request handler panicked: cannot find `!BoundConst { var: 0 }`

[NobodyNada]

rust-analyzer version: 2025-11-24 and later, latest nightly, and master

rustc version: rustc 1.93.0-nightly (cc3eee7fb 2025-11-28)

editor or extension: Neovim

relevant settings: I don't think any of these are relevant, but here's my config:

{
    checkOnSave = true,
    check = {
        allTargets = true,
        command = "clippy"
    },
}

code snippet to reproduce:

pub trait Tr {
    type Assoc;

    fn f(&self, handle: Self::Assoc);
}

pub struct ConstGeneric<const N: usize>;

impl<const N: usize> Tr for &ConstGeneric<N> {
    type Assoc = AssocTy;

    fn f(&self, a: Self::Assoc) {
        a.x
        
    }
}

pub struct AssocTy {
    x: (),
}

Making any edit at all to the above code (including a whitespace change) results in the following error:

rust-analyzer: -32603: request handler panicked: cannot find `!BoundConst { var: 0 }` in param-env: ParamEnv {                 
    clauses: [],                                                                                                               
}

A git bisect points to 3f0a563 (#20974) as the first commit with the error.

[ChayimFriedman2]

This is because we pass ParamEnv::empty() to the solver. We do that in few places. The solver expects to find some predicates in the ParamEnv, and then panics as it does not.

To fix this we should revisit every place we call ParamEnv::empty(). The ones in hir/src/lib.rs are not a problem since they really do not have a ParamEnv. But in the two places in hir-ty, here and here, we should have a ParamEnv. The layout one already has it as a parameter, just need to use it; in projected_ty() however we'll need to pass it from the callers.