[][src]Struct rustc::traits::SelectionContext

pub struct SelectionContext<'cx, 'gcx: 'cx + 'tcx, 'tcx: 'cx> {
    infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
    freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
    intercrate: Option<IntercrateMode>,
    intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
    allow_negative_impls: bool,
    query_mode: TraitQueryMode,
}
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Fields

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Freshener used specifically for skolemizing entries on the obligation stack. This ensures that all entries on the stack at one time will have the same set of skolemized entries, which is important for checking for trait bounds that recursively require themselves.

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If true, indicates that the evaluation should be conservative and consider the possibility of types outside this crate. This comes up primarily when resolving ambiguity. Imagine there is some trait reference $0 : Bar where $0 is an inference variable. If intercrate is true, then we can never say for sure that this reference is not implemented, even if there are no impls at all for Bar, because $0 could be bound to some type that in a downstream crate that implements Bar. This is the suitable mode for coherence. Elsewhere, though, we set this to false, because we are only interested in types that the user could actually have written --- in other words, we consider $0 : Bar to be unimplemented if there is no type that the user could actually name that would satisfy it. This avoids crippling inference, basically.

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Controls whether or not to filter out negative impls when selecting. This is used in librustdoc to distinguish between the lack of an impl and a negative impl

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The mode that trait queries run in, which informs our error handling policy. In essence, canonicalized queries need their errors propagated rather than immediately reported because we do not have accurate spans.

Methods

impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx>
[src]

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Enables tracking of intercrate ambiguity causes. These are used in coherence to give improved diagnostics. We don't do this until we detect a coherence error because it can lead to false overflow results (#47139) and because it costs computation time.

Important traits for Vec<u8>

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Gets the intercrate ambiguity causes collected since tracking was enabled and disables tracking at the same time. If tracking is not enabled, just returns an empty vector.

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Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection context's self.

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Wraps a probe s.t. obligations collected during it are ignored and old obligations are retained.

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Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if the transaction fails and s.t. old obligations are retained.

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Attempts to satisfy the obligation. If successful, this will affect the surrounding type environment by performing unification.

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Evaluates whether the obligation obligation can be satisfied (by any means).

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Evaluates whether the obligation obligation can be satisfied and returns an EvaluationResult.

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Evaluates the predicates in predicates recursively. Note that this applies projections in the predicates, and therefore is run within an inference probe.

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For defaulted traits, we use a co-inductive strategy to solve, so that recursion is ok. This routine returns true if the top of the stack (cycle[0]):

  • is a defaulted trait, and
  • it also appears in the backtrace at some position X; and,
  • all the predicates at positions X.. between X an the top are also defaulted traits.

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Further evaluate candidate to decide whether all type parameters match and whether nested obligations are met. Returns true if candidate remains viable after this further scrutiny.

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Returns true if the global caches can be used. Do note that if the type itself is not in the global tcx, the local caches will be used.

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Given an obligation like <SomeTrait for T>, search the obligations that the caller supplied to find out whether it is listed among them.

Never affects inference environment.

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Check for the artificial impl that the compiler will create for an obligation like X : FnMut<..> where X is a closure type.

Note: the type parameters on a closure candidate are modeled as output type parameters and hence do not affect whether this trait is a match or not. They will be unified during the confirmation step.

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Implement one of the Fn() family for a fn pointer.

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Search for impls that might apply to obligation.

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Search for impls that might apply to obligation.

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Search for unsizing that might apply to obligation.

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Returns true if victim should be dropped in favor of other. Generally speaking we will drop duplicate candidates and prefer where-clause candidates.

See the comment for "SelectionCandidate" for more details.

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Important traits for Vec<u8>

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For default impls, we need to break apart a type into its "constituent types" -- meaning, the types that it contains.

Here are some (simple) examples:

(i32, u32) -> [i32, u32]
Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]

Important traits for Vec<u8>

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Important traits for Vec<u8>

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This handles the case where a auto trait Foo impl is being used. The idea is that the impl applies to X : Foo if the following conditions are met:

  1. For each constituent type Y in X, Y : Foo holds
  2. For each where-clause C declared on Foo, [Self => X] C holds.

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See confirm_auto_impl_candidate

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In the case of closure types and fn pointers, we currently treat the input type parameters on the trait as outputs. This means that when we have a match we have only considered the self type, so we have to go back and make sure to relate the argument types too. This is kind of wrong, but since we control the full set of impls, also not that wrong, and it DOES yield better error messages (since we don't report errors as if there is no applicable impl, but rather report errors are about mismatched argument types.

Here is an example. Imagine we have a closure expression and we desugared it so that the type of the expression is Closure, and Closure expects an int as argument. Then it is "as if" the compiler generated this impl:

impl Fn(int) for Closure { ... }

Now imagine our obligation is Fn(usize) for Closure. So far we have matched the self-type Closure. At this point we'll compare the int to usize and generate an error.

Note that this checking occurs after the impl has selected, because these output type parameters should not affect the selection of the impl. Therefore, if there is a mismatch, we report an error to the user.

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Normalize where_clause_trait_ref and try to match it against obligation. If successful, return any predicates that result from the normalization. Normalization is necessary because where-clauses are stored in the parameter environment unnormalized.

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Returns Ok if poly_trait_ref being true implies that the obligation is satisfied.

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Important traits for Vec<u8>

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Returns the obligations that are implied by instantiating an impl or trait. The obligations are substituted and fully normalized. This is used when confirming an impl or default impl.

Auto Trait Implementations

impl<'cx, 'gcx, 'tcx> !Send for SelectionContext<'cx, 'gcx, 'tcx>

impl<'cx, 'gcx, 'tcx> !Sync for SelectionContext<'cx, 'gcx, 'tcx>

Blanket Implementations

impl<T> MaybeResult for T
[src]

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impl<'a, T> Captures for T where
    T: ?Sized
[src]

impl<T> From for T
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Performs the conversion.

impl<T, U> Into for T where
    U: From<T>, 
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Performs the conversion.

impl<T, U> TryFrom for T where
    T: From<U>, 
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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> Borrow for T where
    T: ?Sized
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Important traits for &'a mut R

Immutably borrows from an owned value. Read more

impl<T, U> TryInto for T where
    U: TryFrom<T>, 
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The type returned in the event of a conversion error.

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Performs the conversion.

impl<T> BorrowMut for T where
    T: ?Sized
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Important traits for &'a mut R

Mutably borrows from an owned value. Read more

impl<T> Any for T where
    T: 'static + ?Sized
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this method will likely be replaced by an associated static

Gets the TypeId of self. Read more

impl<E> SpecializationError for E
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Create an error for a missing method specialization. Defaults to panicking with type, trait & method names. S is the encoder/decoder state type, T is the type being encoded/decoded, and the arguments are the names of the trait and method that should've been overridden. Read more

impl<T> Erased for T
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impl<T> Send for T where
    T: ?Sized
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impl<T> Sync for T where
    T: ?Sized
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impl<T> Erased for T