Emit retags in codegen to support BorrowSanitizer (part 4)

[icmccorm]

Tracking issue: #154760
Zulip Thread

This is one of several PRs that add experimental support for emitting retags as function calls in codegen.

This PR emits our retag intrinsics based on the structure of a RetagPlan. The feature works end-to-end now! But, it has two limitations:

• We do not track interior mutable or pinned ranges precisely. This will come in the next PR, which should be the last one that we need to complete this feature (excluding documentation and compile tests).

• We recurse into repr(simd) types when building a RetagPlan, but we do not emit retags for their fields. We would need BuilderMethods::insert_element, which isn't available. I'm not sure what the best workaround would be for this.

Related: #157825, #156210, #156208

Cc: @RalfJung
r? @saethlin

[saethlin]

I don't think this is quite right. For example:

#![crate_type = "lib"]

pub enum Inner {
    A(&'static u8),
    B(&'static u16),
}

pub enum Outer {
    P(&'static u32),
    Q(Inner),
}

#[inline(never)]
pub fn demo(_x: Outer) {}

produces a block for the Outer::Q case that recurses into the Inner, but that block has no predecessors.

View changes since the review

[icmccorm]

Good catch! This should be fixed now with 19ba1b0.