Reintroduce a FCW system to borrowck

[nxsaken]

Metadata
Point of contact	@oli-obk
Team(s)	compiler, types
Goal document	2026/borrowck-fcw

Summary

When borrowck was transitioned to NLL, we had a system for turning some errors into warnings, if they only happened
with NLL but not old borrowck and generally compare two impls of borrowck to make sure the errors match up.

Due to recent changes to borrowck (opaque type handling and borrowcking typeck children individually),
the remains of the system for comparing two runs of borrowck can't easily be used anymore for those purposes.

Tasks and status

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Note: we have updated the body to match the 2026 goal. Your original text is preserved below.

Details

Metadata
Point of contact	@oli-obk
Team(s)	compiler, types
Goal document	2026/borrowck-fcw

Summary

When borrowck was transitioned to NLL, we had a system for turning some errors into warnings, if they only happened
with NLL but not old borrowck and generally compare two impls of borrowck to make sure the errors match up.

Due to recent changes to borrowck (opaque type handling and borrowcking typeck children individually),
the remains of the system for comparing two runs of borrowck can't easily be used anymore for those purposes.

Tasks and status

[nxsaken]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[oli-obk]

Two preliminary groundwork PRs have landed:

• Various borrowck cleanups and param_env/opaque_types_defined_by query simplifications for typeck children rust#158360
• Emit all borrowck errors at the end of borrowck rust#158916

I'm now going to pick up rust-lang/rust#156776 (just the FnDef part for now) and change it so that it generates a new kind of outlives bound that borrowck turns into a FCW if it would cause an error