[beta-1.91] Warn on future errors from temporary lifetimes shortening in Rust 1.92
Pursuant to [discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/channel/474880-t-compiler.2Fbackports/topic/.23145838.3A.20beta-nominated/near/540530631), this implements a future-compatibility warning lint `macro_extended_temporary_scopes` for errors in Rust 1.92 caused by rust-lang/rust#145838:
```
warning: temporary lifetime shortening in Rust 1.92
--> $DIR/macro-extended-temporary-scopes.rs:54:14
|
LL | &struct_temp().field
| ^^^^^^^^^^^^^ this expression creates a temporary value...
...
LL | } else {
| - ...which will be dropped at the end of this block in Rust 1.92
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see <https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#macro-extended-temporary-scopes>
= note: consider using a `let` binding to create a longer lived value
```
Implementation-wise, this reuses the existing temporary scoping FCW machinery introduced for the `tail_expr_drop_order` edition lint: this adds `BackwardIncompatibleDropHint` statements to the MIR at the end of the shortened scopes for affected temporaries; these are then checked in borrowck to warn if the temporary is used after the future drop hint. There are trade-offs here: on one hand, I believe this gives some assurance over ad-hoc pattern-recognition that there are no false positives[^1]. On the other hand, this fails to lint on future dangling raw pointers and it complicates the potential addition of explanatory diagnostics or suggestions[^2]. I'm hopeful that the limitation around dangling pointers won't be relevant in real code, though; the only real instance we've seen of breakage so far is future errors in formatting macro invocations, which this should be able to catch.
Release logistics notes:
- This PR targets the beta branch directly, since the breakage it's a FCW for is landing in the next Rust version.
- rust-lang/rust#146098 undoes the breakage this is a FCW for. If that behavior is merged and stabilizes in Rust 1.92, this PR should be reverted (or shouldn't be merged) in order to avoid spurious warnings.
cc `@traviscross`
`@rustbot` label +T-lang
[^1]: In particular, more syntactic approaches are complicated by having to avoid warning on promoted constants; they'd either be full of holes, they'd need a lot of extra logic, or they'd need to hack more MIR-to-HIR mapping into `PromoteTemps`.
[^2]: It's definitely possible to add more context and a suggestion, but the ways I've thought of to do so are either too hacky or too complex to feel appropriate for a last-minute direct-to-beta lint.
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