diff options
11 files changed, 358 insertions, 194 deletions
diff --git a/src/librustc_middle/ty/diagnostics.rs b/src/librustc_middle/ty/diagnostics.rs index 790eb8f49af..613d66d59c5 100644 --- a/src/librustc_middle/ty/diagnostics.rs +++ b/src/librustc_middle/ty/diagnostics.rs @@ -2,7 +2,12 @@ use crate::ty::sty::InferTy; use crate::ty::TyKind::*; -use crate::ty::TyS; +use crate::ty::{TyCtxt, TyS}; +use rustc_errors::{Applicability, DiagnosticBuilder}; +use rustc_hir as hir; +use rustc_hir::def_id::DefId; +use rustc_hir::{QPath, TyKind, WhereBoundPredicate, WherePredicate}; +use rustc_span::{BytePos, Span}; impl<'tcx> TyS<'tcx> { /// Similar to `TyS::is_primitive`, but also considers inferred numeric values to be primitive. @@ -67,3 +72,180 @@ impl<'tcx> TyS<'tcx> { } } } + +/// Suggest restricting a type param with a new bound. +pub fn suggest_constraining_type_param( + tcx: TyCtxt<'_>, + generics: &hir::Generics<'_>, + err: &mut DiagnosticBuilder<'_>, + param_name: &str, + constraint: &str, + def_id: Option<DefId>, +) -> bool { + let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name); + + let param = if let Some(param) = param { + param + } else { + return false; + }; + + const MSG_RESTRICT_BOUND_FURTHER: &str = "consider further restricting this bound"; + let msg_restrict_type = format!("consider restricting type parameter `{}`", param_name); + let msg_restrict_type_further = + format!("consider further restricting type parameter `{}`", param_name); + + if def_id == tcx.lang_items().sized_trait() { + // Type parameters are already `Sized` by default. + err.span_label(param.span, &format!("this type parameter needs to be `{}`", constraint)); + return true; + } + let mut suggest_restrict = |span| { + err.span_suggestion_verbose( + span, + MSG_RESTRICT_BOUND_FURTHER, + format!(" + {}", constraint), + Applicability::MachineApplicable, + ); + }; + + if param_name.starts_with("impl ") { + // If there's an `impl Trait` used in argument position, suggest + // restricting it: + // + // fn foo(t: impl Foo) { ... } + // -------- + // | + // help: consider further restricting this bound with `+ Bar` + // + // Suggestion for tools in this case is: + // + // fn foo(t: impl Foo) { ... } + // -------- + // | + // replace with: `impl Foo + Bar` + + suggest_restrict(param.span.shrink_to_hi()); + return true; + } + + if generics.where_clause.predicates.is_empty() + // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the + // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`. + && !matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. }) + { + if let Some(bounds_span) = param.bounds_span() { + // If user has provided some bounds, suggest restricting them: + // + // fn foo<T: Foo>(t: T) { ... } + // --- + // | + // help: consider further restricting this bound with `+ Bar` + // + // Suggestion for tools in this case is: + // + // fn foo<T: Foo>(t: T) { ... } + // -- + // | + // replace with: `T: Bar +` + suggest_restrict(bounds_span.shrink_to_hi()); + } else { + // If user hasn't provided any bounds, suggest adding a new one: + // + // fn foo<T>(t: T) { ... } + // - help: consider restricting this type parameter with `T: Foo` + err.span_suggestion_verbose( + param.span.shrink_to_hi(), + &msg_restrict_type, + format!(": {}", constraint), + Applicability::MachineApplicable, + ); + } + + true + } else { + // This part is a bit tricky, because using the `where` clause user can + // provide zero, one or many bounds for the same type parameter, so we + // have following cases to consider: + // + // 1) When the type parameter has been provided zero bounds + // + // Message: + // fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... } + // - help: consider restricting this type parameter with `where X: Bar` + // + // Suggestion: + // fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... } + // - insert: `, X: Bar` + // + // + // 2) When the type parameter has been provided one bound + // + // Message: + // fn foo<T>(t: T) where T: Foo { ... } + // ^^^^^^ + // | + // help: consider further restricting this bound with `+ Bar` + // + // Suggestion: + // fn foo<T>(t: T) where T: Foo { ... } + // ^^ + // | + // replace with: `T: Bar +` + // + // + // 3) When the type parameter has been provided many bounds + // + // Message: + // fn foo<T>(t: T) where T: Foo, T: Bar {... } + // - help: consider further restricting this type parameter with `where T: Zar` + // + // Suggestion: + // fn foo<T>(t: T) where T: Foo, T: Bar {... } + // - insert: `, T: Zar` + + let mut param_spans = Vec::new(); + + for predicate in generics.where_clause.predicates { + if let WherePredicate::BoundPredicate(WhereBoundPredicate { + span, bounded_ty, .. + }) = predicate + { + if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind { + if let Some(segment) = path.segments.first() { + if segment.ident.to_string() == param_name { + param_spans.push(span); + } + } + } + } + } + + let where_clause_span = generics.where_clause.span_for_predicates_or_empty_place(); + // Account for `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas. + let mut trailing_comma = false; + if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(where_clause_span) { + trailing_comma = snippet.ends_with(','); + } + let where_clause_span = if trailing_comma { + let hi = where_clause_span.hi(); + Span::new(hi - BytePos(1), hi, where_clause_span.ctxt()) + } else { + where_clause_span.shrink_to_hi() + }; + + match ¶m_spans[..] { + &[¶m_span] => suggest_restrict(param_span.shrink_to_hi()), + _ => { + err.span_suggestion_verbose( + where_clause_span, + &msg_restrict_type_further, + format!(", {}: {}", param_name, constraint), + Applicability::MachineApplicable, + ); + } + } + + true + } +} diff --git a/src/librustc_middle/ty/error.rs b/src/librustc_middle/ty/error.rs index 4e1a8b0e92f..329fd928c87 100644 --- a/src/librustc_middle/ty/error.rs +++ b/src/librustc_middle/ty/error.rs @@ -1,4 +1,5 @@ use crate::traits::{ObligationCause, ObligationCauseCode}; +use crate::ty::diagnostics::suggest_constraining_type_param; use crate::ty::{self, BoundRegion, Region, Ty, TyCtxt}; use rustc_ast::ast; use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect}; @@ -401,8 +402,43 @@ impl<'tcx> TyCtxt<'tcx> { (ty::Projection(_), ty::Projection(_)) => { db.note("an associated type was expected, but a different one was found"); } - (ty::Param(_), ty::Projection(_)) | (ty::Projection(_), ty::Param(_)) => { - db.note("you might be missing a type parameter or trait bound"); + (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) { + db.span_label(p_span, "this type parameter"); + } + let hir = self.hir(); + let mut note = true; + if let Some(generics) = hir + .as_local_hir_id(generics.type_param(p, self).def_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 = proj.trait_ref(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 path = if path.ends_with('>') { + format!("{}, {} = {}>", &path[..path.len() - 1], item_name, p) + } else { + format!("{}<{} = {}>", path, item_name, p) + }; + note = !suggest_constraining_type_param( + self, + generics, + db, + &format!("{}", proj.self_ty()), + &path, + None, + ); + } + if note { + db.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)) => { diff --git a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs index 5bc9f6df889..14a094b9d52 100644 --- a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs +++ b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs @@ -10,10 +10,9 @@ use rustc_middle::mir::{ FakeReadCause, Local, LocalDecl, LocalInfo, LocalKind, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind, VarBindingForm, }; -use rustc_middle::ty::{self, Ty}; +use rustc_middle::ty::{self, suggest_constraining_type_param, Ty}; use rustc_span::source_map::DesugaringKind; use rustc_span::Span; -use rustc_trait_selection::traits::error_reporting::suggest_constraining_type_param; use crate::dataflow::drop_flag_effects; use crate::dataflow::indexes::{MoveOutIndex, MovePathIndex}; diff --git a/src/librustc_trait_selection/traits/error_reporting/mod.rs b/src/librustc_trait_selection/traits/error_reporting/mod.rs index fa2af24c945..19ed6b50f92 100644 --- a/src/librustc_trait_selection/traits/error_reporting/mod.rs +++ b/src/librustc_trait_selection/traits/error_reporting/mod.rs @@ -15,17 +15,16 @@ use rustc_data_structures::fx::FxHashMap; use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorReported}; use rustc_hir as hir; use rustc_hir::def_id::{DefId, LOCAL_CRATE}; -use rustc_hir::{Node, QPath, TyKind, WhereBoundPredicate, WherePredicate}; +use rustc_hir::Node; use rustc_middle::mir::interpret::ErrorHandled; use rustc_middle::ty::error::ExpectedFound; -use rustc_middle::ty::fast_reject; use rustc_middle::ty::fold::TypeFolder; -use rustc_middle::ty::SubtypePredicate; use rustc_middle::ty::{ - self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness, + self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, + TypeFoldable, WithConstness, }; use rustc_session::DiagnosticMessageId; -use rustc_span::{BytePos, ExpnKind, Span, DUMMY_SP}; +use rustc_span::{ExpnKind, Span, DUMMY_SP}; use std::fmt; use crate::traits::query::evaluate_obligation::InferCtxtExt as _; @@ -1700,180 +1699,3 @@ impl ArgKind { } } } - -/// Suggest restricting a type param with a new bound. -pub fn suggest_constraining_type_param( - tcx: TyCtxt<'_>, - generics: &hir::Generics<'_>, - err: &mut DiagnosticBuilder<'_>, - param_name: &str, - constraint: &str, - def_id: Option<DefId>, -) -> bool { - let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name); - - let param = if let Some(param) = param { - param - } else { - return false; - }; - - const MSG_RESTRICT_BOUND_FURTHER: &str = "consider further restricting this bound"; - let msg_restrict_type = format!("consider restricting type parameter `{}`", param_name); - let msg_restrict_type_further = - format!("consider further restricting type parameter `{}`", param_name); - - if def_id == tcx.lang_items().sized_trait() { - // Type parameters are already `Sized` by default. - err.span_label(param.span, &format!("this type parameter needs to be `{}`", constraint)); - return true; - } - let mut suggest_restrict = |span| { - err.span_suggestion_verbose( - span, - MSG_RESTRICT_BOUND_FURTHER, - format!(" + {}", constraint), - Applicability::MachineApplicable, - ); - }; - - if param_name.starts_with("impl ") { - // If there's an `impl Trait` used in argument position, suggest - // restricting it: - // - // fn foo(t: impl Foo) { ... } - // -------- - // | - // help: consider further restricting this bound with `+ Bar` - // - // Suggestion for tools in this case is: - // - // fn foo(t: impl Foo) { ... } - // -------- - // | - // replace with: `impl Foo + Bar` - - suggest_restrict(param.span.shrink_to_hi()); - return true; - } - - if generics.where_clause.predicates.is_empty() - // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the - // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`. - && !matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. }) - { - if let Some(bounds_span) = param.bounds_span() { - // If user has provided some bounds, suggest restricting them: - // - // fn foo<T: Foo>(t: T) { ... } - // --- - // | - // help: consider further restricting this bound with `+ Bar` - // - // Suggestion for tools in this case is: - // - // fn foo<T: Foo>(t: T) { ... } - // -- - // | - // replace with: `T: Bar +` - suggest_restrict(bounds_span.shrink_to_hi()); - } else { - // If user hasn't provided any bounds, suggest adding a new one: - // - // fn foo<T>(t: T) { ... } - // - help: consider restricting this type parameter with `T: Foo` - err.span_suggestion_verbose( - param.span.shrink_to_hi(), - &msg_restrict_type, - format!(": {}", constraint), - Applicability::MachineApplicable, - ); - } - - true - } else { - // This part is a bit tricky, because using the `where` clause user can - // provide zero, one or many bounds for the same type parameter, so we - // have following cases to consider: - // - // 1) When the type parameter has been provided zero bounds - // - // Message: - // fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... } - // - help: consider restricting this type parameter with `where X: Bar` - // - // Suggestion: - // fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... } - // - insert: `, X: Bar` - // - // - // 2) When the type parameter has been provided one bound - // - // Message: - // fn foo<T>(t: T) where T: Foo { ... } - // ^^^^^^ - // | - // help: consider further restricting this bound with `+ Bar` - // - // Suggestion: - // fn foo<T>(t: T) where T: Foo { ... } - // ^^ - // | - // replace with: `T: Bar +` - // - // - // 3) When the type parameter has been provided many bounds - // - // Message: - // fn foo<T>(t: T) where T: Foo, T: Bar {... } - // - help: consider further restricting this type parameter with `where T: Zar` - // - // Suggestion: - // fn foo<T>(t: T) where T: Foo, T: Bar {... } - // - insert: `, T: Zar` - - let mut param_spans = Vec::new(); - - for predicate in generics.where_clause.predicates { - if let WherePredicate::BoundPredicate(WhereBoundPredicate { - span, bounded_ty, .. - }) = predicate - { - if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind { - if let Some(segment) = path.segments.first() { - if segment.ident.to_string() == param_name { - param_spans.push(span); - } - } - } - } - } - - let where_clause_span = generics.where_clause.span_for_predicates_or_empty_place(); - // Account for `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas. - let mut trailing_comma = false; - if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(where_clause_span) { - trailing_comma = snippet.ends_with(','); - } - let where_clause_span = if trailing_comma { - let hi = where_clause_span.hi(); - Span::new(hi - BytePos(1), hi, where_clause_span.ctxt()) - } else { - where_clause_span.shrink_to_hi() - }; - - match ¶m_spans[..] { - &[¶m_span] => suggest_restrict(param_span.shrink_to_hi()), - _ => { - err.span_suggestion_verbose( - where_clause_span, - &msg_restrict_type_further, - format!(", {}: {}", param_name, constraint), - Applicability::MachineApplicable, - ); - } - } - - true - } -} diff --git a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs index ce7b1390d46..74dd47a91c2 100644 --- a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs +++ b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs @@ -3,7 +3,6 @@ use super::{ }; use crate::infer::InferCtxt; -use crate::traits::error_reporting::suggest_constraining_type_param; use rustc_errors::{error_code, struct_span_err, Applicability, DiagnosticBuilder, Style}; use rustc_hir as hir; @@ -13,7 +12,8 @@ use rustc_hir::intravisit::Visitor; use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node}; use rustc_middle::ty::TypeckTables; use rustc_middle::ty::{ - self, AdtKind, DefIdTree, Infer, InferTy, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness, + self, suggest_constraining_type_param, AdtKind, DefIdTree, Infer, InferTy, ToPredicate, Ty, + TyCtxt, TypeFoldable, WithConstness, }; use rustc_span::symbol::{kw, sym, Symbol}; use rustc_span::{MultiSpan, Span, DUMMY_SP}; diff --git a/src/librustc_typeck/check/op.rs b/src/librustc_typeck/check/op.rs index 4cc68e7e54d..89a6b98cd8f 100644 --- a/src/librustc_typeck/check/op.rs +++ b/src/librustc_typeck/check/op.rs @@ -10,10 +10,9 @@ use rustc_middle::ty::adjustment::{ Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, }; use rustc_middle::ty::TyKind::{Adt, Array, Char, FnDef, Never, Ref, Str, Tuple, Uint}; -use rustc_middle::ty::{self, Ty, TypeFoldable}; +use rustc_middle::ty::{self, suggest_constraining_type_param, Ty, TypeFoldable}; use rustc_span::Span; use rustc_trait_selection::infer::InferCtxtExt; -use rustc_trait_selection::traits::error_reporting::suggest_constraining_type_param; impl<'a, 'tcx> FnCtxt<'a, 'tcx> { /// Checks a `a <op>= b` diff --git a/src/test/ui/generic-associated-types/construct_with_other_type.stderr b/src/test/ui/generic-associated-types/construct_with_other_type.stderr index bad746f7ef1..b9468b3330b 100644 --- a/src/test/ui/generic-associated-types/construct_with_other_type.stderr +++ b/src/test/ui/generic-associated-types/construct_with_other_type.stderr @@ -2,11 +2,16 @@ error[E0271]: type mismatch resolving `for<'a> <<T as Baz>::Baa<'a> as std::ops: --> $DIR/construct_with_other_type.rs:19:9 | LL | impl<T> Baz for T where T: Foo { - | ^^^ expected type parameter `T`, found associated type + | - ^^^ expected type parameter `T`, found associated type + | | + | this type parameter | = note: expected associated type `<T as Foo>::Bar<'_, 'static>` found associated type `<<T as Baz>::Quux<'_> as Foo>::Bar<'_, 'static>` - = note: you might be missing a type parameter or trait bound +help: consider further restricting this bound + | +LL | impl<T> Baz for T where T: Foo + Baz<Quux = T> { + | ^^^^^^^^^^^^^^^ error: aborting due to previous error diff --git a/src/test/ui/generic-associated-types/missing-bounds.fixed b/src/test/ui/generic-associated-types/missing-bounds.fixed new file mode 100644 index 00000000000..c2e619d1a7e --- /dev/null +++ b/src/test/ui/generic-associated-types/missing-bounds.fixed @@ -0,0 +1,35 @@ +// run-rustfix + +use std::ops::Add; + +struct A<B>(B); + +impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + A(self.0 + rhs.0) //~ ERROR mismatched types + } +} + +struct C<B>(B); + +impl<B: Add + std::ops::Add<Output = B>> Add for C<B> { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + Self(self.0 + rhs.0) //~ ERROR mismatched types + } +} + +struct D<B>(B); + +impl<B: std::ops::Add<Output = B>> Add for D<B> { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B` + } +} + +fn main() {} diff --git a/src/test/ui/generic-associated-types/missing-bounds.rs b/src/test/ui/generic-associated-types/missing-bounds.rs new file mode 100644 index 00000000000..1852ef62fe6 --- /dev/null +++ b/src/test/ui/generic-associated-types/missing-bounds.rs @@ -0,0 +1,35 @@ +// run-rustfix + +use std::ops::Add; + +struct A<B>(B); + +impl<B> Add for A<B> where B: Add { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + A(self.0 + rhs.0) //~ ERROR mismatched types + } +} + +struct C<B>(B); + +impl<B: Add> Add for C<B> { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + Self(self.0 + rhs.0) //~ ERROR mismatched types + } +} + +struct D<B>(B); + +impl<B> Add for D<B> { + type Output = Self; + + fn add(self, rhs: Self) -> Self { + Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B` + } +} + +fn main() {} diff --git a/src/test/ui/generic-associated-types/missing-bounds.stderr b/src/test/ui/generic-associated-types/missing-bounds.stderr new file mode 100644 index 00000000000..630ceac093e --- /dev/null +++ b/src/test/ui/generic-associated-types/missing-bounds.stderr @@ -0,0 +1,49 @@ +error[E0308]: mismatched types + --> $DIR/missing-bounds.rs:11:11 + | +LL | impl<B> Add for A<B> where B: Add { + | - this type parameter +... +LL | A(self.0 + rhs.0) + | ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type + | + = note: expected type parameter `B` + found associated type `<B as std::ops::Add>::Output` +help: consider further restricting this bound + | +LL | impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> { + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +error[E0308]: mismatched types + --> $DIR/missing-bounds.rs:21:14 + | +LL | impl<B: Add> Add for C<B> { + | - this type parameter +... +LL | Self(self.0 + rhs.0) + | ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type + | + = note: expected type parameter `B` + found associated type `<B as std::ops::Add>::Output` +help: consider further restricting this bound + | +LL | impl<B: Add + std::ops::Add<Output = B>> Add for C<B> { + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +error[E0369]: cannot add `B` to `B` + --> $DIR/missing-bounds.rs:31:21 + | +LL | Self(self.0 + rhs.0) + | ------ ^ ----- B + | | + | B + | +help: consider restricting type parameter `B` + | +LL | impl<B: std::ops::Add<Output = B>> Add for D<B> { + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +error: aborting due to 3 previous errors + +Some errors have detailed explanations: E0308, E0369. +For more information about an error, try `rustc --explain E0308`. diff --git a/src/test/ui/issues/issue-24204.stderr b/src/test/ui/issues/issue-24204.stderr index d69efc86005..d5cbcf786bf 100644 --- a/src/test/ui/issues/issue-24204.stderr +++ b/src/test/ui/issues/issue-24204.stderr @@ -7,7 +7,9 @@ LL | type A: MultiDispatch<Self::B, O = Self>; | -------- required by this bound in `Trait` ... LL | fn test<T: Trait<B=i32>>(b: i32) -> T where T::A: MultiDispatch<i32> { T::new(b) } - | ^^^^^^^^^^^^ expected type parameter `T`, found associated type + | - ^^^^^^^^^^^^ expected type parameter `T`, found associated type + | | + | this type parameter | = note: expected type parameter `T` found associated type `<<T as Trait>::A as MultiDispatch<i32>>::O` |