This seemed like a good way to kick the tires on the
elided-lifetimes-in-paths lint (#52069)—seems to work! This was also
pretty tedious—it sure would be nice if `cargo fix` worked on this
codebase (#53896)!
rustc: future-proof error reporting for polymorphic constants in types.
Currently, we have 3 categories of positions where a constant can be used (`const` and associated `const` can be considered "aliases" for an expression):
* runtime - if the function is polymorphic, we could even just warn and emit a panic
* `static` - always monomorphic, so we can error at definition site
* type-system - **must** *enforce* evaluation success where it was written
That last one is the tricky one, because we can't easily turn *the presence* a type with an erroring const into a runtime panic, and we'd have to do post-monomorphization errors (which we'd rather avoid).
<hr/>
The solution we came up with, as part of the plans for const generics, is to require successful evaluation wherever a constant shows up in a type (currently in array lengths, and values for const parameters in the future), *through* the WF system, which means that in certain situations (e.g. function signatures) we can assume evaluation *will* succeed, and require it of users (e.g. callers) instead (we've been doing this for lifetime bounds, for a long time now, and it's pretty ergonomic).
So once we do sth about #43408, this example *should* work, by propagating the responsability, to callers of `foo::<X>`, of proving `std::mem::size_of::<X>()` succeeds (and those callers can do the same).
```rust
pub fn foo<T>(_: [u8; std::mem::size_of::<T>()]) {}
```
But this one *shouldn't*, as there is nothing in the signature/bounds to indicate it:
```rust
pub fn bar<T>() {
let _: [u8; std::mem::size_of::<T>()];
}
```
<hr/>
I've come across some bit of code in the compiler that ignores const-evaluation errors *even when* they come from a constant in a type, and I've added an ICE *only when* there are no other reported errors (e.g. it's fine to ignore evaluation errors if the constant doesn't even type-check).
r? @nikomatsakis cc @oli-obk @RalfJung @Centril
Implement Unsized Rvalues
This PR is the first step to implement RFC1909: unsized rvalues (#48055).
## Implemented
- `Sized` is removed for arguments and local bindings. (under `#![feature(unsized_locals)]`)
- Unsized locations are allowed in MIR
- Unsized places and operands are correctly translated at codegen
## Not implemented in this PR
- Additional `Sized` checks:
- tuple struct constructor (accidentally compiles now)
- closure arguments at closure generation (accidentally compiles now)
- upvars (ICEs now)
- Generating vtable for `fn method(self)` (ICEs now)
- VLAs: `[e; n]` where `n` isn't const
- Reduce unnecessary allocations
## Current status
- [x] Fix `__rust_probestack` (rust-lang-nursery/compiler-builtins#244)
- [x] Get the fix merged
- [x] `#![feature(unsized_locals)]`
- [x] Give it a tracking issue number
- [x] Lift sized checks in typeck and MIR-borrowck
- [ ] <del>Forbid `A(unsized-expr)`</del> will be another PR
- [x] Minimum working codegen
- [x] Add more examples and fill in unimplemented codegen paths
- [ ] <del>Loosen object-safety rules (will be another PR)</del>
- [ ] <del>Implement `Box<FnOnce>` (will be another PR)</del>
- [ ] <del>Reduce temporaries (will be another PR)</del>
add suggestion applicabilities to librustc and libsyntax
A down payment on #50723. Interested in feedback on whether my `MaybeIncorrect` vs. `MachineApplicable` judgement calls are well-calibrated (and that we have a consensus on what this means).
r? @Manishearth
cc @killercup @estebank
Consider this a down payment on #50723. To recap, an `Applicability`
enum was recently (#50204) added, to convey to Rustfix and other tools
whether we think it's OK for them to blindly apply the suggestion, or
whether to prompt a human for guidance (because the suggestion might
contain placeholders that we can't infer, or because we think it has a
sufficiently high probability of being wrong even though it's—
presumably—right often enough to be worth emitting in the first place).
When a suggestion is marked as `MaybeIncorrect`, we try to use comments
to indicate precisely why (although there are a few places where we just
say `// speculative` because the present author's subjective judgement
balked at the idea that the suggestion has no false positives).
The `run-rustfix` directive is opporunistically set on some relevant UI
tests (and a couple tests that were in the `test/ui/suggestions`
directory, even if the suggestions didn't originate in librustc or
libsyntax). This is less trivial than it sounds, because a surprising
number of test files aren't equipped to be tested as fixed even when
they contain successfully fixable errors, because, e.g., there are more,
not-directly-related errors after fixing. Some test files need an
attribute or underscore to avoid unused warnings tripping up the "fixed
code is still producing diagnostics" check despite the fixes being
correct; this is an interesting contrast-to/inconsistency-with the
behavior of UI tests (which secretly pass `-A unused`), a behavior which
we probably ought to resolve one way or the other (filed issue #50926).
A few suggestion labels are reworded (e.g., to avoid phrasing it as a
question, which which is discouraged by the style guidelines listed in
`.span_suggestion`'s doc-comment).
