Enum rustc_trait_selection::traits::solve::CandidateSource

pub enum CandidateSource {
    Impl(DefId),
    BuiltinImpl(BuiltinImplSource),
    ParamEnv(usize),
    AliasBound,
}

Possible ways the given goal can be proven.

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();
}

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