The ICE from rust-lang#84044 no longer occurs.

Add tests for three old ICEs

Closes rust-lang#84044
Closes rust-lang#91594
Closes rust-lang#89066

Make `NonNull::new` `const`

Tracking issue: rust-lang#93235

…1-dead

Add note suggesting that predicate may be satisfied, but is not `const`

Not sure if we should be printing this in addition to, or perhaps _instead_ of the help message:
```
help: the trait `~const Add` is not implemented for `NonConstAdd`
```

Also added `ParamEnv::is_const` and `PolyTraitPredicate::is_const_if_const` and, in a separate commit, used those in other places instead of `== hir::Constness::Const`, etc.

r? ```@fee1-dead```

…oli-obk

Do not register infer var for GAT projection in RPIT

Fixes rust-lang#93340
Fixes rust-lang#91603

r? ``@oli-obk``

…dtolnay

kmc-solid: Implement `net::FileDesc::duplicate`

This PR implements `std::sys::solid::net::FileDesc::duplicate`, which was accidentally left out when this target was added by rust-lang#86191.

Move unstable is_{arch}_feature_detected! macros to std::arch

These macros are unstable, except for `is_x86_feature_detected` which is still exported from the crate root for backwards-compatibility.

This should unblock the stabilization of `is_aarch64_feature_detected`.

r? ``@m-ou-se``

fix nit

…ulacrum

remove unused `jemallocator` crate

When it was noticed that the rustc binary wasn't actually using jemalloc via `#[global_allocator]` and that was removed, the dependency remained.

Tests pass locally with a `jemalloc = true` build, but I'll trigger a try build to ensure I haven't missed an edge-case somewhere.

r? ``@ghost`` until that completes