Related: #66426
This commit adds handling for opaque types during inference variable
fallback. Type variables generated from the instantiatino of opaque
types now fallback to the opque type itself.
Normally, the type variable for an instantiated opaque type is either
unified with the concrete type, or with the opaque type itself (e.g when
a function returns an opaque type by calling another function).
However, it's possible for the type variable to be left completely
unconstrained. This can occur in code like this:
```rust
pub type Foo = impl Copy;
fn produce() -> Option<Foo> {
None
}
```
Here, we'll instantatiate the `Foo` in `Option<Foo>` to a fresh type
variable, but we will never unify it with anything due to the fact
that we return a `None`.
This results in the error message:
`type annotations needed: cannot resolve `_: std::marker::Copy``
pointing at `pub type Foo = impl Copy`.
This message is not only confusing, it's incorrect. When an opaque type
inference variable is completely unconstrained, we can always fall back
to using the opaque type itself. This effectively turns that particular
use of the opaque type into a non-defining use, even if it appears in a
defining scope.
Make a test compatible across python versions.
Progress on #65063
This PR allows this test to work on both python2 and python3, ~~and it also allows `./x.py test` to fully complete on my system without python2 installed at all.~~
rustdoc: Stabilize `edition` annotation.
The rustdoc `edition` annotation is currently ignored on stable. This means that the tests will be ignored, unless there is a `rust` annotation, then it will use the global edition. I suspect this was just an oversight during the edition stabilization, but I don't know. Example:
```rust
/// ```edition2018
/// // This code block was ignored on stable.
/// ```
/// ```rust,edition2018
/// // This code block would use whatever edition is passed on the command line.
/// ```
```
AFAIK, it is not possible to write a test that verifies stable behavior, as all tests appear to set RUSTC_BOOTSTRAP which forces all tests to run as "nightly", even on a stable release.
Closes#65980
Suggest borrowing when it would satisfy an unmet trait bound
When there are multiple implementors for the same trait that is present
in an unmet binding, modify the E0277 error to refer to the parent
obligation and verify whether borrowing the argument being passed in
would satisfy the unmet bound. If it would, suggest it.
Fix#56368.
Make dataflow-based const qualification the canonical one
For over a month, dataflow-based const qualification has been running in parallel with `qualify_consts` to check the bodies of `const` and `static`s. This PR removes the old qualification pass completely in favor of the dataflow-based one.
**edit:**
This PR also stops checking `QUALIF_ERROR_BIT` during promotion. This check appears to no longer serve a purpose now that the CTFE engine is more robust.
As a side-effect, this resolves#66167.
r? @eddyb
