Introduce macro sub-namespaces and `macro_use` prelude
This PR implements two mechanisms needed for correct macro name resolution: macro sub-namespace and `macro_use` prelude.
- [macro sub-namespaces][subns-ref]
Macros have two sub-namespaces: one for function-like macro and the other for those in attributes (including custom derive macros). When we're resolving a macro name for function-like macro, we should ignore non-function-like macros, and vice versa.
This helps resolve single-segment macro names because we can (and should, as rustc does) fallback to names in preludes when the name in the current module scope is in different sub-namespace.
- [`macro_use` prelude][prelude-ref]
`#[macro_use]`'d extern crate declarations (including the standard library) bring their macros into scope, but they should not be prioritized over local macros (those defined in place and those explicitly imported).
We have been bringing them into legacy (textual) macro scope, which has the highest precedence in name resolution. This PR introduces the `macro_use` prelude in crate-level `DefMap`s, whose precedence is lower than local macros but higher than the standard library prelude.
The first 3 commits are drive-by fixes/refactors.
Fixes#8828 (prelude)
Fixes#12505 (prelude)
Fixes#12734 (prelude)
Fixes#13683 (prelude)
Fixes#13821 (prelude)
Fixes#13974 (prelude)
Fixes#14254 (namespace)
[subns-ref]: https://doc.rust-lang.org/reference/names/namespaces.html#sub-namespaces
[prelude-ref]: https://doc.rust-lang.org/reference/names/preludes.html#macro_use-prelude
Register obligations during path inference
Fixes#14635
When we infer path expressions that resolve to some generic item, we need to consider their generic bounds. For example, when we resolve a path `Into::into` to `fn into<?0, ?1>` (note that `?0` is the self type of trait ref), we should register an obligation `?0: Into<?1>` or else their relationship would be lost.
Relevant part in rustc is [`add_required_obligations_with_code()`] that's called in [`instantiate_value_path()`].
[`instantiate_value_path()`]: 3462f79e94/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L1052)
[`add_required_obligations_with_code()`]: 3462f79e94/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs (L1411)
Normalize associated types in paths in expressions
Part of #14393
When we resolve paths in expressions (either path expressions or paths in struct expressions), there's a need of projection normalization, which `TyLoweringContext` cannot do on its own. We've been properly applying normalization for paths in struct expressions without type anchor, but not for others:
```rust
enum E {
S { v: i32 }
Empty,
}
impl Foo for Bar {
type Assoc = E;
fn foo() {
let _ = Self::Assoc::S { v: 42 }; // path in struct expr without type anchor; we already support this
let _ = <Self>::Assoc::S { v: 42 }; // path in struct expr with type anchor; resolves with this PR
let _ = Self::Assoc::Empty; // path expr; resolves with this PR
}
}
```
With this PR we correctly resolve the whole path, but we need some more tweaks in HIR and/or IDE layers to properly resolve a qualifier (prefix) of such paths and provide IDE features that are pointed out in #14393 to be currently broken.
MIR episode 3
This PR adds lowering for try operator and overloaded dereference, and adds evaluating support for function pointers and trait objects. It also adds a flag to `analysis-stats` to show percentage of functions that it fails to emit mir for them, which is currently `20%` (which is somehow lying, as most of the supported `80%` are tests). The most offenders are closure (1975 items) and overloaded index (415 items). I will try to add overloaded index before monday to have it in this PR, and tackle the closure in the next episode.
