Description
Proposal
Add a new lint CONST_ITEM_MUTATION. Given an item const FOO: SomeType = .., this lint fires on:
- Attempting to write directly to a field (
FOO.field = some_val) or
array entry (FOO.array_field[0] = val) - Taking a mutable reference to the
constitem (&mut FOO), including
through an autoderefFOO.some_mut_self_method()
The lint message explains that since each use of a constant creates a
new temporary, the original const item will not be modified.
The need for such a lint was brought up in rust-lang/rust#74053, rust-lang/rust#55721, and rust-lang/rust-clippy#829
While this could be a clippy lint, I believe that this is important enough to go in the compiler. Without this lint, users may write useless code which is silent accepted (e.g. FOO[0] = my_val or MY_CONST_VEC.push(val)). Such code may come about as a result of unrelated refactoring (e.g. changing a static to a const, and changing a method to use &mut self instead of interior mutability).
Mentors or Reviewers
I've implemented this lint in rust-lang/rust#75573
cc @oli-obk
Process
The main points of the Major Change Process is as follows:
- File an issue describing the proposal.
- A compiler team member or contributor who is knowledgeable in the area can second by writing
@rustbot second.- Finding a "second" suffices for internal changes. If however you are proposing a new public-facing feature, such as a
-C flag, then full team check-off is required. - Compiler team members can initiate a check-off via
@rfcbot fcp mergeon either the MCP or the PR.
- Finding a "second" suffices for internal changes. If however you are proposing a new public-facing feature, such as a
- Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.
You can read more about Major Change Proposals on forge.
Comments
This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.