Introduce debug information to statements in MIR

[dianqk]

Proposal

Currently, the MIR optimization passes are unable to eliminate dead statements that contain debug information. For example:

struct Foo {
    v1: i32,
    v2: i32
}

#[no_mangle]
fn foo(foo: Foo) {
    let _val = foo.v2; // Because we need the debug information to know the value of `_val` at this point, we cannot eliminate this statement.
}

To enable more optimizations within MIR, I propose adding debug information to Statement. This will allow us to preserve necessary debug information while eliminating the statements when they are dead.

Mentors or Reviewers

@wesleywiser (because of your work on MIR optimizations and debug information)

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.

[rustbot]

Important

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.

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cc @rust-lang/compiler