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-rw-r--r--compiler/rustc_middle/src/ty/error.rs965
1 files changed, 965 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/ty/error.rs b/compiler/rustc_middle/src/ty/error.rs
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index 000000000..4b0bc3c11
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/error.rs
@@ -0,0 +1,965 @@
+use crate::traits::{ObligationCause, ObligationCauseCode};
+use crate::ty::diagnostics::suggest_constraining_type_param;
+use crate::ty::print::{FmtPrinter, Printer};
+use crate::ty::{self, BoundRegionKind, Region, Ty, TyCtxt};
+use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
+use rustc_errors::{pluralize, Diagnostic, MultiSpan};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::{BytePos, Span};
+use rustc_target::spec::abi;
+
+use std::borrow::Cow;
+use std::fmt;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable, TypeVisitable)]
+pub struct ExpectedFound<T> {
+ pub expected: T,
+ pub found: T,
+}
+
+impl<T> ExpectedFound<T> {
+ pub fn new(a_is_expected: bool, a: T, b: T) -> Self {
+ if a_is_expected {
+ ExpectedFound { expected: a, found: b }
+ } else {
+ ExpectedFound { expected: b, found: a }
+ }
+ }
+}
+
+// Data structures used in type unification
+#[derive(Clone, Debug, TypeFoldable, TypeVisitable)]
+pub enum TypeError<'tcx> {
+ Mismatch,
+ ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
+ PolarityMismatch(ExpectedFound<ty::ImplPolarity>),
+ UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
+ AbiMismatch(ExpectedFound<abi::Abi>),
+ Mutability,
+ ArgumentMutability(usize),
+ TupleSize(ExpectedFound<usize>),
+ FixedArraySize(ExpectedFound<u64>),
+ ArgCount,
+ FieldMisMatch(Symbol, Symbol),
+
+ RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
+ RegionsInsufficientlyPolymorphic(BoundRegionKind, Region<'tcx>),
+ RegionsOverlyPolymorphic(BoundRegionKind, Region<'tcx>),
+ RegionsPlaceholderMismatch,
+
+ Sorts(ExpectedFound<Ty<'tcx>>),
+ ArgumentSorts(ExpectedFound<Ty<'tcx>>, usize),
+ IntMismatch(ExpectedFound<ty::IntVarValue>),
+ FloatMismatch(ExpectedFound<ty::FloatTy>),
+ Traits(ExpectedFound<DefId>),
+ VariadicMismatch(ExpectedFound<bool>),
+
+ /// Instantiating a type variable with the given type would have
+ /// created a cycle (because it appears somewhere within that
+ /// type).
+ CyclicTy(Ty<'tcx>),
+ CyclicConst(ty::Const<'tcx>),
+ ProjectionMismatched(ExpectedFound<DefId>),
+ ExistentialMismatch(
+ ExpectedFound<&'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>>,
+ ),
+ ObjectUnsafeCoercion(DefId),
+ ConstMismatch(ExpectedFound<ty::Const<'tcx>>),
+
+ IntrinsicCast,
+ /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
+ TargetFeatureCast(DefId),
+}
+
+/// Explains the source of a type err in a short, human readable way. This is meant to be placed
+/// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
+/// afterwards to present additional details, particularly when it comes to lifetime-related
+/// errors.
+impl<'tcx> fmt::Display for TypeError<'tcx> {
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ use self::TypeError::*;
+ fn report_maybe_different(
+ f: &mut fmt::Formatter<'_>,
+ expected: &str,
+ found: &str,
+ ) -> fmt::Result {
+ // A naive approach to making sure that we're not reporting silly errors such as:
+ // (expected closure, found closure).
+ if expected == found {
+ write!(f, "expected {}, found a different {}", expected, found)
+ } else {
+ write!(f, "expected {}, found {}", expected, found)
+ }
+ }
+
+ let br_string = |br: ty::BoundRegionKind| match br {
+ ty::BrNamed(_, name) => format!(" {}", name),
+ _ => String::new(),
+ };
+
+ match *self {
+ CyclicTy(_) => write!(f, "cyclic type of infinite size"),
+ CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
+ Mismatch => write!(f, "types differ"),
+ ConstnessMismatch(values) => {
+ write!(f, "expected {} bound, found {} bound", values.expected, values.found)
+ }
+ PolarityMismatch(values) => {
+ write!(f, "expected {} polarity, found {} polarity", values.expected, values.found)
+ }
+ UnsafetyMismatch(values) => {
+ write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+ }
+ AbiMismatch(values) => {
+ write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+ }
+ ArgumentMutability(_) | Mutability => write!(f, "types differ in mutability"),
+ TupleSize(values) => write!(
+ f,
+ "expected a tuple with {} element{}, found one with {} element{}",
+ values.expected,
+ pluralize!(values.expected),
+ values.found,
+ pluralize!(values.found)
+ ),
+ FixedArraySize(values) => write!(
+ f,
+ "expected an array with a fixed size of {} element{}, found one with {} element{}",
+ values.expected,
+ pluralize!(values.expected),
+ values.found,
+ pluralize!(values.found)
+ ),
+ ArgCount => write!(f, "incorrect number of function parameters"),
+ FieldMisMatch(adt, field) => write!(f, "field type mismatch: {}.{}", adt, field),
+ RegionsDoesNotOutlive(..) => write!(f, "lifetime mismatch"),
+ // Actually naming the region here is a bit confusing because context is lacking
+ RegionsInsufficientlyPolymorphic(..) => {
+ write!(f, "one type is more general than the other")
+ }
+ RegionsOverlyPolymorphic(br, _) => write!(
+ f,
+ "expected concrete lifetime, found bound lifetime parameter{}",
+ br_string(br)
+ ),
+ RegionsPlaceholderMismatch => write!(f, "one type is more general than the other"),
+ ArgumentSorts(values, _) | Sorts(values) => ty::tls::with(|tcx| {
+ report_maybe_different(
+ f,
+ &values.expected.sort_string(tcx),
+ &values.found.sort_string(tcx),
+ )
+ }),
+ Traits(values) => ty::tls::with(|tcx| {
+ report_maybe_different(
+ f,
+ &format!("trait `{}`", tcx.def_path_str(values.expected)),
+ &format!("trait `{}`", tcx.def_path_str(values.found)),
+ )
+ }),
+ IntMismatch(ref values) => {
+ let expected = match values.expected {
+ ty::IntVarValue::IntType(ty) => ty.name_str(),
+ ty::IntVarValue::UintType(ty) => ty.name_str(),
+ };
+ let found = match values.found {
+ ty::IntVarValue::IntType(ty) => ty.name_str(),
+ ty::IntVarValue::UintType(ty) => ty.name_str(),
+ };
+ write!(f, "expected `{}`, found `{}`", expected, found)
+ }
+ FloatMismatch(ref values) => {
+ write!(
+ f,
+ "expected `{}`, found `{}`",
+ values.expected.name_str(),
+ values.found.name_str()
+ )
+ }
+ VariadicMismatch(ref values) => write!(
+ f,
+ "expected {} fn, found {} function",
+ if values.expected { "variadic" } else { "non-variadic" },
+ if values.found { "variadic" } else { "non-variadic" }
+ ),
+ ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
+ write!(
+ f,
+ "expected {}, found {}",
+ tcx.def_path_str(values.expected),
+ tcx.def_path_str(values.found)
+ )
+ }),
+ ExistentialMismatch(ref values) => report_maybe_different(
+ f,
+ &format!("trait `{}`", values.expected),
+ &format!("trait `{}`", values.found),
+ ),
+ ConstMismatch(ref values) => {
+ write!(f, "expected `{}`, found `{}`", values.expected, values.found)
+ }
+ IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
+ TargetFeatureCast(_) => write!(
+ f,
+ "cannot coerce functions with `#[target_feature]` to safe function pointers"
+ ),
+ ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
+ }
+ }
+}
+
+impl<'tcx> TypeError<'tcx> {
+ pub fn must_include_note(&self) -> bool {
+ use self::TypeError::*;
+ match self {
+ CyclicTy(_) | CyclicConst(_) | UnsafetyMismatch(_) | ConstnessMismatch(_)
+ | PolarityMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
+ | ArgumentSorts(..) | Sorts(_) | IntMismatch(_) | FloatMismatch(_)
+ | VariadicMismatch(_) | TargetFeatureCast(_) => false,
+
+ Mutability
+ | ArgumentMutability(_)
+ | TupleSize(_)
+ | ArgCount
+ | FieldMisMatch(..)
+ | RegionsDoesNotOutlive(..)
+ | RegionsInsufficientlyPolymorphic(..)
+ | RegionsOverlyPolymorphic(..)
+ | RegionsPlaceholderMismatch
+ | Traits(_)
+ | ProjectionMismatched(_)
+ | ExistentialMismatch(_)
+ | ConstMismatch(_)
+ | IntrinsicCast
+ | ObjectUnsafeCoercion(_) => true,
+ }
+ }
+}
+
+impl<'tcx> Ty<'tcx> {
+ pub fn sort_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
+ match *self.kind() {
+ ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => {
+ format!("`{}`", self).into()
+ }
+ ty::Tuple(ref tys) if tys.is_empty() => format!("`{}`", self).into(),
+
+ ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did())).into(),
+ ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
+ ty::Array(t, n) => {
+ if t.is_simple_ty() {
+ return format!("array `{}`", self).into();
+ }
+
+ let n = tcx.lift(n).unwrap();
+ if let ty::ConstKind::Value(v) = n.kind() {
+ if let Some(n) = v.try_to_machine_usize(tcx) {
+ return format!("array of {} element{}", n, pluralize!(n)).into();
+ }
+ }
+ "array".into()
+ }
+ ty::Slice(ty) if ty.is_simple_ty() => format!("slice `{}`", self).into(),
+ ty::Slice(_) => "slice".into(),
+ ty::RawPtr(_) => "*-ptr".into(),
+ ty::Ref(_, ty, mutbl) => {
+ let tymut = ty::TypeAndMut { ty, mutbl };
+ let tymut_string = tymut.to_string();
+ if tymut_string != "_"
+ && (ty.is_simple_text() || tymut_string.len() < "mutable reference".len())
+ {
+ format!("`&{}`", tymut_string).into()
+ } else {
+ // Unknown type name, it's long or has type arguments
+ match mutbl {
+ hir::Mutability::Mut => "mutable reference",
+ _ => "reference",
+ }
+ .into()
+ }
+ }
+ ty::FnDef(..) => "fn item".into(),
+ ty::FnPtr(_) => "fn pointer".into(),
+ ty::Dynamic(ref inner, ..) if let Some(principal) = inner.principal() => {
+ format!("trait object `dyn {}`", tcx.def_path_str(principal.def_id())).into()
+ }
+ ty::Dynamic(..) => "trait object".into(),
+ ty::Closure(..) => "closure".into(),
+ ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
+ ty::GeneratorWitness(..) => "generator witness".into(),
+ ty::Tuple(..) => "tuple".into(),
+ ty::Infer(ty::TyVar(_)) => "inferred type".into(),
+ ty::Infer(ty::IntVar(_)) => "integer".into(),
+ ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
+ ty::Placeholder(..) => "placeholder type".into(),
+ ty::Bound(..) => "bound type".into(),
+ ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
+ ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
+ ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
+ ty::Projection(_) => "associated type".into(),
+ ty::Param(p) => format!("type parameter `{}`", p).into(),
+ ty::Opaque(..) => "opaque type".into(),
+ ty::Error(_) => "type error".into(),
+ }
+ }
+
+ pub fn prefix_string(self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
+ match *self.kind() {
+ ty::Infer(_)
+ | ty::Error(_)
+ | ty::Bool
+ | ty::Char
+ | ty::Int(_)
+ | ty::Uint(_)
+ | ty::Float(_)
+ | ty::Str
+ | ty::Never => "type".into(),
+ ty::Tuple(ref tys) if tys.is_empty() => "unit type".into(),
+ ty::Adt(def, _) => def.descr().into(),
+ ty::Foreign(_) => "extern type".into(),
+ ty::Array(..) => "array".into(),
+ ty::Slice(_) => "slice".into(),
+ ty::RawPtr(_) => "raw pointer".into(),
+ ty::Ref(.., mutbl) => match mutbl {
+ hir::Mutability::Mut => "mutable reference",
+ _ => "reference",
+ }
+ .into(),
+ ty::FnDef(..) => "fn item".into(),
+ ty::FnPtr(_) => "fn pointer".into(),
+ ty::Dynamic(..) => "trait object".into(),
+ ty::Closure(..) => "closure".into(),
+ ty::Generator(def_id, ..) => tcx.generator_kind(def_id).unwrap().descr().into(),
+ ty::GeneratorWitness(..) => "generator witness".into(),
+ ty::Tuple(..) => "tuple".into(),
+ ty::Placeholder(..) => "higher-ranked type".into(),
+ ty::Bound(..) => "bound type variable".into(),
+ ty::Projection(_) => "associated type".into(),
+ ty::Param(_) => "type parameter".into(),
+ ty::Opaque(..) => "opaque type".into(),
+ }
+ }
+}
+
+impl<'tcx> TyCtxt<'tcx> {
+ pub fn note_and_explain_type_err(
+ self,
+ diag: &mut Diagnostic,
+ err: &TypeError<'tcx>,
+ cause: &ObligationCause<'tcx>,
+ sp: Span,
+ body_owner_def_id: DefId,
+ ) {
+ use self::TypeError::*;
+ debug!("note_and_explain_type_err err={:?} cause={:?}", err, cause);
+ match err {
+ ArgumentSorts(values, _) | Sorts(values) => {
+ match (values.expected.kind(), values.found.kind()) {
+ (ty::Closure(..), ty::Closure(..)) => {
+ diag.note("no two closures, even if identical, have the same type");
+ diag.help("consider boxing your closure and/or using it as a trait object");
+ }
+ (ty::Opaque(..), ty::Opaque(..)) => {
+ // Issue #63167
+ diag.note("distinct uses of `impl Trait` result in different opaque types");
+ }
+ (ty::Float(_), ty::Infer(ty::IntVar(_)))
+ if let Ok(
+ // Issue #53280
+ snippet,
+ ) = self.sess.source_map().span_to_snippet(sp) =>
+ {
+ if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
+ diag.span_suggestion(
+ sp,
+ "use a float literal",
+ format!("{}.0", snippet),
+ MachineApplicable,
+ );
+ }
+ }
+ (ty::Param(expected), ty::Param(found)) => {
+ let generics = self.generics_of(body_owner_def_id);
+ let e_span = self.def_span(generics.type_param(expected, self).def_id);
+ if !sp.contains(e_span) {
+ diag.span_label(e_span, "expected type parameter");
+ }
+ let f_span = self.def_span(generics.type_param(found, self).def_id);
+ if !sp.contains(f_span) {
+ diag.span_label(f_span, "found type parameter");
+ }
+ diag.note(
+ "a type parameter was expected, but a different one was found; \
+ you might be missing a type parameter or trait bound",
+ );
+ diag.note(
+ "for more information, visit \
+ https://doc.rust-lang.org/book/ch10-02-traits.html\
+ #traits-as-parameters",
+ );
+ }
+ (ty::Projection(_), ty::Projection(_)) => {
+ diag.note("an associated type was expected, but a different one was found");
+ }
+ (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
+ let generics = self.generics_of(body_owner_def_id);
+ let p_span = self.def_span(generics.type_param(p, self).def_id);
+ if !sp.contains(p_span) {
+ diag.span_label(p_span, "this type parameter");
+ }
+ let hir = self.hir();
+ let mut note = true;
+ if let Some(generics) = generics
+ .type_param(p, self)
+ .def_id
+ .as_local()
+ .map(|id| hir.local_def_id_to_hir_id(id))
+ .and_then(|id| self.hir().find(self.hir().get_parent_node(id)))
+ .as_ref()
+ .and_then(|node| node.generics())
+ {
+ // Synthesize the associated type restriction `Add<Output = Expected>`.
+ // FIXME: extract this logic for use in other diagnostics.
+ let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
+ let path =
+ self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
+ let item_name = self.item_name(proj.item_def_id);
+ let item_args = self.format_generic_args(assoc_substs);
+
+ let path = if path.ends_with('>') {
+ format!(
+ "{}, {}{} = {}>",
+ &path[..path.len() - 1],
+ item_name,
+ item_args,
+ p
+ )
+ } else {
+ format!("{}<{}{} = {}>", path, item_name, item_args, p)
+ };
+ note = !suggest_constraining_type_param(
+ self,
+ generics,
+ diag,
+ &format!("{}", proj.self_ty()),
+ &path,
+ None,
+ );
+ }
+ if note {
+ diag.note("you might be missing a type parameter or trait bound");
+ }
+ }
+ (ty::Param(p), ty::Dynamic(..) | ty::Opaque(..))
+ | (ty::Dynamic(..) | ty::Opaque(..), ty::Param(p)) => {
+ let generics = self.generics_of(body_owner_def_id);
+ let p_span = self.def_span(generics.type_param(p, self).def_id);
+ if !sp.contains(p_span) {
+ diag.span_label(p_span, "this type parameter");
+ }
+ diag.help("type parameters must be constrained to match other types");
+ if self.sess.teach(&diag.get_code().unwrap()) {
+ diag.help(
+ "given a type parameter `T` and a method `foo`:
+```
+trait Trait<T> { fn foo(&self) -> T; }
+```
+the only ways to implement method `foo` are:
+- constrain `T` with an explicit type:
+```
+impl Trait<String> for X {
+ fn foo(&self) -> String { String::new() }
+}
+```
+- add a trait bound to `T` and call a method on that trait that returns `Self`:
+```
+impl<T: std::default::Default> Trait<T> for X {
+ fn foo(&self) -> T { <T as std::default::Default>::default() }
+}
+```
+- change `foo` to return an argument of type `T`:
+```
+impl<T> Trait<T> for X {
+ fn foo(&self, x: T) -> T { x }
+}
+```",
+ );
+ }
+ diag.note(
+ "for more information, visit \
+ https://doc.rust-lang.org/book/ch10-02-traits.html\
+ #traits-as-parameters",
+ );
+ }
+ (ty::Param(p), ty::Closure(..) | ty::Generator(..)) => {
+ let generics = self.generics_of(body_owner_def_id);
+ let p_span = self.def_span(generics.type_param(p, self).def_id);
+ if !sp.contains(p_span) {
+ diag.span_label(p_span, "this type parameter");
+ }
+ diag.help(&format!(
+ "every closure has a distinct type and so could not always match the \
+ caller-chosen type of parameter `{}`",
+ p
+ ));
+ }
+ (ty::Param(p), _) | (_, ty::Param(p)) => {
+ let generics = self.generics_of(body_owner_def_id);
+ let p_span = self.def_span(generics.type_param(p, self).def_id);
+ if !sp.contains(p_span) {
+ diag.span_label(p_span, "this type parameter");
+ }
+ }
+ (ty::Projection(proj_ty), _) => {
+ self.expected_projection(
+ diag,
+ proj_ty,
+ values,
+ body_owner_def_id,
+ cause.code(),
+ );
+ }
+ (_, ty::Projection(proj_ty)) => {
+ let msg = format!(
+ "consider constraining the associated type `{}` to `{}`",
+ values.found, values.expected,
+ );
+ if !(self.suggest_constraining_opaque_associated_type(
+ diag,
+ &msg,
+ proj_ty,
+ values.expected,
+ ) || self.suggest_constraint(
+ diag,
+ &msg,
+ body_owner_def_id,
+ proj_ty,
+ values.expected,
+ )) {
+ diag.help(&msg);
+ diag.note(
+ "for more information, visit \
+ https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
+ );
+ }
+ }
+ _ => {}
+ }
+ debug!(
+ "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
+ values.expected,
+ values.expected.kind(),
+ values.found,
+ values.found.kind(),
+ );
+ }
+ CyclicTy(ty) => {
+ // Watch out for various cases of cyclic types and try to explain.
+ if ty.is_closure() || ty.is_generator() {
+ diag.note(
+ "closures cannot capture themselves or take themselves as argument;\n\
+ this error may be the result of a recent compiler bug-fix,\n\
+ see issue #46062 <https://github.com/rust-lang/rust/issues/46062>\n\
+ for more information",
+ );
+ }
+ }
+ TargetFeatureCast(def_id) => {
+ let target_spans =
+ self.get_attrs(*def_id, sym::target_feature).map(|attr| attr.span);
+ diag.note(
+ "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
+ );
+ diag.span_labels(target_spans, "`#[target_feature]` added here");
+ }
+ _ => {}
+ }
+ }
+
+ fn suggest_constraint(
+ self,
+ diag: &mut Diagnostic,
+ msg: &str,
+ body_owner_def_id: DefId,
+ proj_ty: &ty::ProjectionTy<'tcx>,
+ ty: Ty<'tcx>,
+ ) -> bool {
+ let assoc = self.associated_item(proj_ty.item_def_id);
+ let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
+ if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
+ if let Some(hir_generics) = item.generics() {
+ // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
+ // This will also work for `impl Trait`.
+ let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind() {
+ let generics = self.generics_of(body_owner_def_id);
+ generics.type_param(param_ty, self).def_id
+ } else {
+ return false;
+ };
+ let Some(def_id) = def_id.as_local() else {
+ return false;
+ };
+
+ // First look in the `where` clause, as this might be
+ // `fn foo<T>(x: T) where T: Trait`.
+ for pred in hir_generics.bounds_for_param(def_id) {
+ if self.constrain_generic_bound_associated_type_structured_suggestion(
+ diag,
+ &trait_ref,
+ pred.bounds,
+ &assoc,
+ assoc_substs,
+ ty,
+ msg,
+ false,
+ ) {
+ return true;
+ }
+ }
+ }
+ }
+ false
+ }
+
+ /// An associated type was expected and a different type was found.
+ ///
+ /// We perform a few different checks to see what we can suggest:
+ ///
+ /// - In the current item, look for associated functions that return the expected type and
+ /// suggest calling them. (Not a structured suggestion.)
+ /// - If any of the item's generic bounds can be constrained, we suggest constraining the
+ /// associated type to the found type.
+ /// - If the associated type has a default type and was expected inside of a `trait`, we
+ /// mention that this is disallowed.
+ /// - If all other things fail, and the error is not because of a mismatch between the `trait`
+ /// and the `impl`, we provide a generic `help` to constrain the assoc type or call an assoc
+ /// fn that returns the type.
+ fn expected_projection(
+ self,
+ diag: &mut Diagnostic,
+ proj_ty: &ty::ProjectionTy<'tcx>,
+ values: &ExpectedFound<Ty<'tcx>>,
+ body_owner_def_id: DefId,
+ cause_code: &ObligationCauseCode<'_>,
+ ) {
+ let msg = format!(
+ "consider constraining the associated type `{}` to `{}`",
+ values.expected, values.found
+ );
+ let body_owner = self.hir().get_if_local(body_owner_def_id);
+ let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
+
+ // We don't want to suggest calling an assoc fn in a scope where that isn't feasible.
+ let callable_scope = matches!(
+ body_owner,
+ Some(
+ hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(..), .. })
+ | hir::Node::TraitItem(hir::TraitItem { kind: hir::TraitItemKind::Fn(..), .. })
+ | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }),
+ )
+ );
+ let impl_comparison =
+ matches!(cause_code, ObligationCauseCode::CompareImplItemObligation { .. });
+ let assoc = self.associated_item(proj_ty.item_def_id);
+ if !callable_scope || impl_comparison {
+ // We do not want to suggest calling functions when the reason of the
+ // type error is a comparison of an `impl` with its `trait` or when the
+ // scope is outside of a `Body`.
+ } else {
+ // If we find a suitable associated function that returns the expected type, we don't
+ // want the more general suggestion later in this method about "consider constraining
+ // the associated type or calling a method that returns the associated type".
+ let point_at_assoc_fn = self.point_at_methods_that_satisfy_associated_type(
+ diag,
+ assoc.container_id(self),
+ current_method_ident,
+ proj_ty.item_def_id,
+ values.expected,
+ );
+ // Possibly suggest constraining the associated type to conform to the
+ // found type.
+ if self.suggest_constraint(diag, &msg, body_owner_def_id, proj_ty, values.found)
+ || point_at_assoc_fn
+ {
+ return;
+ }
+ }
+
+ self.suggest_constraining_opaque_associated_type(diag, &msg, proj_ty, values.found);
+
+ if self.point_at_associated_type(diag, body_owner_def_id, values.found) {
+ return;
+ }
+
+ if !impl_comparison {
+ // Generic suggestion when we can't be more specific.
+ if callable_scope {
+ diag.help(&format!(
+ "{} or calling a method that returns `{}`",
+ msg, values.expected
+ ));
+ } else {
+ diag.help(&msg);
+ }
+ diag.note(
+ "for more information, visit \
+ https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
+ );
+ }
+ if self.sess.teach(&diag.get_code().unwrap()) {
+ diag.help(
+ "given an associated type `T` and a method `foo`:
+```
+trait Trait {
+type T;
+fn foo(&self) -> Self::T;
+}
+```
+the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
+```
+impl Trait for X {
+type T = String;
+fn foo(&self) -> Self::T { String::new() }
+}
+```",
+ );
+ }
+ }
+
+ /// When the expected `impl Trait` is not defined in the current item, it will come from
+ /// a return type. This can occur when dealing with `TryStream` (#71035).
+ fn suggest_constraining_opaque_associated_type(
+ self,
+ diag: &mut Diagnostic,
+ msg: &str,
+ proj_ty: &ty::ProjectionTy<'tcx>,
+ ty: Ty<'tcx>,
+ ) -> bool {
+ let assoc = self.associated_item(proj_ty.item_def_id);
+ if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
+ let opaque_local_def_id = def_id.as_local();
+ let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
+ match &self.hir().expect_item(opaque_local_def_id).kind {
+ hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
+ _ => bug!("The HirId comes from a `ty::Opaque`"),
+ }
+ } else {
+ return false;
+ };
+
+ let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
+
+ self.constrain_generic_bound_associated_type_structured_suggestion(
+ diag,
+ &trait_ref,
+ opaque_hir_ty.bounds,
+ assoc,
+ assoc_substs,
+ ty,
+ msg,
+ true,
+ )
+ } else {
+ false
+ }
+ }
+
+ fn point_at_methods_that_satisfy_associated_type(
+ self,
+ diag: &mut Diagnostic,
+ assoc_container_id: DefId,
+ current_method_ident: Option<Symbol>,
+ proj_ty_item_def_id: DefId,
+ expected: Ty<'tcx>,
+ ) -> bool {
+ let items = self.associated_items(assoc_container_id);
+ // Find all the methods in the trait that could be called to construct the
+ // expected associated type.
+ // FIXME: consider suggesting the use of associated `const`s.
+ let methods: Vec<(Span, String)> = items
+ .items
+ .iter()
+ .filter(|(name, item)| {
+ ty::AssocKind::Fn == item.kind && Some(**name) != current_method_ident
+ })
+ .filter_map(|(_, item)| {
+ let method = self.fn_sig(item.def_id);
+ match *method.output().skip_binder().kind() {
+ ty::Projection(ty::ProjectionTy { item_def_id, .. })
+ if item_def_id == proj_ty_item_def_id =>
+ {
+ Some((
+ self.def_span(item.def_id),
+ format!("consider calling `{}`", self.def_path_str(item.def_id)),
+ ))
+ }
+ _ => None,
+ }
+ })
+ .collect();
+ if !methods.is_empty() {
+ // Use a single `help:` to show all the methods in the trait that can
+ // be used to construct the expected associated type.
+ let mut span: MultiSpan =
+ methods.iter().map(|(sp, _)| *sp).collect::<Vec<Span>>().into();
+ let msg = format!(
+ "{some} method{s} {are} available that return{r} `{ty}`",
+ some = if methods.len() == 1 { "a" } else { "some" },
+ s = pluralize!(methods.len()),
+ are = pluralize!("is", methods.len()),
+ r = if methods.len() == 1 { "s" } else { "" },
+ ty = expected
+ );
+ for (sp, label) in methods.into_iter() {
+ span.push_span_label(sp, label);
+ }
+ diag.span_help(span, &msg);
+ return true;
+ }
+ false
+ }
+
+ fn point_at_associated_type(
+ self,
+ diag: &mut Diagnostic,
+ body_owner_def_id: DefId,
+ found: Ty<'tcx>,
+ ) -> bool {
+ let Some(hir_id) = body_owner_def_id.as_local() else {
+ return false;
+ };
+ let hir_id = self.hir().local_def_id_to_hir_id(hir_id);
+ // When `body_owner` is an `impl` or `trait` item, look in its associated types for
+ // `expected` and point at it.
+ let parent_id = self.hir().get_parent_item(hir_id);
+ let item = self.hir().find_by_def_id(parent_id);
+ debug!("expected_projection parent item {:?}", item);
+ match item {
+ Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(.., items), .. })) => {
+ // FIXME: account for `#![feature(specialization)]`
+ for item in &items[..] {
+ match item.kind {
+ hir::AssocItemKind::Type => {
+ // FIXME: account for returning some type in a trait fn impl that has
+ // an assoc type as a return type (#72076).
+ if let hir::Defaultness::Default { has_value: true } =
+ self.impl_defaultness(item.id.def_id)
+ {
+ if self.type_of(item.id.def_id) == found {
+ diag.span_label(
+ item.span,
+ "associated type defaults can't be assumed inside the \
+ trait defining them",
+ );
+ return true;
+ }
+ }
+ }
+ _ => {}
+ }
+ }
+ }
+ Some(hir::Node::Item(hir::Item {
+ kind: hir::ItemKind::Impl(hir::Impl { items, .. }),
+ ..
+ })) => {
+ for item in &items[..] {
+ if let hir::AssocItemKind::Type = item.kind {
+ if self.type_of(item.id.def_id) == found {
+ diag.span_label(item.span, "expected this associated type");
+ return true;
+ }
+ }
+ }
+ }
+ _ => {}
+ }
+ false
+ }
+
+ /// Given a slice of `hir::GenericBound`s, if any of them corresponds to the `trait_ref`
+ /// requirement, provide a structured suggestion to constrain it to a given type `ty`.
+ ///
+ /// `is_bound_surely_present` indicates whether we know the bound we're looking for is
+ /// inside `bounds`. If that's the case then we can consider `bounds` containing only one
+ /// trait bound as the one we're looking for. This can help in cases where the associated
+ /// type is defined on a supertrait of the one present in the bounds.
+ fn constrain_generic_bound_associated_type_structured_suggestion(
+ self,
+ diag: &mut Diagnostic,
+ trait_ref: &ty::TraitRef<'tcx>,
+ bounds: hir::GenericBounds<'_>,
+ assoc: &ty::AssocItem,
+ assoc_substs: &[ty::GenericArg<'tcx>],
+ ty: Ty<'tcx>,
+ msg: &str,
+ is_bound_surely_present: bool,
+ ) -> bool {
+ // FIXME: we would want to call `resolve_vars_if_possible` on `ty` before suggesting.
+
+ let trait_bounds = bounds.iter().filter_map(|bound| match bound {
+ hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => Some(ptr),
+ _ => None,
+ });
+
+ let matching_trait_bounds = trait_bounds
+ .clone()
+ .filter(|ptr| ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id))
+ .collect::<Vec<_>>();
+
+ let span = match &matching_trait_bounds[..] {
+ &[ptr] => ptr.span,
+ &[] if is_bound_surely_present => match &trait_bounds.collect::<Vec<_>>()[..] {
+ &[ptr] => ptr.span,
+ _ => return false,
+ },
+ _ => return false,
+ };
+
+ self.constrain_associated_type_structured_suggestion(
+ diag,
+ span,
+ assoc,
+ assoc_substs,
+ ty,
+ msg,
+ )
+ }
+
+ /// Given a span corresponding to a bound, provide a structured suggestion to set an
+ /// associated type to a given type `ty`.
+ fn constrain_associated_type_structured_suggestion(
+ self,
+ diag: &mut Diagnostic,
+ span: Span,
+ assoc: &ty::AssocItem,
+ assoc_substs: &[ty::GenericArg<'tcx>],
+ ty: Ty<'tcx>,
+ msg: &str,
+ ) -> bool {
+ if let Ok(has_params) =
+ self.sess.source_map().span_to_snippet(span).map(|snippet| snippet.ends_with('>'))
+ {
+ let (span, sugg) = if has_params {
+ let pos = span.hi() - BytePos(1);
+ let span = Span::new(pos, pos, span.ctxt(), span.parent());
+ (span, format!(", {} = {}", assoc.ident(self), ty))
+ } else {
+ let item_args = self.format_generic_args(assoc_substs);
+ (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident(self), item_args, ty))
+ };
+ diag.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
+ return true;
+ }
+ false
+ }
+
+ fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
+ FmtPrinter::new(self, hir::def::Namespace::TypeNS)
+ .path_generic_args(Ok, args)
+ .expect("could not write to `String`.")
+ .into_buffer()
+ }
+}
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