Uplift `temporary-cstring-as-ptr` lint from `clippy` into rustc

[nathanwhit]

Proposal

Move the temporary-cstring-as-ptr lint from clippy into the compiler. The lint would be deny by default, and fire when attempting to take the pointer of a temporary CString.

For instance, the following code

let foo = CString::new("bar").unwrap().as_ptr();

would trigger the lint. This mistake is common and potentially dangerous, to the extent the documentation specifically warns about it, so it would be ideal for the compiler to help prevent this particular footgun.

Mentors or Reviewers

I've performed this change in rust-lang/rust#75671. 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 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.

[oli-obk]

@rustbot second