Use a loop to avoid emitting long sequences of retags.

[icmccorm]

See MCP #958 for context on -Zcodegen-emit-retag.

If we need to retag an array of references, then we will retag every element of the array. This does not scale for large arrays. For example:

const N: usize = 100_000;

fn example(array: [&u32; N]) {}

fn main() {
    example([&0; N]);
}

This program will cause rustc to hang at -O0 with -Zcodegen-emit-retag. In the body of example, we need to ensure that every element of array receives a function-entry retag, so we emit 100,000 retag function calls within a single basic block!

This PR fixes this issue by adjusting how we retag the contents of arrays. Instead of retagging ever element in succession, we emit a loop that applies a single retag call to every element of the array. I have tested it on BorrowSanitizer's test suite.

Also, I saw the new #FIXME comment about supporting repr(simd) types. Some of the changes that I made here will be helpful for fixing that issue in a subsequent PR.

cc: @mojeanmac
r? @saethlin

[saethlin]

@bors r+ rollup

[rust-bors]

📌 Commit e7d3ffc has been approved by saethlin

It is now in the queue for this repository.