[7/n] rustc: desugar UFCS in HIR and don't use DefMap for associated resolutions.
_This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37412) | [next](https://github.com/rust-lang/rust/pull/37688)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._
<hr>
Previously, a path like `T::Assoc::method`, while equivalent to `<<T>::Assoc>::method`, wasn't desugared in any way at the HIR level and everything inspecting it had to either deal with knowing only `T` (before typeck) or knowing only the definition of `method` (after typeck).
Such a path also had only one `NodeId` and associated resolution during typeck modified `DefMap`, in a way that would be hard for incremental recompilation to track, and inconvenient for partial type conversions from HIR to `Ty`, which are required to break faux-cycles in on-demand type collection.
The desugarings performed by this PR are as follows:
* `use a::{b,c};` is flattened to `use a as _; use a::b; use a::c;`
* as resolution is complete, `use a as _;` doesn't do anything, except get checked for stability
* `Vec::new` (an expression) becomes `Vec<..>::new<..>`, to distinguish it from `<Vec>::new<..>`
* the "infer all parameters" `<..>` form is internal and not even pretty-printed
* used when there are no type parameters at all, in an expression or pattern path segment
* `T::A::B` becomes `<<T>::A>::B` in a type, and `<<T<..>>::A<..>>::B<..>` in an expression/pattern
* one additional `hir::Ty` node is created for each prefix, starting with the fully-resolved type (`T`) and extending it with each segment (e.g. `<T>::A`)
* fully-resolved paths contain their `Def` in HIR, getting rid of the `DefMap` and absolving incremental recompilation of needing to manually look up nodes to handle that side information
Not keeping the `DefMap` around meant that associated resolutions had to be stored somewhere else:
* expressions and patterns use a new `NodeId -> Def` map in `ty::Tables`
* compatible with the future per-body (constant / `fn` / closure) `Tables`
* types are accessible via `Ty` and the usual per-item generics / predicates / type
* `rustdoc` and `save-analysis` are the only situations which insist on mapping syntactical types to semantical ones, or at least understand the resolution of associated types, therefore the type conversion cache, i.e. a `NodeId -> Ty` map, is exposed by typeck for this purpose
* stability had to be split into a pass that runs on HIR and checks the results of name resolution, and impromptu checks triggered by `typeck` for associated paths, methods, fields, etc.
* privacy using semantic types results in accurate reachability for `impl Trait`, which fixes#35870, and thorough introspection of associated types, which may allow relaxing private-in-public checking on bounds, while keeping the intended ban on projections with private type parameters
cc @petrochenkov
Use displacement instead of initial bucket in HashMap code
Use displacement instead of initial bucket in HashMap code. It makes the code a bit cleaner and also saves a few instructions (handy since it'll be using some to do some sort of adaptive behavior soon).
Fix error explanation formatting
Errors E0101, E0458, E0535 and E0537 have incorrectly formatted bulleted lists in their explanations. As the result, they are not rendered as lists in the documentation.
The fix applies the correct formatting to those lists.
don't double-apply variant padding to const enums
`build_const_struct` already returns the struct with padding - don't double-apply it in the `General` case.
This should hopefully be the last time we have this sort of bug.
Fixes#38002.
Beta-nominating because regression.
r? @eddyb
Add rustbuild command `bench`
Add command bench to rustbuild, so that `./x.py bench <path>` can compile and run benchmarks.
`./x.py bench --stage 1 src/libcollections` and `./x.py bench --stage 1 src/libstd` should both compile well. Just `./x.py bench` runs all benchmarks for the libstd crates.
Fixes#37897
Follow our own recommendations in the examples
Remove exclamation marks from the the example error descriptions:
> The description [...] should not contain newlines or sentence-ending punctuation
Define `bound` argument in std::sync::mpsc::sync_channel in the documentation
The `bound` argument in `std::sync::mpsc::sync:channel(bound: usize)` was not defined in the documentation.
Fix two small issues in iterator docs
- `collect()` is a regular method, not an adaptor (does not return an Iterator). I just randomly picked `filter` as a third common adaptor to mention instead.
- Fix example in `Map`'s docs so that it uses the DoubleEndedIterator implementation
Use literal 5 instead of five in book section 4.1
The other two code snippets in this sentence are valid code, so it makes more sense to use the literal `5` rather than the invalid symbol `five`.
