by factoring out `fn ctor_fields_span` into a separate function

…, r=nikomatsakis

Use TypeRelating for instantiating query responses

`eq` can add constraints to `RegionConstraintData`, which isn't allowed during borrow checking outside of a `CustomTypeOp`. Use `TypeRelating` instead to always push constraints to the obligations list.

closes rust-lang#69490

Improve expression & attribute parsing

This PR includes misc improvements to expression and attribute parsing.

1. Some code simplifications
2. Better recovery for various block forms, e.g. `loop statements }` (missing `{` after `loop`). (See e.g., `block-no-opening-brace.rs` among others for examples.)
3. Added recovery for e.g., `unsafe $b` where `$b` refers to a `block` macro fragment. (See `bad-interpolated-block.rs` for examples.)
4. ^--- These are done so that code sharing in block parsing is increased.
5. Added recovery for e.g., `'label: loop { ... }` (See `labeled-no-colon-expr.rs`.)
6. Added recovery for e.g., `&'lifetime expr` (See `regions-out-of-scope-slice.rs`.)
7. Added recovery for e.g., `fn foo() = expr;` (See `fn-body-eq-expr-semi.rs`.)
8. Simplified attribute parsing code & slightly improved diagnostics.
9. Added recovery for e.g., `Box<('a) + Trait>`.
10. Added recovery for e.g, `if true #[attr] {} else #[attr] {} else #[attr] if true {}`.

r? @estebank

resolve: Print import chains on privacy errors

A part of rust-lang#67951 that doesn't require hacks.
r? @estebank

…andry

Use smaller discriminants for generators

Closes rust-lang#69815

I'm not yet sure about the runtime performance impact of this, so I'll try running this on some benchmarks (if I can find any). (Update: No impact on the benchmarks I've measured on)

* [x] Add test with a generator that has exactly 256 total states
* [x] Add test with a generator that has more than 256 states so that it needs to use a u16 discriminant
* [x] Add tests for the size of `Option<[generator]>`
* [x] Add tests for the `discriminant_value` intrinsic in all cases

…ov,eddyb

Expansion-driven outline module parsing

After this PR, the parser will not do any conditional compilation or loading of external module files when `mod foo;` is encountered. Instead, the parser only leaves `mod foo;` in place in the AST, with no items filled in. Expansion later kicks in and will load the actual files and do the parsing. This entails that the following is now valid:

```rust
#[cfg(FALSE)]
mod foo {
    mod bar {
        mod baz; // `foo/bar/baz.rs` doesn't exist, but no error!
    }
}
```

Fixes rust-lang#64197.

r? @petrochenkov

fix rust-lang#62456

closes rust-lang#62456