update error E0450 to new format
Fixes#35925 as part of #35233.
I've solve the bonus, and I wonder if any simpler way to do this. But may be possible simplify if let expressions?
r? @jonathandturner
Combine types and regions in Substs into one interleaved list.
Previously, `Substs` would contain types and regions, in two separate vectors, for example:
```rust
<X as Trait<'a, 'b, A, B>>::method::<'p, 'q, T, U>
/* corresponds to */
Substs { regions: ['a, 'b, 'p, 'q], types: [X, A, B, T, U] }
```
This PR continues the work started in #35605 by further removing the distinction.
A new abstraction over types and regions is introduced in the compiler, `Kind`.
Each `Kind` is a pointer (`&TyS` or `&Region`), with the lowest two bits used as a tag.
Two bits were used instead of just one (type = `0`, region = `1`) to allow adding more kinds.
`Substs` contain only a `Vec<Kind>`, with `Self` first, followed by regions and types (in the definition order):
```rust
Substs { params: [X, 'a, 'b, A, B, 'p, 'q, T, U] }
```
The resulting interleaved list has the property of being the concatenation of parameters for the (potentially) nested generic items it describes, and can be sliced back into those components:
```rust
params[0..5] = [X, 'a, 'b, A, B] // <X as Trait<'a, 'b, A, B>>
params[5..9] = ['p, 'q, T, U] // <_>::method::<'p, 'q, T, U>
```
r? @nikomatsakis
Improve error message when failing to parse a block
We want to catch this error:
```
if (foo)
bar;
```
as it's valid syntax in other languages, and say how to fix it.
Unfortunately it didn't care if the suggestion made sense and just
highlighted the unexpected token.
Now it attempts to parse a statement, and if it succeeds, it shows the
help message.
Fixes#35907
Remove the old AST-based backend from rustc_trans.
Starting with Rust 1.13, `--disable-orbit` , `-Z orbit=off` and `#[rustc_no_mir]` have been removed.
Only the new MIR backend is left in the compiler, and only early const_eval uses ASTs from other crates.
Filling drop (previously "zeroing drop"), `#[unsafe_no_drop_flag]` and associated unstable APIs are gone.
Implementing `Drop` doesn't add a flag anymore to the type, all of the dynamic drop is function local.
This is a [breaking-change], please use `Option::None` and/or `mem::forget` if you are unsure about your ability to prevent/control the drop of a value. In the future, `union` will be usable in some such cases.
**NOTE**: DO NOT MERGE before we get the new beta as the stage0, there's some cruft to remove.
All of this will massively simplify any efforts to implement (and as such it blocks) features such as `union`s, safe use of `#[packed]` or new type layout optimizations, not to mention many other experiments.
We want to catch this error:
```
if (foo)
bar;
```
as it's valid syntax in other languages, and say how to fix it.
Unfortunately it didn't care if the suggestion made sense and just
highlighted the unexpected token.
Now it attempts to parse a statement, and if it succeeds, it shows the
help message.
Fixes#35907
Carrier trait (third attempt)
This adds a `Carrier` trait to operate with `?`. The only public implementation is for `Result`, so effectively the trait does not exist, however, it ensures future compatibility for the `?` operator. This is not intended to be used, nor is it intended to be a long-term solution.
Although this exact PR has not been through Crater, I do not expect it to be a breaking change based on putting numerous similar PRs though Crater in the past.
cc:
* [? tracking issue](https://github.com/rust-lang/rust/issues/31436)
* [previous PR](https://github.com/rust-lang/rust/pull/35056)
* [RFC issue](https://github.com/rust-lang/rfcs/issues/1718) for discussion of long-term Carrier trait solutions.
r? @nikomatsakis
