This commit introduces `QPath::LangItem` to the HIR and uses it in AST
lowering instead of constructing a `hir::Path` from a slice of symbols.

This might be better for performance, but is also much cleaner as the
previous approach is fragile. In addition, it resolves a bug (#61019)
where an extern crate imported as "std" would result in the paths
created during AST lowering being resolved incorrectly (or not at all).
Co-authored-by: NMatthew Jasper <mjjasper1@gmail.com>
Signed-off-by: NDavid Wood <david@davidtw.co>

- Move the type parameter from `encode` and `decode` methods to
  the trait.
- Remove `UseSpecialized(En|De)codable` traits.
- Remove blanket impls for references.
- Add `RefDecodable` trait to allow deserializing to arena-allocated
  references safely.
- Remove ability to (de)serialize HIR.
- Create proc-macros `(Ty)?(En|De)codable` to help implement these new
  traits.

This commit restricts the substitution polymorphization added in #75255
to only apply to the tupled upvar substitution, rather than all
substitutions, fixing a bunch of regressions when polymorphization is
enabled.
Signed-off-by: NDavid Wood <david@davidtw.co>

Renamed remaining references to "undef" to "uninit" when referring to Miri.

Impacted directories are:

- src/librustc_codegen_llvm/consts.rs
- src/librustc_middle/mir/interpret/
- src/librustc_middle/ty/print/pretty.rs
- src/librustc_mir/
- src/tools/clippy/clippy_lints/src/consts.rs

Upon building Miri based on the new changes it was verified that no changes needed to be made with the Miri project.

Related issue #71193

By moving `{known,used}_attrs` from `SessionGlobals` to `Session`. This
means they are accessed via the `Session`, rather than via TLS. A few
`Attr` methods and `librustc_ast` functions are now methods of
`Session`.

All of this required passing a `Session` to lots of functions that didn't
already have one. Some of these functions also had arguments removed, because
those arguments could be accessed directly via the `Session` argument.

`contains_feature_attr()` was dead, and is removed.

Some functions were moved from `librustc_ast` elsewhere because they now need
to access `Session`, which isn't available in that crate.
- `entry_point_type()` --> `librustc_builtin_macros`
- `global_allocator_spans()` --> `librustc_metadata`
- `is_proc_macro_attr()` --> `Session`

This commit extends previous polymorphization of substs to polymorphize
`FnDef`.
Signed-off-by: NDavid Wood <david@davidtw.co>

This commit avoids unnecessary calls to `mk_closure` and `mk_generator`
by checking if the polymorphized substs match the original substs.
Signed-off-by: NDavid Wood <david@davidtw.co>

This commit adds a `MAY_POLYMORPHIZE` which checks for closures and
generators so that polymorphization of substs does not need to traverse
every substs.
Signed-off-by: NDavid Wood <david@davidtw.co>

By always polymorphizing substitutions, functions which take closures as
arguments (e.g. `impl Fn()`) can have fewer mono items when some of the
argument closures can be polymorphized.
Signed-off-by: NDavid Wood <david@davidtw.co>

This commit modifies how instances are polymorphized so that closures
and generators have any closures or generators captured within their
upvars also polymorphized - this avoids symbol clashes with the new
symbol mangling scheme.
Signed-off-by: NDavid Wood <david@davidtw.co>

The prior PR corrected for errors encountered when trying to generate
the coverage map on source code inlined from external crates (including
macros and generics) by avoiding adding external DefIds to the coverage
map.

This made it possible to generate a coverage report including external
crates, but the external crate coverage was incomplete (did not include
coverage for the DefIds that were eliminated.

The root issue was that the coverage map was converting Span locations
to source file and locations, using the SourceMap for the current crate,
and this would not work for spans from external crates (compliled with a
different SourceMap).

The solution was to convert the Spans to filename and location during
MIR generation instead, so precompiled external crates would already
have the correct source code locations embedded in their MIR, when
imported into another crate.

This commit changes polymorphization to return a `FiniteBitSet<u32>`
rather than a `FiniteBitSet<u64>` because most functions do not use
anywhere near sixty-four generic parameters so keeping a `u64` around is
unnecessary in most cases.
Signed-off-by: NDavid Wood <david@davidtw.co>

They're only used in error messages printed if there's an internal
compiler error, and the cost of maintaining them is high enough to show
up in profiles.

We store an `ImplicitCtxt` pointer in a thread-local value (TLV). This allows
implicit access to a `GlobalCtxt` and some other things.

We also store a `GlobalCtxt` pointer in `GCX_PTR`. This is always the same
`GlobalCtxt` as the one within the `ImplicitCtxt` pointer in TLV. `GCX_PTR`
is only used in the parallel compiler's `handle_deadlock()` function.

This commit does the following.
- It removes `GCX_PTR`.
- It also adds `ImplicitCtxt::new()`, which constructs an `ImplicitCtxt` from a
  `GlobalCtxt`. `ImplicitCtxt::new()` + `tls::enter_context()` is now
  equivalent to the old `tls::enter_global()`.
- Makes `tls::get_tlv()` public for the parallel compiler, because it's
  now used in `handle_deadlock()`.