Experimental Tracking Issue for Heterogeneous Try Blocks

This is a tracking issue for experimental work related to https://github.com/rust-lang/rfcs/pull/3721#issuecomment-3208342727
The feature gate for the issue is #![feature(try_blocks_heterogeneous)].

For homogeneous try blocks, see #31436

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
  • #149489
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• #154127
• #154128

Implementation history

• #149489

Start with the linked RFC discussion and implementation issue #149489, then review unresolved questions #154127 and #154128. The remaining work is represented by the unchecked documentation, style, and stabilization steps; completion requires resolving those items and completing stabilization.