Properly infer types with type casts
This PR reenables `Expectation::Castable` (previous attempt at #14104, reverted by #14120) and implements type cast checks, which enable us to infer a bit more.
Castable expectations are relatively weak -- they only influence the inference if we cannot infer the types by other means. Therefore, we need to defer possible type unification with the casted type until we type check all expressions of the body. This PR adds a struct and slots in `InferenceContext` for the deferred cast checks (c.f. [`CastCheck`] in `rustc_hir_typeck`).
I only implemented the bits that affect the inference result. It should be possible to return type adjustments for well-formed casts and report diagnostics for invalid casts, but I'm leaving them for future work for now.
Fixes#11571Fixes#15246
[`CastCheck`]: da1d099f91/compiler/rustc_hir_typeck/src/cast.rs (L55)
Normalize expected ty in call arguments
fix#15321
I'm not sure if we should do this, or add a normalize in the beginning of `infer_expr_inner`, or somewhere else. r? `@lowr`
Support GATs in bounds for associated types
Chalk has a dedicated IR for bounds on trait associated type: `rust_ir::InlineBound`. We have been failing to convert GATs inside those bounds from our IR to chalk IR. This PR provides an easy fix for it: properly take GATs into account during the conversion.
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
