error[E0425]: cannot find value `title` in this scope
--> $DIR/struct-pattern-with-missing-fields-resolve-error.rs:18:30
|
LL | if let Website { url, .. } = website {
| ------------------- this pattern doesn't include `title`, which is available in `Website`
LL | println!("[{}]({})", title, url);
| ^^^^^ not found in this scope
```
Fix#74863.
rustdoc-search: handle `impl Into<X>` better
This PR fixes two bugs I ran into while searching the compiler docs:
- It omitted an `impl Trait` entry in the type signature field, producing `TyCtxt, , Symbol -> bool`
- It didn't let me search for `TyCtxt, DefId, Symbol -> bool` even though that's a perfectly good description of the function I was looking for (the function actually used `impl Into<DefId>`
r? ``@GuillaumeGomez`` cc ``@lolbinarycat``
Correctly handle comments in attributes in doctests source code
Fixes https://github.com/rust-lang/rust/issues/134221.
The problem was that attributes are "inlined" (backlines are stripped), then when there is an inline comment inside it, the attribute is never considered valid (since unclosed). Fix was to simply put back backlines in case it's a multiline attribute.
r? ``@notriddle``
rustc_mir_build: Clarify that 'mirrored' does not mean 'flipped' or 'reversed'
My intuition for 'mirrored' is that it means 'flipped' or 'reversed'. Clarify that that is not what is meant to 'mirror' the THIR from the HIR.
Rollup of 5 pull requests
Successful merges:
- #134314 (Make sure to use normalized ty for unevaluated const in default struct value)
- #134342 (crashes: more tests)
- #134357 (Fix `trimmed_def_paths` ICE in the function ptr comparison lint)
- #134369 (Update spelling of "referring")
- #134372 (Disable `tests/ui/associated-consts/issue-93775.rs` on windows msvc)
Failed merges:
- #134365 (Rename `rustc_mir_build::build` to `builder`)
r? `@ghost`
`@rustbot` modify labels: rollup
Update spelling of "referring"
I noticed that `referring` was spelled incorrectly in the output of `unexpected 'cfg' condition name` warnings; it looks like it was also incorrectly spelled in a doc comment. I've update both instances.
Fix `trimmed_def_paths` ICE in the function ptr comparison lint
This PR fixes an ICE with `trimmed_def_paths` ICE in the function ptr comparison lint, specifically when pretty-printing user types but then not using the resulting pretty-printing.
Fixes#134345
r? `@saethlin`
Make sure to use normalized ty for unevaluated const in default struct value
This cleans up the way that we construct the `mir::Const::Unevaluated` for default struct values. We were previously using `from_unevaluated`, which doesn't normalize the type, and is really only used for inline assembly. Other codepaths (such as `ExprKind::NamedConst`) use the type from the body.
Also, let's stop using `literal_operand`, which also is really not meant for calls other than for literal comparisons in pattern lowering.
Also move all of the tests to a separate subdirectory so they don't need to have the same prefix on all the test files.
Fixes#134298
r? estebank or reassign
Hir attributes
This PR needs some explanation, it's somewhat large.
- This is step one as described in https://github.com/rust-lang/compiler-team/issues/796. I've added a new `hir::Attribute` which is a lowered version of `ast::Attribute`. Right now, this has few concrete effects, however every place that after this PR parses a `hir::Attribute` should later get a pre-parsed attribute as described in https://github.com/rust-lang/compiler-team/issues/796 and transitively https://github.com/rust-lang/rust/issues/131229.
- an extension trait `AttributeExt` is added, which is implemented for both `ast::Attribute` and `hir::Atribute`. This makes `hir::Attributes` mostly compatible with code that used to parse `ast::Attribute`. All its methods are also added as inherent methods to avoid having to import the trait everywhere in the compiler.
- Incremental can not not hash `ast::Attribute` at all.
Pass `TyCtxt` to early diagostics decoration
This PR pass a `TyCtxt` to the early diagnostics decoration code so that diagnostics code that take advantage of (a very limited but still useful) `TyCtxt` in their note, help, suggestions, ...
This is particulary useful for #133221 which wants to get the crate name of a `DefId`, which is possible with `tcx.crate_name(...)`.
I highly recommend reviewing this PR commit by commit.
r? `@jieyouxu`
bootstrap: make ./x test error-index work
This makes it more likely that someone who sees an error index CI failure will be able to figure out how to reproduce that locally. Note that bootstrap already prints "Testing stage2 error-index", which is misleading since the test is actually called error_index_generator.
