Exponential compile time with `let` in unsupported position within closure

[jruderman]

Found with a modified fuzz-rustc

Code

Variant A: slow and high memory use

fn main () {
    let z = 0_u32; 

    let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    || let _ =
    z;
}

Variant B: slow only

fn main () {
    let _clos = || { 
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        let _ =
        0_u32; 
    };
}

Where it's slow

From -Z time-passes:

• item_bodies_checking

From sample-head.txt:

• ExprUseVisitor when called by rustc_hir_typeck::upvar::FnCtxt::analyze_closure

Regression

Regression in nightly-2021-08-17, perhaps from #80357

Version

rustc 1.67.0-nightly (ff8c8dfbe 2022-11-22)
binary: rustc
commit-hash: ff8c8dfbe66701531e3e5e335c28c544d0fbc945
commit-date: 2022-11-22
host: x86_64-apple-darwin
release: 1.67.0-nightly
LLVM version: 15.0.4

(Doesn't reproduce on playground)

@rustbot label +I-compiletime