# Type Alias rustc_middle::traits::solve::CandidateSource

source ·

    pub type CandidateSource<'tcx> = CandidateSource<TyCtxt<'tcx>>;

## Aliased Type§

    enum CandidateSource<'tcx> {
        Impl(DefId),
        BuiltinImpl(BuiltinImplSource),
        ParamEnv(usize),
        AliasBound,
        CoherenceUnknowable,
    }

## Variants§

§

### Impl(DefId)

A user written impl.

##### §Examples

    fn main() {
        let x: Vec<u32> = Vec::new();
        // This uses the impl from the standard library to prove `Vec<T>: Clone`.
        let y = x.clone();
    }

§

### BuiltinImpl(BuiltinImplSource)

A builtin impl generated by the compiler. When adding a new special trait, try to use actual impls whenever possible. Builtin impls should only be used in cases where the impl cannot be manually be written.

Notable examples are auto traits, `Sized`, and `DiscriminantKind`. For a list of all traits with builtin impls, check out the `EvalCtxt::assemble_builtin_impl_candidates` method.

§

### ParamEnv(usize)

An assumption from the environment.

More precisely we’ve used the `n-th` assumption in the `param_env`.

##### §Examples

    fn is_clone<T: Clone>(x: T) -> (T, T) {
        // This uses the assumption `T: Clone` from the `where`-bounds
        // to prove `T: Clone`.
        (x.clone(), x)
    }

§

### AliasBound

If the self type is an alias type, e.g. an opaque type or a projection, we know the bounds on that alias to hold even without knowing its concrete underlying type.

More precisely this candidate is using the `n-th` bound in the `item_bounds` of the self type.

##### §Examples

    trait Trait {
        type Assoc: Clone;
    }

    fn foo<T: Trait>(x: <T as Trait>::Assoc) {
        // We prove `<T as Trait>::Assoc` by looking at the bounds on `Assoc` in
        // in the trait definition.
        let _y = x.clone();
    }

§

### CoherenceUnknowable

A candidate that is registered only during coherence to represent some yet-unknown impl that could be produced downstream without violating orphan rules.

## Layout§

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain `repr(...)` attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 16 bytes

Size for each variant:

- `Impl`: 16 bytes
- `BuiltinImpl`: 16 bytes
- `ParamEnv`: 16 bytes
- `AliasBound`: 0 bytes
- `CoherenceUnknowable`: 0 bytes