[MIR] Add explicit SetDiscriminant StatementKind for deaggregating enums
cc #35186
To deaggregate enums, we need to be able to explicitly set the discriminant. This PR implements a new StatementKind that does that.
I think some of the places that have `panics!` now could maybe do something smarter.
Implement `impl Trait` in return type position by anonymization.
This is the first step towards implementing `impl Trait` (cc #34511).
`impl Trait` types are only allowed in function and inherent method return types, and capture all named lifetime and type parameters, being invariant over them.
No lifetimes that are not explicitly named lifetime parameters are allowed to escape from the function body.
The exposed traits are only those listed explicitly, i.e. `Foo` and `Clone` in `impl Foo + Clone`, with the exception of "auto traits" (like `Send` or `Sync`) which "leak" the actual contents.
The implementation strategy is anonymization, i.e.:
```rust
fn foo<T>(xs: Vec<T>) -> impl Iterator<Item=impl FnOnce() -> T> {
xs.into_iter().map(|x| || x)
}
// is represented as:
type A</*invariant over*/ T> where A<T>: Iterator<Item=B<T>>;
type B</*invariant over*/ T> where B<T>: FnOnce() -> T;
fn foo<T>(xs: Vec<T>) -> A<T> {
xs.into_iter().map(|x| || x): $0 where $0: Iterator<Item=$1>, $1: FnOnce() -> T
}
```
`$0` and `$1` are resolved (to `iter::Map<vec::Iter<T>, closure>` and the closure, respectively) and assigned to `A` and `B`, after checking the body of `foo`. `A` and `B` are *never* resolved for user-facing type equality (typeck), but always for the low-level representation and specialization (trans).
The "auto traits" exception is implemented by collecting bounds like `impl Trait: Send` that have failed for the obscure `impl Trait` type (i.e. `A` or `B` above), pretending they succeeded within the function and trying them again after type-checking the whole crate, by replacing `impl Trait` with the real type.
While passing around values which have explicit lifetime parameters (of the function with `-> impl Trait`) in their type *should* work, regionck appears to assign inference variables in *way* too many cases, and never properly resolving them to either explicit lifetime parameters, or `'static`.
We might not be able to handle lifetime parameters in `impl Trait` without changes to lifetime inference, but type parameters can have arbitrary lifetimes in them from the caller, so most type-generic usecases (or not generic at all) should not run into this problem.
cc @rust-lang/lang
E0072 update error format
Part of #35233Fixes#35506
r? @jonathandturner
The bonus for this issue currently seems to be impossible to do reliably, as the compiler seems to lack span information for item names alone, like `Foo` in `struct Foo { ... }`. It would be possible to hack something together by computing span offsets, but that seems like a solution that would be begging for trouble.
A proper solution to this would, of course, be to add span information to the right place (seems to be `rustc::hir::Item::name` but I may be wrong).
Update E0038 to the new error format
Part of #35233
Addresses #35500
"r? @jonathandturner
This doesn't compile yet, and I need help. In my naive solution, adding the span label makes our error message a mutable `errors::DiagnosticBuilder` pointer.
```bash
python src/bootstrap/bootstrap.py --step check-cfail E0038 --stage 1
```
```
Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Building stage0 test artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Building stage0 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
Compiling rustc v0.0.0 (file:///home/nash/code/rust/src/librustc)
src/librustc/traits/error_reporting.rs:735:9: 735:12 error: mismatched types [E0308]
src/librustc/traits/error_reporting.rs:735 err
^~~
src/librustc/traits/error_reporting.rs:735:9: 735:12 help: run `rustc --explain E0308` to see a detailed explanation
src/librustc/traits/error_reporting.rs:735:9: 735:12 note: expected type `core::option::Option<errors::DiagnosticBuilder<'tcx>>`
src/librustc/traits/error_reporting.rs:735:9: 735:12 note: found type `core::option::Option<&mut errors::DiagnosticBuilder<'_>>`
error: aborting due to previous error
error: Could not compile `rustc`.
To learn more, run the command again with --verbose.
command did not execute successfully: "/home/nash/code/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "-j" "4" "--target" "x86_64-unknown-linux-gnu" "--release" "--features" " jemalloc" "--manifest-path" "/home/nash/code/rust/src/rustc/Cargo.toml"
expected success, got: exit code: 101
```
