ICE with const-generic signature

[ranjitjhala]

The code below

#[flux::sig(fn (head: _, rows: _) -> usize)]
fn csv<const N: usize>(head: [&str; N], rows: &[[&str; N]]) -> usize {
    0
}

makes flux keel over with

 --> tests/tests/todo/boo.rs:2:4
  |
2 | fn csv<const N: usize>(head: [&str; N], rows: &[[&str; N]]) -> usize {
  |    ^^^
-Ztrack-diagnostics: created at compiler/rustc_passes/src/dead.rs:1114:18
  |
  = note: `#[warn(dead_code)]` on by default

thread 'rustc' panicked at crates/flux-fhir-analysis/src/conv/struct_compat.rs:526:22:
attempt to subtract with overflow

[nilehmann]

This must be related to these changes #884. Struct compact requires some complicated de Bruijn index juggling

[ranjitjhala]

@nilehmann any idea what is going on here? Seems like some glitch in adjust_bvars ?

[ranjitjhala]

Looks like this works instead

#[flux::sig(fn (rows: &[_]))]
fn csv<const N: usize>(rows: &[[&str; N]]) {}

[nilehmann]

That code fills in holes by matching against the default signature. When we find a match, the hole and the matching type could be under a different number of binders so we try to adjust de Bruijn indices. It's a confusing computation and I just probably got it wrong.