Prepare for lowering or-patterns
This is probably best reviewed commit-by-commit.
* Rustfmt `rustc_mir::build::{self, matches}`
* Remove `-Znll-dont-emit-read-for-match`
* Split `match_expr` into smaller functions
* Feature gate or-patterns in const contexts
cc https://github.com/rust-lang/rust/issues/54883
r? @Centril
Merge `TraitItem` & `ImplItem into `AssocItem`
In this PR we:
- Merge `{Trait,Impl}Item{Kind?}` into `AssocItem{Kind?}` as discussed in https://github.com/rust-lang/rust/issues/65041#issuecomment-538105286.
- This is done by using the cover grammar of both forms.
- In particular, it requires that we syntactically allow (under `#[cfg(FALSE)]`):
- `default`ness on `trait` items,
- `impl` items without a body / definition (`const`, `type`, and `fn`),
- and associated `type`s in `impl`s with bounds, e.g., `type Foo: Ord;`.
- The syntactic restrictions are replaced by semantic ones in `ast_validation`.
- Move syntactic restrictions around C-variadic parameters from the parser into `ast_validation`:
- `fn`s in all contexts now syntactically allow `...`,
- `...` can occur anywhere in the list syntactically (`fn foo(..., x: usize) {}`),
- and `...` can be the sole parameter (`fn foo(...) {}`.
r? @petrochenkov
Enable `if` and `match` in constants behind a feature flag
This PR is an initial implementation of #49146. It introduces a `const_if_match` feature flag and does the following if it is enabled:
- Allows `Downcast` projections, `SwitchInt` terminators and `FakeRead`s for matched places through the MIR const-checker.
- Allows `if` and `match` expressions through the HIR const-checker.
- Stops converting `&&` to `&` and `||` to `|` in `const` and `static` items.
As a result, the following operations are now allowed in a const context behind the feature flag:
- `if` and `match`
- short circuiting logic operators (`&&` and `||`)
- the `assert` and `debug_assert` macros (if the `const_panic` feature flag is also enabled)
However, the following operations remain forbidden:
- `while`, `loop` and `for` (see #52000)
- the `?` operator (calls `From::from` on its error variant)
- the `assert_eq` and `assert_ne` macros, along with their `debug` variants (calls `fmt::Debug`)
This PR is possible now that we use dataflow for const qualification (see #64470 and #66385).
r? @oli-obk
cc @rust-lang/wg-const-eval @eddyb
*Syntactically* permit visibilities on trait items & enum variants
Fixes#65041
Suppose we have `$vis trait_item` or `$vis enum_variant` and `$vis` is a `:vis` macro fragment. Before this PR, this would fail to parse. This is now instead allowed as per language team consensus in https://github.com/rust-lang/rust/issues/65041#issuecomment-538105286. (See added tests for elaboration.)
Moreover, we now also permit visibility modifiers on trait items & enum variants *syntactically* but reject them with semantic checks (in `ast_validation`):
```rust
#[cfg(FALSE)]
trait Foo { pub fn bar(); } // OK
#[cfg(FALSE)]
enum E { pub U } // OK
```
