Non exhaustive reachable patterns lint

[DevinR528]

Proposal

Add a new lint that checks for missing patterns in matches and destructuring assignment of non_exhaustive enums and structs.
This was suggested in the original non_exhaustive PR rust-lang/rust#45394 and there is an open issue rust-lang/rust#84332 for this lint as well as a very early PR rust-lang/rust#86809.

If the attribute is put on the wildcard pattern there is this issue with .. rust-lang/rust#81282.

Example

// crate x
#[non_exhaustive]
pub struct Foo {
    a: u8,
    b: usize,
    c: String,
}

#[non_exhaustive]
pub enum Bar {
    A,
    B,
    C,
}

// crate y
match Bar::A {
    Bar::A => {},
    Bar::B => {},
    #[deny(reachable)] // attribute goes here or on the expresion
    _ => {} // triggers lint "missing Bar::C..."
}

let Foo { 
    a
    b,
    #[warn(reachable)]
    .. // triggers lint "missing field `c`..."
} = structure;

Mentors or Reviewers

@davidtwco who is a reviewer on the lint PR and implemented non_exhaustive.

Process

The main points of the Major Change Process are 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 merge on either the MCP or the PR.
• 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.

[rustbot]

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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[pnkfelix]

This landed in rust-lang/rust#86809