Because:
- `diagnostic_builder.rs` is small (282 lines),
- `Diagnostic` and `DiagnosticBuilder` are closely related types, and
- there's already an `impl DiagnosticBuilder` block in `diagnostic.rs`.

At the same time, reorder a few of things already in `diagnostic.rs`,
e.g. move `struct Diagnostic` just before `impl Diagnostic`.

This commit only moves code around. There are no functional changes.

If we abort immediately after complaining about the obsolete `impl Trait
for ..` syntax, then we avoid reaching HIR lowering. This means we can
use `TyKind::Dummy` instead of `TyKind::Err`.

… the interpreter

To test behaviour that depends on the extern options of intermediate
crates, compiletest auxiliaries must have their own auxiliaries.

Auxiliary compilation previously did not trigger compilation of any
auxiliaries in the auxiliary's headers. In addition, those auxiliaries
would need to be in an `auxiliary/auxiliary` directory, which is
unnecessary and makes some crate graphs harder to write tests for,
such as when A depends on B and C, and B depends on C.

For a test `tests/ui/$path/root.rs`, with the following crate graph:

```
root
|-- grandparent
`-- parent
    `-- grandparent
```

then the intermediate outputs from compiletest will be:

```
build/$target/test/ui/$path/
|-- auxiliary
|   |-- libgrandparent.dylib
|   |-- libparent.dylib
|   |-- grandparent
|   |   |-- grandparent.err
|   |   `-- grandparent.out
|   `-- parent
|       |-- parent.err
|       `-- parent.out
|-- libroot.rmeta
|-- root.err
`-- root.out
```

Signed-off-by: David Wood <david@davidtw.co>

…unds, r=fmease

Support async trait bounds in macros

r? fmease

This is similar to your work on const trait bounds. This theoretically regresses `impl async $ident:ident` in macros, but I doubt this is occurring in practice.

…=matthewjasper

match lowering: test one or pattern at a time

This is a bit more opinionated than the previous PRs. On the face of it this is less efficient and more complex than before, but I personally found the loop that digs into `leaf_candidates` on each iteration very confusing. Instead this does "generate code for this or-pattern" then "generate further code for each branch if needed" in two steps.

Incidentally this way we don't _require_ or patterns to be sorted at the end. It's still an important optimization but I find it clearer to not rely on it for correctness.

r? `@matthewjasper`

…nur-ozkan

bootstrap: apply most of clippy's suggestions

…li-obk

compiletest: support auxiliaries with auxiliaries

To test behaviour that depends on the extern options of intermediate crates, compiletest auxiliaries must have their own auxiliaries.

Auxiliary compilation previously did not trigger compilation of any auxiliaries in the auxiliary's headers. In addition, those auxiliaries would need to be in an `auxiliary/auxiliary` directory, which is unnecessary and makes some crate graphs harder to write tests for, such as when A depends on B and C, and B depends on C.

For a test `tests/ui/$path/root.rs`, with the following crate graph:

```
root
|-- grandparent
`-- parent
    `-- grandparent
```

then the intermediate outputs from compiletest will be:

```
build/$target/test/ui/$path/
|-- auxiliary
|   |-- libgrandparent.dylib
|   |-- libparent.dylib
|   |-- grandparent
|   |   |-- grandparent.err
|   |   `-- grandparent.out
|   `-- parent
|       |-- parent.err
|       `-- parent.out
|-- libroot.rmeta
|-- root.err
`-- root.out
```

…r-errors

miscellaneous type system improvements

see review comments for rationale

r? `@compiler-errors`

…rs, r=compiler-errors

Remove `diagnostic_builder.rs`

rust-lang#120576 moved a big chunk of `DiagnosticBuilder`'s functionality out of `diagnostic_builder.rs` into `diagnostic.rs`, which left `DiagnosticBuilder` spread across the two files.

This PR fixes that messiness by merging what remains of `diagnostic_builder.rs` into `diagnostic.rs`.

This is part of rust-lang/compiler-team#722.

r? `@davidtwco`

…ranteed, r=oli-obk

Remove an `unchecked_error_guaranteed` call.

If we abort immediately after complaining about the obsolete `impl Trait for ..` syntax, then we avoid reaching HIR lowering. This means we can use `TyKind::Dummy` instead of `TyKind::Err`.

r? `@oli-obk`

…r=oli-obk

make it possible for outside crates to inspect a mir::ConstValue with the interpreter

For MiniRust we need to convert MIR constant values into MiniRust constant values. However, it's not currently possible to get nice high-level access to the inerts of a `ConstValue`: we can access the raw contents (the allocation / `ScalarInt`), but if it is e.g. of enum type and we want to determine which variant is encoded, we are stuck. There's only `try_destructure_mir_constant_for_user_output` which is meant for diagnostics, so it doesn't fit.

The interpreter has all the APIs to traverse such a value, so we just need a way to get such a ConstValue into an interpreter instance. This adds the public functions necessary to make that happen.