Comprehensive niche checks for Rust

[nikomatsakis]

Metadata
Point of contact	@1c3t3a
Team(s)	compiler, opsem
Goal document	2025h2/comprehensive-niche-checks

Summary

Add runtime checks to rustc that check for valid niche values. This is an
extension of a previous project goal
that added null pointer and enum checks and generally works towards checking for
Undefined behavior at runtime (in debug builds / behind compiler flags).

The check should fire for invalid values that are already created by inserting
a check at load-time / on function entry.

Tasks and status

• Discussion and moral support (compiler, opsem )
• Implementation (@1c3t3a, @jakos-sec)
• MCP decision (compiler )
• Dedicated reviewer (compiler, opsem )

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Note: we have updated the body to match the 2025h2 goal. Your original text is preserved below.

Details

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!! :)

[1c3t3a]

Key developments: Landed the enum discriminant check and enabled it for transmutes to enums for now (this is not so powerful), currently extending it to union reads and pointer reads.

Blockers: question of how to insert a check if we already observe UB (e.g. the enum is only represented by an i1 in LLVM IR). This is to be addressed by the next project goal: https://rust-lang.github.io/rust-project-goals/2025h2/comprehensive-niche-checks.html.