Null and enum-discriminant runtime checks in debug builds

[nikomatsakis]

Metadata
Point of contact	@1c3t3a
Team(s)	compiler, lang, opsem
Goal document	2025h1/null-enum-discriminant-debug-checks

Summary

Add runtime checks to rustc that check for null pointers on pointer access and
invalid enum discriminants. Similar to integer overflow and pointer alignment
checks, this will only be enabled in debug builds.

Tasks and status

• Discussion and moral support (lang, opsem )
• Implementation (@1c3t3a, @vabr-g)
• Standard reviews (compiler, opsem )

[nikomatsakis]

This issue is intended for status updates only.

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

[1c3t3a]

Status update: The null-checks landed in rust#134424. Next up are the enum discriminant checks.

[1c3t3a]

Upps, giving another status update here:

Key developments: Landed an extension of the alignment check to include (mutable) borrows in rust#137940. Working on the enums check (no draft PR yet). Hope to open a PR by mid next week.

Blockers: None so far.

Help wanted: Happy to join forces on general checks and for advice what other UB would be great to check!! :)