Remove last uses of gensyms
Underscore bindings now use unique `SyntaxContext`s to avoid collisions. This was the last use of gensyms in the compiler, so this PR also removes them.
closes#49300
cc #60869
r? @petrochenkov
Instead just use `pprust::path_to_string(..)` where needed.
This has two benefits:
a) The AST definition is now independent of printing it.
(Therefore we get closer to extracting a data-crate.)
b) Debugging should be easier as program flow is clearer.
Use hygiene for AST passes
AST passes are now able to have resolve consider their expansions as if they were opaque macros defined either in some module in the current crate, or a fake empty module with `#[no_implicit_prelude]`.
* Add an ExpnKind for AST passes.
* Remove gensyms in AST passes.
* Remove gensyms in`#[test]`, `#[bench]` and `#[test_case]`.
* Allow opaque macros to define tests.
* Move tests for unit tests to their own directory.
* Remove `Ident::{gensym, is_gensymed}` - `Ident::gensym_if_underscore` still exists.
cc #60869, #61019
r? @petrochenkov
Move some code into `build_reduced_graph.rs` to keep `BuildReducedGraphVisitor` it private
Also move the def collector call to `build_reduced_graph.rs`, it belongs there.
Traces already contain module info without that.
It's easy to forget to call `finalize_*` on a module.
In particular, macros enum and trait modules weren't finalized.
By happy accident macros weren't placed into those modules until now.
Move methods logically belonging to build-reduced-graph into `impl BuildReducedGraphVisitor` and `build_reduced_graph.rs`
Move types mostly specific to late resolution closer to the late resolution visitor
Move `Resolver` fields specific to late resolution to the new visitor.
The `current_module` field from `Resolver` is replaced with two `current_module`s in `LateResolutionVisitor` and `BuildReducedGraphVisitor`.
Outside of those visitors `current_module` is replaced by passing `parent_scope` to more functions and using the parent module from it.
Visibility resolution no longer have access to later resolution methods and has to use early resolution, so its diagnostics in case of errors regress slightly.
