Implement the alternative try block desugaring

[scottmcm]

As discussed in rust-lang/rfcs#3721 (comment), update the try in nightly to match the RFC as a way to experiment.

This addresses the following unresolved issue from #31436

Address issues with type inference (try { expr? }? currently requires an explicit type annotation somewhere).

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[scottmcm]

With the rustc_force_inline this is no longer different, as it's always MIR-inlined.

[scottmcm]

...or maybe not because rustc_force_inline doesn't work in CI #148915

[petrochenkov]

Does in work locally but not on CI? Very strange.
I'd expect this error to happen because residual_into_try_type is not marked as reachable from other crates for some reason (I thought lang items were always assumed to be reachable). What happens if it's publicly reexported?

[scottmcm]

Yeah, it all built just fine locally, though admittedly I didn't run all the tests so I might have missed something.

Making it exported-but-unstable is a good idea; I'll give that a shot and see! Thanks.

[rust-log-analyzer]

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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[RUSTC-TIMING] rustc_demangle test:false 0.697
error: `ops::try_trait::residual_into_try_type` could not be inlined into `Box::<T>::try_map` but is required to be inlined
   --> library/alloc/src/boxed.rs:462:21
    |
462 |                     f(value)?,
    |                     ^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
    |
    = note: could not be inlined due to: implementation limitation -- MIR unavailable

error: `ops::try_trait::residual_into_try_type` could not be inlined into `Box::<T>::try_map` but is required to be inlined
   --> library/alloc/src/boxed.rs:466:28
    |
466 |             try { Box::new(f(*this)?) }
    |                            ^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
    |
    = note: could not be inlined due to: implementation limitation -- MIR unavailable

error: `ops::try_trait::residual_into_try_type` could not be inlined into `Rc::<T>::try_map` but is required to be inlined
   --> library/alloc/src/rc.rs:728:27
    |
728 |             try { Rc::new(f(&*this)?) }
    |                           ^^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
    |
    = note: could not be inlined due to: implementation limitation -- MIR unavailable

error: `ops::try_trait::residual_into_try_type` could not be inlined into `UniqueRc::<T>::try_map` but is required to be inlined
    --> library/alloc/src/rc.rs:4172:33
     |
4172 |             try { UniqueRc::new(f(UniqueRc::unwrap(this))?) }
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
     |
     = note: could not be inlined due to: implementation limitation -- MIR unavailable

error: `ops::try_trait::residual_into_try_type` could not be inlined into `Arc::<T>::try_map` but is required to be inlined
   --> library/alloc/src/sync.rs:741:28
    |
741 |             try { Arc::new(f(&*this)?) }
    |                            ^^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
    |
    = note: could not be inlined due to: implementation limitation -- MIR unavailable

error: `ops::try_trait::residual_into_try_type` could not be inlined into `UniqueArc::<T>::try_map` but is required to be inlined
    --> library/alloc/src/sync.rs:4586:34
     |
4586 |             try { UniqueArc::new(f(UniqueArc::unwrap(this))?) }
     |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...`ops::try_trait::residual_into_try_type` called here
     |
     = note: could not be inlined due to: implementation limitation -- MIR unavailable

[RUSTC-TIMING] alloc test:false 5.510
error: could not compile `alloc` (lib) due to 6 previous errors

[petrochenkov]

error: ops::try_trait::residual_into_try_type could not be inlined into Box::<T>::try_map but is required to be inlined
note: could not be inlined due to: implementation limitation -- MIR unavailable