Struct rustc_typeck::check::Inherited[][src]

pub struct Inherited<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
    infcx: InferCtxt<'a, 'gcx, 'tcx>,
    tables: MaybeInProgressTables<'a, 'tcx>,
    locals: RefCell<NodeMap<Ty<'tcx>>>,
    fulfillment_cx: RefCell<Box<TraitEngine<'tcx>>>,
    deferred_call_resolutions: RefCell<DefIdMap<Vec<DeferredCallResolution<'gcx, 'tcx>>>>,
    deferred_cast_checks: RefCell<Vec<CastCheck<'tcx>>>,
    deferred_generator_interiors: RefCell<Vec<(BodyId, Ty<'tcx>)>>,
    anon_types: RefCell<DefIdMap<AnonTypeDecl<'tcx>>>,
    implicit_region_bound: Option<Region<'tcx>>,
    body_id: Option<BodyId>,
}
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closures defined within the function. For example:

fn foo() {
    bar(move|| { ... })
}

Here, the function foo() and the closure passed to bar() will each have their own FnCtxt, but they will share the inherited fields.

Fields

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Each type parameter has an implicit region bound that indicates it must outlive at least the function body (the user may specify stronger requirements). This field indicates the region of the callee. If it is None, then the parameter environment is for an item or something where the "callee" is not clear.

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Methods

impl<'a, 'gcx, 'tcx> Inherited<'a, 'gcx, 'tcx>
[src]

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impl<'a, 'gcx, 'tcx> Inherited<'a, 'gcx, 'tcx>
[src]

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Methods from Deref<Target = InferCtxt<'a, 'gcx, 'tcx>>

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Replace all anonymized types in value with fresh inference variables and creates appropriate obligations. For example, given the input:

impl Iterator<Item = impl Debug>

this method would create two type variables, ?0 and ?1. It would return the type ?0 but also the obligations:

?0: Iterator<Item = ?1>
?1: Debug

Moreover, it returns a AnonTypeMap that would map ?0 to info about the impl Iterator<..> type and ?1 to info about the impl Debug type.

Parameters

  • parent_def_id -- we will only instantiate anonymous types with this parent. This is typically the def-id of the function in whose return type anon types are being instantiated.
  • body_id -- the body-id with which the resulting obligations should be associated
  • param_env -- the in-scope parameter environment to be used for obligations
  • value -- the value within which we are instantiating anon types

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Given the map anon_types containing the existential impl Trait types whose underlying, hidden types are being inferred, this method adds constraints to the regions appearing in those underlying hidden types to ensure that they at least do not refer to random scopes within the current function. These constraints are not (quite) sufficient to guarantee that the regions are actually legal values; that final condition is imposed after region inference is done.

The Problem

Let's work through an example to explain how it works. Assume the current function is as follows:

fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)

Here, we have two impl Trait types whose values are being inferred (the impl Bar<'a> and the impl Bar<'b>). Conceptually, this is sugar for a setup where we define underlying abstract types (Foo1, Foo2) and then, in the return type of foo, we reference those definitions:

abstract type Foo1<'x>: Bar<'x>;
abstract type Foo2<'x>: Bar<'x>;
fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
                   //  ^^^^ ^^
                   //  |    |
                   //  |    substs
                   //  def_id

As indicating in the comments above, each of those references is (in the compiler) basically a substitution (substs) applied to the type of a suitable def_id (which identifies Foo1 or Foo2).

Now, at this point in compilation, what we have done is to replace each of the references (Foo1<'a>, Foo2<'b>) with fresh inference variables C1 and C2. We wish to use the values of these variables to infer the underlying types of Foo1 and Foo2. That is, this gives rise to higher-order (pattern) unification constraints like:

for<'a> (Foo1<'a> = C1)
for<'b> (Foo1<'b> = C2)

For these equation to be satisfiable, the types C1 and C2 can only refer to a limited set of regions. For example, C1 can only refer to 'static and 'a, and C2 can only refer to 'static and 'b. The job of this function is to impose that constraint.

Up to this point, C1 and C2 are basically just random type inference variables, and hence they may contain arbitrary regions. In fact, it is fairly likely that they do! Consider this possible definition of foo:

fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
        (&*x, &*y)
    }

Here, the values for the concrete types of the two impl traits will include inference variables:

&'0 i32
&'1 i32

Ordinarily, the subtyping rules would ensure that these are sufficiently large. But since impl Bar<'a> isn't a specific type per se, we don't get such constraints by default. This is where this function comes into play. It adds extra constraints to ensure that all the regions which appear in the inferred type are regions that could validly appear.

This is actually a bit of a tricky constraint in general. We want to say that each variable (e.g., '0) can only take on values that were supplied as arguments to the abstract type (e.g., 'a for Foo1<'a>) or 'static, which is always in scope. We don't have a constraint quite of this kind in the current region checker.

The Solution

We make use of the constraint that we do have in the <= relation. To do that, we find the "minimum" of all the arguments that appear in the substs: that is, some region which is less than all the others. In the case of Foo1<'a>, that would be 'a (it's the only choice, after all). Then we apply that as a least bound to the variables (e.g., 'a <= '0).

In some cases, there is no minimum. Consider this example:

fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }

Here we would report an error, because 'a and 'b have no relation to one another.

The free_region_relations parameter

The free_region_relations argument is used to find the "minimum" of the regions supplied to a given abstract type. It must be a relation that can answer whether 'a <= 'b, where 'a and 'b are regions that appear in the "substs" for the abstract type references (the <'a> in Foo1<'a>).

Note that we do not impose the constraints based on the generic regions from the Foo1 definition (e.g., 'x). This is because the constraints we are imposing here is basically the concern of the one generating the constraining type C1, which is the current function. It also means that we can take "implied bounds" into account in some cases:

trait SomeTrait<'a, 'b> { }
fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }

Here, the fact that 'b: 'a is known only because of the implied bounds from the &'a &'b u32 parameter, and is not "inherent" to the abstract type definition.

Parameters

  • anon_types -- the map produced by instantiate_anon_types
  • free_region_relations -- something that can be used to relate the free regions ('a) that appear in the impl trait.

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Given the fully resolved, instantiated type for an anonymous type, i.e., the value of an inference variable like C1 or C2 (*), computes the "definition type" for an abstract type definition -- that is, the inferred value of Foo1<'x> or Foo2<'x> that we would conceptually use in its definition:

abstract type Foo1<'x>: Bar<'x> = AAA; <-- this type AAA
abstract type Foo2<'x>: Bar<'x> = BBB; <-- or this type BBB
fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }

Note that these values are defined in terms of a distinct set of generic parameters ('x instead of 'a) from C1 or C2. The main purpose of this function is to do that translation.

(*) C1 and C2 were introduced in the comments on constrain_anon_types. Read that comment for more context.

Parameters

  • def_id, the impl Trait type
  • anon_defn, the anonymous definition created in instantiate_anon_types
  • instantiated_ty, the inferred type C1 -- fully resolved, lifted version of anon_defn.concrete_ty

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Canonicalizes a query value V. When we canonicalize a query, we not only canonicalize unbound inference variables, but we also replace all free regions whatsoever. So for example a query like T: Trait<'static> would be canonicalized to

T: Trait<'?0>

with a mapping M that maps '?0 to 'static.

To get a good understanding of what is happening here, check out the chapter in the rustc guide.

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Canonicalizes a query response V. When we canonicalize a query response, we only canonicalize unbound inference variables, and we leave other free regions alone. So, continuing with the example from canonicalize_query, if there was an input query T: Trait<'static>, it would have been canonicalized to

T: Trait<'?0>

with a mapping M that maps '?0 to 'static. But if we found that there exists only one possible impl of Trait, and it looks like

impl<T> Trait<'static> for T { .. }

then we would prepare a query result R that (among other things) includes a mapping to '?0 := 'static. When canonicalizing this query result R, we would leave this reference to 'static alone.

To get a good understanding of what is happening here, check out the chapter in the rustc guide.

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A hacky variant of canonicalize_query that does not canonicalize 'static. Unfortunately, the existing leak check treaks 'static differently in some cases (see also #33684), so if we are performing an operation that may need to prove "leak-check" related things, we leave 'static alone.

FIXME(#48536) -- once we have universes, we can remove this and just use canonicalize_query.

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This method is meant to be invoked as the final step of a canonical query implementation. It is given:

  • the instantiated variables inference_vars created from the query key
  • the result answer of the query
  • a fulfillment context fulfill_cx that may contain various obligations which have yet to be proven.

Given this, the function will process the obligations pending in fulfill_cx:

  • If all the obligations can be proven successfully, it will package up any resulting region obligations (extracted from infcx) along with the fully resolved value answer into a query result (which is then itself canonicalized).
  • If some obligations can be neither proven nor disproven, then the same thing happens, but the resulting query is marked as ambiguous.
  • Finally, if any of the obligations result in a hard error, then Err(NoSolution) is returned.

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Given the (canonicalized) result to a canonical query, instantiates the result so it can be used, plugging in the values from the canonical query. (Note that the result may have been ambiguous; you should check the certainty level of the query before applying this function.)

To get a good understanding of what is happening here, check out the chapter in the rustc guide.

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An alternative to instantiate_query_result_and_region_obligations that is more efficient for NLL. NLL is a bit more advanced in the "transition to chalk" than the rest of the compiler. During the NLL type check, all of the "processing" of types and things happens in queries -- the NLL checker itself is only interested in the region obligations ('a: 'b or T: 'b) that come out of these queries, which it wants to convert into MIR-based constraints and solve. Therefore, it is most convenient for the NLL Type Checker to directly consume the QueryRegionConstraint values that arise from doing a query. This is contrast to other parts of the compiler, which would prefer for those QueryRegionConstraint to be converted into the older infcx-style constraints (e.g., calls to sub_regions or register_region_obligation).

Therefore, instantiate_nll_query_result_and_region_obligations performs the same basic operations as instantiate_query_result_and_region_obligations but it returns its result differently:

  • It creates a substitution S that maps from the original query variables to the values computed in the query result. If any errors arise, they are propagated back as an Err result.
  • In the case of a successful substitution, we will append QueryRegionConstraint values onto the output_query_region_constraints vector for the solver to use (if an error arises, some values may also be pushed, but they should be ignored).
  • It can happen (though it rarely does currently) that equating types and things will give rise to subobligations that must be processed. In this case, those subobligations are propagated back in the return value.
  • Finally, the query result (of type R) is propagated back, after applying the substitution S.

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Creates a substitution S for the canonical value with fresh inference variables and applies it to the canonical value. Returns both the instantiated result and the substitution S.

This is useful at the start of a query: it basically brings the canonical value "into scope" within your new infcx. At the end of processing, the substitution S (once canonicalized) then represents the values that you computed for each of the canonical inputs to your query.

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This rather funky routine is used while processing expected types. What happens here is that we want to propagate a coercion through the return type of a fn to its argument. Consider the type of Option::Some, which is basically for<T> fn(T) -> Option<T>. So if we have an expression Some(&[1, 2, 3]), and that has the expected type Option<&[u32]>, we would like to type check &[1, 2, 3] with the expectation of &[u32]. This will cause us to coerce from &[u32; 3] to &[u32] and make the users life more pleasant.

The way we do this is using fudge_regions_if_ok. What the routine actually does is to start a snapshot and execute the closure f. In our example above, what this closure will do is to unify the expectation (Option<&[u32]>) with the actual return type (Option<?T>, where ?T represents the variable instantiated for T). This will cause ?T to be unified with &?a [u32], where ?a is a fresh lifetime variable. The input type (?T) is then returned by f().

At this point, fudge_regions_if_ok will normalize all type variables, converting ?T to &?a [u32] and end the snapshot. The problem is that we can't just return this type out, because it references the region variable ?a, and that region variable was popped when we popped the snapshot.

So what we do is to keep a list (region_vars, in the code below) of region variables created during the snapshot (here, ?a). We fold the return value and replace any such regions with a new region variable (e.g., ?b) and return the result (&?b [u32]). This can then be used as the expectation for the fn argument.

The important point here is that, for soundness purposes, the regions in question are not particularly important. We will use the expected types to guide coercions, but we will still type-check the resulting types from those coercions against the actual types (?T, Option<?T) -- and remember that after the snapshot is popped, the variable ?T is no longer unified.

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Replace all regions bound by binder with skolemized regions and return a map indicating which bound-region was replaced with what skolemized region. This is the first step of checking subtyping when higher-ranked things are involved.

Important: you must call this function from within a snapshot. Moreover, before committing the snapshot, you must eventually call either plug_leaks or pop_skolemized to remove the skolemized regions. If you rollback the snapshot (or are using a probe), then the pop occurs as part of the rollback, so an explicit call is not needed (but is also permitted).

For more information about how skolemization for HRTBs works, see the rustc guide.

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Searches the region constraints created since snapshot was started and checks to determine whether any of the skolemized regions created in skol_map would "escape" -- meaning that they are related to other regions in some way. If so, the higher-ranked subtyping doesn't hold. See README.md for more details.

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This code converts from skolemized regions back to late-bound regions. It works by replacing each region in the taint set of a skolemized region with a bound-region. The bound region will be bound by the outer-most binder in value; the caller must ensure that there is such a binder and it is the right place.

This routine is only intended to be used when the leak-check has passed; currently, it's used in the trait matching code to create a set of nested obligations frmo an impl that matches against something higher-ranked. More details can be found in librustc/middle/traits/README.md.

As a brief example, consider the obligation for<'a> Fn(&'a int) -> &'a int, and the impl:

impl<A,R> Fn<A,R> for SomethingOrOther
    where A : Clone
{ ... }

Here we will have replaced 'a with a skolemized region '0. This means that our substitution will be {A=>&'0 int, R=>&'0 int}.

When we apply the substitution to the bounds, we will wind up with &'0 int : Clone as a predicate. As a last step, we then go and replace '0 with a late-bound region 'a. The depth is matched to the depth of the predicate, in this case 1, so that the final predicate is for<'a> &'a int : Clone.

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Pops the skolemized regions found in skol_map from the region inference context. Whenever you create skolemized regions via skolemize_late_bound_regions, they must be popped before you commit the enclosing snapshot (if you do not commit, e.g. within a probe or as a result of an error, then this is not necessary, as popping happens as part of the rollback).

Note: popping also occurs implicitly as part of leak_check.

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Registers that the given region obligation must be resolved from within the scope of body_id. These regions are enqueued and later processed by regionck, when full type information is available (see region_obligations field for more information).

Important traits for Vec<u8>

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Trait queries just want to pass back type obligations "as is"

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Process the region obligations that must be proven (during regionck) for the given body_id, given information about the region bounds in scope and so forth. This function must be invoked for all relevant body-ids before region inference is done (or else an assert will fire).

See the region_obligations field of InferCtxt for some comments about how this function fits into the overall expected flow of the the inferencer. The key point is that it is invoked after all type-inference variables have been bound -- towards the end of regionck. This also ensures that the region-bound-pairs are available (see comments above regarding closures).

Parameters

  • region_bound_pairs: the set of region bounds implied by the parameters and where-clauses. In particular, each pair ('a, K) in this list tells us that the bounds in scope indicate that K: 'a, where K is either a generic parameter like T or a projection like T::Item.
  • implicit_region_bound: if some, this is a region bound that is considered to hold for all type parameters (the function body).
  • param_env is the parameter environment for the enclosing function.
  • body_id is the body-id whose region obligations are being processed.

Returns

This function may have to perform normalizations, and hence it returns an InferOk with subobligations that must be processed.

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Processes a single ad-hoc region obligation that was not registered in advance.

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Important traits for Vec<u8>

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Execute f and commit the bindings

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Execute f and commit the bindings if closure f returns Ok(_)

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Execute f then unroll any bindings it creates

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Create a fresh region variable with the next available index.

Parameters

  • origin: information about why we created this variable, for use during diagnostics / error-reporting.

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Number of region variables created so far.

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Just a convenient wrapper of next_region_var for using during NLL.

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Given a set of generics defined on a type or impl, returns a substitution mapping each type/region parameter to a fresh inference variable.

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True if errors have been reported since this infcx was created. This is sometimes used as a heuristic to skip reporting errors that often occur as a result of earlier errors, but where it's hard to be 100% sure (e.g., unresolved inference variables, regionck errors).

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Set the "tainted by errors" flag to true. We call this when we observe an error from a prior pass.

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Process the region constraints and report any errors that result. After this, no more unification operations should be done -- or the compiler will panic -- but it is legal to use resolve_type_vars_if_possible as well as fully_resolve.

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Like resolve_regions_and_report_errors, but skips error reporting if NLL is enabled. This is used for fn bodies where the same error may later be reported by the NLL-based inference.

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Obtains (and clears) the current set of region constraints. The inference context is still usable: further unifications will simply add new constraints.

This method is not meant to be used with normal lexical region resolution. Rather, it is used in the NLL mode as a kind of interim hack: basically we run normal type-check and generate region constraints as normal, but then we take them and translate them into the form that the NLL solver understands. See the NLL module for mode details.

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Gives temporary access to the region constraint data.

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Takes ownership of the list of variable regions. This implies that all the region constriants have already been taken, and hence that resolve_regions_and_report_errors can never be called. This is used only during NLL processing to "hand off" ownership of the set of region vairables into the NLL region context.

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Where possible, replaces type/int/float variables in value with their final value. Note that region variables are unaffected. If a type variable has not been unified, it is left as is. This is an idempotent operation that does not affect inference state in any way and so you can do it at will.

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Returns true if T contains unresolved type variables. In the process of visiting T, this will resolve (where possible) type variables in T, but it never constructs the final, resolved type, so it's more efficient than resolve_type_vars_if_possible().

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Attempts to resolve all type/region variables in value. Region inference must have been run already (e.g., by calling resolve_regions_and_report_errors). If some variable was never unified, an Err results.

This method is idempotent, but it not typically not invoked except during the writeback phase.

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Given a higher-ranked projection predicate like:

for<'a> <T as Fn<&'a u32>>::Output = &'a u32

and a target trait-ref like:

<T as Fn<&'x u32>>

find a substitution S for the higher-ranked regions (here, ['a => 'x]) such that the predicate matches the trait-ref, and then return the value (here, &'a u32) but with the substitution applied (hence, &'x u32).

See higher_ranked_match in higher_ranked/mod.rs for more details.

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See verify_generic_bound method in region_constraints

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Obtains the latest type of the given closure; this may be a closure in the current function, in which case its ClosureKind may not yet be known.

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Obtain the signature of a closure. For closures, unlike tcx.fn_sig(def_id), this method will work during the type-checking of the enclosing function and return the closure signature in its partially inferred state.

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Normalizes associated types in value, potentially returning new obligations that must further be processed.

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Clears the selection, evaluation, and projection cachesThis is useful when repeatedly attemping to select an Obligation while changing only its ParamEnv, since FulfillmentContext doesn't use 'probe'

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Create and return a new subunivese of the current universe; update self.universe to that new subuniverse. At present, used only in the NLL subtyping code, which uses the new universe-based scheme instead of the more limited leak-check scheme.

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Reports that an overflow has occurred and halts compilation. We halt compilation unconditionally because it is important that overflows never be masked -- they basically represent computations whose result could not be truly determined and thus we can't say if the program type checks or not -- and they are unusual occurrences in any case.

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Reports that a cycle was detected which led to overflow and halts compilation. This is equivalent to report_overflow_error except that we can give a more helpful error message (and, in particular, we do not suggest increasing the overflow limit, which is not going to help).

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Given some node representing a fn-like thing in the HIR map, returns a span and ArgKind information that describes the arguments it expects. This can be supplied to report_arg_count_mismatch.

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Reports an error when the number of arguments needed by a trait match doesn't match the number that the expression provides.

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Evaluates whether the predicate can be satisfied (by any means) in the given ParamEnv.

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Evaluates whether the predicate can be satisfied in the given ParamEnv, and returns false if not certain. However, this is not entirely accurate if inference variables are involved.

Important traits for Vec<u8>

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Implied bounds are region relationships that we deduce automatically. The idea is that (e.g.) a caller must check that a function's argument types are well-formed immediately before calling that fn, and hence the callee can assume that its argument types are well-formed. This may imply certain relationships between generic parameters. For example:

fn foo<'a,T>(x: &'a T)

can only be called with a 'a and T such that &'a T is WF. For &'a T to be WF, T: 'a must hold. So we can assume T: 'a.

Parameters

  • param_env, the where-clauses in scope
  • body_id, the body-id to use when normalizing assoc types. Note that this may cause outlives obligations to be injected into the inference context with this body-id.
  • ty, the type that we are supposed to assume is WF.
  • span, a span to use when normalizing, hopefully not important, might be useful if a bug! occurs.

Trait Implementations

impl<'a, 'gcx, 'tcx> Deref for Inherited<'a, 'gcx, 'tcx>
[src]

The resulting type after dereferencing.

Dereferences the value.

Auto Trait Implementations

impl<'a, 'gcx, 'tcx> !Send for Inherited<'a, 'gcx, 'tcx>

impl<'a, 'gcx, 'tcx> !Sync for Inherited<'a, 'gcx, 'tcx>