NoMatchingImplFound in comptime code only

[jfecher]

Aim

use std::collections::umap::UHashMap;
use std::hash::BuildHasherDefault;
use std::hash::poseidon2::Poseidon2Hasher;

unconstrained fn main() {
    comptime {
        let mut map: UHashMap<u32, Field, BuildHasherDefault<Poseidon2Hasher>> =
            UHashMap::default();

        map.insert(1, 2);
    }
}

Expected Behavior

The code to execute.

Bug

error: No matching impl found for `B: BuildHasher<H>`
    ┌─ std/collections/umap.nr:406:26
    │
406 │         let mut hasher = self._build_hasher.build_hasher();
    │                          --------------------------------- No impl for `B: BuildHasher<H>`
    │

This error does not occur when the comptime block is removed, so there is a difference between how the monomorphizer and interpreter handle generics.

I've looked into this issue and determined it to be the following:

• When the interpreter and elaborator call a function they bind monomorphization bindings at the beginning and undo them at the end.
• In the interpreter, we call UHashMap::insert which binds (among other bindings) K'367 = u32, then calls UHashMap::try_resize, which calls UHashMap::len
• UHashMap::len is on the same impl and binds the same K'367 = u32 on entrance, but then also unbinds it on exit.
• Now when we return to insert and try_resize, our key type is unbound instead of being bound to a u32.
• Monomorphization does not have this issue because it does not actually call functions in a call stack. Instead, it queues the need to monomorphize that function with the specified type. So the scenario where a finished function will undo the bindings of another does not occur because the monomorphizer's state is always wiped clean between each function.

To Reproduce

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