Struct rustc_middle::ty::WithOptConstParam

pub struct WithOptConstParam<T> {
    pub did: T,
    pub const_param_did: Option<DefId>,
}

A DefId which, in case it is a const argument, is potentially bundled with the DefId of the generic parameter it instantiates.

This is used to avoid calls to type_of for const arguments during typeck which cause cycle errors.

struct A;
impl A {
    fn foo<const N: usize>(&self) -> [u8; N] { [0; N] }
    //           ^ const parameter
}
struct B;
impl B {
    fn foo<const M: u8>(&self) -> usize { 42 }
    //           ^ const parameter
}

fn main() {
    let a = A;
    let _b = a.foo::<{ 3 + 7 }>();
    //               ^^^^^^^^^ const argument
}

Let’s look at the call a.foo::<{ 3 + 7 }>() here. We do not know which foo is used until we know the type of a.

We only know the type of a once we are inside of typeck(main). We also end up normalizing the type of _b during typeck(main) which requires us to evaluate the const argument.

To evaluate that const argument we need to know its type, which we would get using type_of(const_arg). This requires us to resolve foo as it can be either usize or u8 in this example. However, resolving foo once again requires typeck(main) to get the type of a, which results in a cycle.

In short we must not call type_of(const_arg) during typeck(main).

When first creating the ty::Const of the const argument inside of typeck we have already resolved foo so we know which const parameter this argument instantiates. This means that we also know the expected result of type_of(const_arg) even if we aren’t allowed to call that query: it is equal to type_of(const_param) which is trivial to compute.

If we now want to use that constant in a place which potentially needs its type we also pass the type of its const_param. This is the point of WithOptConstParam, except that instead of a Ty we bundle the DefId of the const parameter. Meaning that we need to use type_of(const_param_did) if const_param_did is Some to get the type of did.

Fields

did: T

const_param_did: Option<DefId>

The DefId of the corresponding generic parameter in case did is a const argument.

Note that even if did is a const argument, this may still be None. All queries taking WithOptConstParam start by calling tcx.opt_const_param_of(def.did) to potentially update param_did in the case it is None.

Implementations

impl<T> WithOptConstParam<T>

pub fn unknown(did: T) -> WithOptConstParam<T>

Creates a new WithOptConstParam setting const_param_did to None.

impl WithOptConstParam<LocalDefId>

pub fn try_lookup( did: LocalDefId, tcx: TyCtxt<'_> ) -> Option<(LocalDefId, DefId)>

Returns Some((did, param_did)) if def_id is a const argument, None otherwise.

pub fn try_upgrade( self, tcx: TyCtxt<'_> ) -> Option<WithOptConstParam<LocalDefId>>

In case self is unknown but self.did is a const argument, this returns a WithOptConstParam with the correct const_param_did.

pub fn to_global(self) -> WithOptConstParam<DefId>

pub fn def_id_for_type_of(self) -> DefId

impl WithOptConstParam<DefId>

pub fn as_local(self) -> Option<WithOptConstParam<LocalDefId>>

pub fn as_const_arg(self) -> Option<(LocalDefId, DefId)>

pub fn is_local(self) -> bool

pub fn def_id_for_type_of(self) -> DefId

Trait Implementations

impl<T: Clone> Clone for WithOptConstParam<T>

fn clone(&self) -> WithOptConstParam<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug> Debug for WithOptConstParam<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'tcx, T, __D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<__D> for WithOptConstParam<T>where T: Decodable<__D>,

fn decode(__decoder: &mut __D) -> Self

impl<'tcx, T, __E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<__E> for WithOptConstParam<T>where T: Encodable<__E>,

fn encode(&self, __encoder: &mut __E)

impl<T: Hash> Hash for WithOptConstParam<T>

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<'__ctx, T> HashStable<StableHashingContext<'__ctx>> for WithOptConstParam<T>where T: HashStable<StableHashingContext<'__ctx>>,

fn hash_stable( &self, __hcx: &mut StableHashingContext<'__ctx>, __hasher: &mut StableHasher )

impl Key for WithOptConstParam<LocalDefId>

type CacheSelector = DefaultCacheSelector<WithOptConstParam<LocalDefId>>

fn query_crate_is_local(&self) -> bool

Given an instance of this key, what crate is it referring to? This is used to find the provider.

fn default_span(&self, tcx: TyCtxt<'_>) -> Span

In the event that a cycle occurs, if no explicit span has been given for a query with key self, what span should we use?

fn key_as_def_id(&self) -> Option<DefId>

If the key is a DefId or DefId–equivalent, return that DefId. Otherwise, return None.

fn ty_adt_id(&self) -> Option<DefId>

impl<'__lifted, T> Lift<'__lifted> for WithOptConstParam<T>where T: Lift<'__lifted>,

type Lifted = WithOptConstParam<<T as Lift<'__lifted>>::Lifted>

fn lift_to_tcx( self, __tcx: TyCtxt<'__lifted> ) -> Option<WithOptConstParam<T::Lifted>>

impl<T: Ord> Ord for WithOptConstParam<T>

fn cmp(&self, other: &WithOptConstParam<T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Selfwhere Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Selfwhere Self: Sized + PartialOrd<Self>,

Restrict a value to a certain interval.

impl<T: PartialEq> PartialEq<WithOptConstParam<T>> for WithOptConstParam<T>

fn eq(&self, other: &WithOptConstParam<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T: PartialOrd> PartialOrd<WithOptConstParam<T>> for WithOptConstParam<T>

fn partial_cmp(&self, other: &WithOptConstParam<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'tcx, T> TypeFoldable<TyCtxt<'tcx>> for WithOptConstParam<T>where T: TypeFoldable<TyCtxt<'tcx>>,

fn try_fold_with<__F: FallibleTypeFolder<TyCtxt<'tcx>>>( self, __folder: &mut __F ) -> Result<Self, __F::Error>

The entry point for folding. To fold a value t with a folder f call: t.try_fold_with(f).

fn fold_with<F>(self, folder: &mut F) -> Selfwhere F: TypeFolder<I>,

A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with.

impl<'tcx, T> TypeVisitable<TyCtxt<'tcx>> for WithOptConstParam<T>where T: TypeVisitable<TyCtxt<'tcx>>,

fn visit_with<__V: TypeVisitor<TyCtxt<'tcx>>>( &self, __visitor: &mut __V ) -> ControlFlow<__V::BreakTy>

The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v).

impl<T: Copy> Copy for WithOptConstParam<T>

impl<T: Eq> Eq for WithOptConstParam<T>

impl<T> StructuralEq for WithOptConstParam<T>

impl<T> StructuralPartialEq for WithOptConstParam<T>

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.