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//! Method lookup: the secret sauce of Rust. See the [rustc dev guide] for more information.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/method-lookup.html
mod confirm;
mod prelude_edition_lints;
pub(crate) mod probe;
mod suggest;
use rustc_errors::{Applicability, Diag, SubdiagMessage};
use rustc_hir as hir;
use rustc_hir::def::{CtorOf, DefKind, Namespace};
use rustc_hir::def_id::DefId;
use rustc_infer::infer::{self, InferOk};
use rustc_middle::query::Providers;
use rustc_middle::traits::ObligationCause;
use rustc_middle::ty::{
self, GenericArgs, GenericArgsRef, GenericParamDefKind, Ty, TypeVisitableExt,
};
use rustc_middle::{bug, span_bug};
use rustc_span::symbol::Ident;
use rustc_span::Span;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::{self, NormalizeExt};
use tracing::{debug, instrument};
use self::probe::{IsSuggestion, ProbeScope};
pub(crate) use self::MethodError::*;
use crate::FnCtxt;
pub(crate) fn provide(providers: &mut Providers) {
probe::provide(providers);
}
#[derive(Clone, Copy, Debug)]
pub(crate) struct MethodCallee<'tcx> {
/// Impl method ID, for inherent methods, or trait method ID, otherwise.
pub def_id: DefId,
pub args: GenericArgsRef<'tcx>,
/// Instantiated method signature, i.e., it has been
/// instantiated, normalized, and has had late-bound
/// lifetimes replaced with inference variables.
pub sig: ty::FnSig<'tcx>,
}
#[derive(Debug)]
pub(crate) enum MethodError<'tcx> {
// Did not find an applicable method, but we did find various near-misses that may work.
NoMatch(NoMatchData<'tcx>),
// Multiple methods might apply.
Ambiguity(Vec<CandidateSource>),
// Found an applicable method, but it is not visible. The third argument contains a list of
// not-in-scope traits which may work.
PrivateMatch(DefKind, DefId, Vec<DefId>),
// Found a `Self: Sized` bound where `Self` is a trait object.
IllegalSizedBound {
candidates: Vec<DefId>,
needs_mut: bool,
bound_span: Span,
self_expr: &'tcx hir::Expr<'tcx>,
},
// Found a match, but the return type is wrong
BadReturnType,
}
// Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
// could lead to matches if satisfied, and a list of not-in-scope traits which may work.
#[derive(Debug)]
pub(crate) struct NoMatchData<'tcx> {
pub static_candidates: Vec<CandidateSource>,
pub unsatisfied_predicates:
Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
pub out_of_scope_traits: Vec<DefId>,
pub similar_candidate: Option<ty::AssocItem>,
pub mode: probe::Mode,
}
// A pared down enum describing just the places from which a method
// candidate can arise. Used for error reporting only.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub(crate) enum CandidateSource {
Impl(DefId),
Trait(DefId /* trait id */),
}
impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
/// Determines whether the type `self_ty` supports a visible method named `method_name` or not.
#[instrument(level = "debug", skip(self))]
pub fn method_exists_for_diagnostic(
&self,
method_name: Ident,
self_ty: Ty<'tcx>,
call_expr_id: hir::HirId,
return_type: Option<Ty<'tcx>>,
) -> bool {
match self.probe_for_name(
probe::Mode::MethodCall,
method_name,
return_type,
IsSuggestion(true),
self_ty,
call_expr_id,
ProbeScope::TraitsInScope,
) {
Ok(pick) => {
pick.maybe_emit_unstable_name_collision_hint(
self.tcx,
method_name.span,
call_expr_id,
);
true
}
Err(NoMatch(..)) => false,
Err(Ambiguity(..)) => true,
Err(PrivateMatch(..)) => false,
Err(IllegalSizedBound { .. }) => true,
Err(BadReturnType) => false,
}
}
/// Adds a suggestion to call the given method to the provided diagnostic.
#[instrument(level = "debug", skip(self, err, call_expr))]
pub(crate) fn suggest_method_call(
&self,
err: &mut Diag<'_>,
msg: impl Into<SubdiagMessage> + std::fmt::Debug,
method_name: Ident,
self_ty: Ty<'tcx>,
call_expr: &hir::Expr<'tcx>,
span: Option<Span>,
) {
let params = self
.lookup_probe_for_diagnostic(
method_name,
self_ty,
call_expr,
ProbeScope::TraitsInScope,
None,
)
.map(|pick| {
let sig = self.tcx.fn_sig(pick.item.def_id);
sig.skip_binder().inputs().skip_binder().len().saturating_sub(1)
})
.unwrap_or(0);
// Account for `foo.bar<T>`;
let sugg_span = span.unwrap_or(call_expr.span).shrink_to_hi();
let (suggestion, applicability) = (
format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
);
err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
}
/// Performs method lookup. If lookup is successful, it will return the callee
/// and store an appropriate adjustment for the self-expr. In some cases it may
/// report an error (e.g., invoking the `drop` method).
///
/// # Arguments
///
/// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
///
/// * `self`: the surrounding `FnCtxt` (!)
/// * `self_ty`: the (unadjusted) type of the self expression (`foo`)
/// * `segment`: the name and generic arguments of the method (`bar::<T1, ...Tn>`)
/// * `span`: the span for the method call
/// * `call_expr`: the complete method call: (`foo.bar::<T1,...Tn>(...)`)
/// * `self_expr`: the self expression (`foo`)
/// * `args`: the expressions of the arguments (`a, b + 1, ...`)
#[instrument(level = "debug", skip(self))]
pub fn lookup_method(
&self,
self_ty: Ty<'tcx>,
segment: &'tcx hir::PathSegment<'tcx>,
span: Span,
call_expr: &'tcx hir::Expr<'tcx>,
self_expr: &'tcx hir::Expr<'tcx>,
args: &'tcx [hir::Expr<'tcx>],
) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
let scope = if let Some(only_method) = segment.res.opt_def_id() {
ProbeScope::Single(only_method)
} else {
ProbeScope::TraitsInScope
};
let pick = self.lookup_probe(segment.ident, self_ty, call_expr, scope)?;
self.lint_edition_dependent_dot_call(
self_ty, segment, span, call_expr, self_expr, &pick, args,
);
for &import_id in &pick.import_ids {
debug!("used_trait_import: {:?}", import_id);
self.typeck_results.borrow_mut().used_trait_imports.insert(import_id);
}
self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span, None);
let result = self.confirm_method(span, self_expr, call_expr, self_ty, &pick, segment);
debug!("result = {:?}", result);
if let Some(span) = result.illegal_sized_bound {
let mut needs_mut = false;
if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
let trait_type = Ty::new_ref(self.tcx, *region, *t_type, mutability.invert());
// We probe again to see if there might be a borrow mutability discrepancy.
match self.lookup_probe(
segment.ident,
trait_type,
call_expr,
ProbeScope::TraitsInScope,
) {
Ok(ref new_pick) if pick.differs_from(new_pick) => {
needs_mut = new_pick.self_ty.ref_mutability() != self_ty.ref_mutability();
}
_ => {}
}
}
// We probe again, taking all traits into account (not only those in scope).
let candidates = match self.lookup_probe_for_diagnostic(
segment.ident,
self_ty,
call_expr,
ProbeScope::AllTraits,
None,
) {
// If we find a different result the caller probably forgot to import a trait.
Ok(ref new_pick) if pick.differs_from(new_pick) => {
vec![new_pick.item.container_id(self.tcx)]
}
Err(Ambiguity(ref sources)) => sources
.iter()
.filter_map(|source| {
match *source {
// Note: this cannot come from an inherent impl,
// because the first probing succeeded.
CandidateSource::Impl(def) => self.tcx.trait_id_of_impl(def),
CandidateSource::Trait(_) => None,
}
})
.collect(),
_ => Vec::new(),
};
return Err(IllegalSizedBound { candidates, needs_mut, bound_span: span, self_expr });
}
Ok(result.callee)
}
pub(crate) fn lookup_method_for_diagnostic(
&self,
self_ty: Ty<'tcx>,
segment: &hir::PathSegment<'tcx>,
span: Span,
call_expr: &'tcx hir::Expr<'tcx>,
self_expr: &'tcx hir::Expr<'tcx>,
) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
let pick = self.lookup_probe_for_diagnostic(
segment.ident,
self_ty,
call_expr,
ProbeScope::TraitsInScope,
None,
)?;
Ok(self
.confirm_method_for_diagnostic(span, self_expr, call_expr, self_ty, &pick, segment)
.callee)
}
#[instrument(level = "debug", skip(self, call_expr))]
pub fn lookup_probe(
&self,
method_name: Ident,
self_ty: Ty<'tcx>,
call_expr: &hir::Expr<'_>,
scope: ProbeScope,
) -> probe::PickResult<'tcx> {
let pick = self.probe_for_name(
probe::Mode::MethodCall,
method_name,
None,
IsSuggestion(false),
self_ty,
call_expr.hir_id,
scope,
)?;
pick.maybe_emit_unstable_name_collision_hint(self.tcx, method_name.span, call_expr.hir_id);
Ok(pick)
}
pub(crate) fn lookup_probe_for_diagnostic(
&self,
method_name: Ident,
self_ty: Ty<'tcx>,
call_expr: &hir::Expr<'_>,
scope: ProbeScope,
return_type: Option<Ty<'tcx>>,
) -> probe::PickResult<'tcx> {
let pick = self.probe_for_name(
probe::Mode::MethodCall,
method_name,
return_type,
IsSuggestion(true),
self_ty,
call_expr.hir_id,
scope,
)?;
Ok(pick)
}
pub(super) fn obligation_for_method(
&self,
cause: ObligationCause<'tcx>,
trait_def_id: DefId,
self_ty: Ty<'tcx>,
opt_input_types: Option<&[Ty<'tcx>]>,
) -> (traits::PredicateObligation<'tcx>, ty::GenericArgsRef<'tcx>) {
// Construct a trait-reference `self_ty : Trait<input_tys>`
let args = GenericArgs::for_item(self.tcx, trait_def_id, |param, _| {
match param.kind {
GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
GenericParamDefKind::Type { .. } => {
if param.index == 0 {
return self_ty.into();
} else if let Some(input_types) = opt_input_types {
return input_types[param.index as usize - 1].into();
}
}
}
self.var_for_def(cause.span, param)
});
let trait_ref = ty::TraitRef::new_from_args(self.tcx, trait_def_id, args);
// Construct an obligation
let poly_trait_ref = ty::Binder::dummy(trait_ref);
(traits::Obligation::new(self.tcx, cause, self.param_env, poly_trait_ref), args)
}
/// `lookup_method_in_trait` is used for overloaded operators.
/// It does a very narrow slice of what the normal probe/confirm path does.
/// In particular, it doesn't really do any probing: it simply constructs
/// an obligation for a particular trait with the given self type and checks
/// whether that trait is implemented.
#[instrument(level = "debug", skip(self))]
pub(super) fn lookup_method_in_trait(
&self,
cause: ObligationCause<'tcx>,
m_name: Ident,
trait_def_id: DefId,
self_ty: Ty<'tcx>,
opt_input_types: Option<&[Ty<'tcx>]>,
) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
let (obligation, args) =
self.obligation_for_method(cause, trait_def_id, self_ty, opt_input_types);
// FIXME(effects) find a better way to do this
// Operators don't have generic methods, but making them `#[const_trait]` gives them
// `const host: bool`.
let args = if self.tcx.is_const_trait(trait_def_id) {
self.tcx.mk_args_from_iter(
args.iter()
.chain([self.tcx.expected_host_effect_param_for_body(self.body_id).into()]),
)
} else {
args
};
self.construct_obligation_for_trait(m_name, trait_def_id, obligation, args)
}
// FIXME(#18741): it seems likely that we can consolidate some of this
// code with the other method-lookup code. In particular, the second half
// of this method is basically the same as confirmation.
fn construct_obligation_for_trait(
&self,
m_name: Ident,
trait_def_id: DefId,
obligation: traits::PredicateObligation<'tcx>,
args: ty::GenericArgsRef<'tcx>,
) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
debug!(?obligation);
// Now we want to know if this can be matched
if !self.predicate_may_hold(&obligation) {
debug!("--> Cannot match obligation");
// Cannot be matched, no such method resolution is possible.
return None;
}
// Trait must have a method named `m_name` and it should not have
// type parameters or early-bound regions.
let tcx = self.tcx;
let Some(method_item) = self.associated_value(trait_def_id, m_name) else {
bug!("expected associated item for operator trait")
};
let def_id = method_item.def_id;
if method_item.kind != ty::AssocKind::Fn {
span_bug!(tcx.def_span(def_id), "expected `{m_name}` to be an associated function");
}
debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
let mut obligations = vec![];
// FIXME(effects): revisit when binops get `#[const_trait]`
// Instantiate late-bound regions and instantiate the trait
// parameters into the method type to get the actual method type.
//
// N.B., instantiate late-bound regions before normalizing the
// function signature so that normalization does not need to deal
// with bound regions.
let fn_sig = tcx.fn_sig(def_id).instantiate(self.tcx, args);
let fn_sig =
self.instantiate_binder_with_fresh_vars(obligation.cause.span, infer::FnCall, fn_sig);
let InferOk { value, obligations: o } =
self.at(&obligation.cause, self.param_env).normalize(fn_sig);
let fn_sig = {
obligations.extend(o);
value
};
// Register obligations for the parameters. This will include the
// `Self` parameter, which in turn has a bound of the main trait,
// so this also effectively registers `obligation` as well. (We
// used to register `obligation` explicitly, but that resulted in
// double error messages being reported.)
//
// Note that as the method comes from a trait, it should not have
// any late-bound regions appearing in its bounds.
let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, args);
let InferOk { value, obligations: o } =
self.at(&obligation.cause, self.param_env).normalize(bounds);
let bounds = {
obligations.extend(o);
value
};
assert!(!bounds.has_escaping_bound_vars());
let predicates_cause = obligation.cause.clone();
obligations.extend(traits::predicates_for_generics(
move |_, _| predicates_cause.clone(),
self.param_env,
bounds,
));
// Also add an obligation for the method type being well-formed.
let method_ty = Ty::new_fn_ptr(tcx, ty::Binder::dummy(fn_sig));
debug!(
"lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
method_ty, obligation
);
obligations.push(traits::Obligation::new(
tcx,
obligation.cause,
self.param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
method_ty.into(),
))),
));
let callee = MethodCallee { def_id, args, sig: fn_sig };
debug!("callee = {:?}", callee);
Some(InferOk { obligations, value: callee })
}
/// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
/// lookup is successful, it will return the type of definition and the [`DefId`] of the found
/// function definition.
///
/// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
///
/// # Arguments
///
/// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
///
/// * `self`: the surrounding `FnCtxt` (!)
/// * `span`: the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
/// * `method_name`: the identifier of the function within the container type (`bar`)
/// * `self_ty`: the type to search within (`Foo`)
/// * `self_ty_span` the span for the type being searched within (span of `Foo`)
/// * `expr_id`: the [`hir::HirId`] of the expression composing the entire call
#[instrument(level = "debug", skip(self), ret)]
pub fn resolve_fully_qualified_call(
&self,
span: Span,
method_name: Ident,
self_ty: Ty<'tcx>,
self_ty_span: Span,
expr_id: hir::HirId,
) -> Result<(DefKind, DefId), MethodError<'tcx>> {
let tcx = self.tcx;
// Check if we have an enum variant.
let mut struct_variant = None;
if let ty::Adt(adt_def, _) = self_ty.kind() {
if adt_def.is_enum() {
let variant_def = adt_def
.variants()
.iter()
.find(|vd| tcx.hygienic_eq(method_name, vd.ident(tcx), adt_def.did()));
if let Some(variant_def) = variant_def {
if let Some((ctor_kind, ctor_def_id)) = variant_def.ctor {
tcx.check_stability(
ctor_def_id,
Some(expr_id),
span,
Some(method_name.span),
);
return Ok((DefKind::Ctor(CtorOf::Variant, ctor_kind), ctor_def_id));
} else {
struct_variant = Some((DefKind::Variant, variant_def.def_id));
}
}
}
}
let pick = self.probe_for_name(
probe::Mode::Path,
method_name,
None,
IsSuggestion(false),
self_ty,
expr_id,
ProbeScope::TraitsInScope,
);
let pick = match (pick, struct_variant) {
// Fall back to a resolution that will produce an error later.
(Err(_), Some(res)) => return Ok(res),
(pick, _) => pick?,
};
pick.maybe_emit_unstable_name_collision_hint(self.tcx, span, expr_id);
self.lint_fully_qualified_call_from_2018(
span,
method_name,
self_ty,
self_ty_span,
expr_id,
&pick,
);
debug!(?pick);
{
let mut typeck_results = self.typeck_results.borrow_mut();
for import_id in pick.import_ids {
debug!(used_trait_import=?import_id);
typeck_results.used_trait_imports.insert(import_id);
}
}
let def_kind = pick.item.kind.as_def_kind();
tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
Ok((def_kind, pick.item.def_id))
}
/// Finds item with name `item_name` defined in impl/trait `def_id`
/// and return it, or `None`, if no such item was defined there.
fn associated_value(&self, def_id: DefId, item_name: Ident) -> Option<ty::AssocItem> {
self.tcx
.associated_items(def_id)
.find_by_name_and_namespace(self.tcx, item_name, Namespace::ValueNS, def_id)
.copied()
}
}