Add more information to error message for unconstrained_parameter E0207

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -16,7 +16,7 @@ use rustc_errors::struct_span_err;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{LocalDefId, LocalModDefId};
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::{Span, Symbol};
 
 mod min_specialization;
@@ -82,6 +82,14 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
         );
         return;
     }
+
+    let other_span = tcx.source_span(impl_def_id);
+    let other_span = tcx.sess.source_map().span_until_char(other_span, '{');
+    let prev_span = tcx.sess.source_map().span_through_char(other_span, '>');
+    let other_span = prev_span.between(other_span.shrink_to_hi());
+
+    let impl_subject = tcx.impl_subject(impl_def_id.into()).instantiate_identity();
+
     let impl_generics = tcx.generics_of(impl_def_id);
     let impl_predicates = tcx.predicates_of(impl_def_id);
     let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::instantiate_identity);
@@ -119,7 +127,31 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
             ty::GenericParamDefKind::Type { .. } => {
                 let param_ty = ty::ParamTy::for_def(param);
                 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
-                    report_unused_parameter(tcx, tcx.def_span(param.def_id), "type", param_ty.name);
+                    let relevant_spans: Vec<_> = impl_predicates
+                        .predicates
+                        .iter()
+                        .filter_map(|(clause, span)| match clause.kind().skip_binder() {
+                            ty::ClauseKind::Trait(t) => {
+                                if param_ty.to_ty(tcx) == t.self_ty() {
+                                    Some(Span::clone(span))
+                                } else {
+                                    None
+                                }
+                            }
+                            _ => None,
+                        })
+                        .collect();
+
+                    report_unused_parameter(
+                        tcx,
+                        tcx.def_span(param.def_id),
+                        "type",
+                        param_ty.name,
+                        impl_self_ty,
+                        impl_subject,
+                        other_span,
+                        relevant_spans,
+                    );
                 }
             }
             ty::GenericParamDefKind::Lifetime => {
@@ -132,6 +164,10 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
                         tcx.def_span(param.def_id),
                         "lifetime",
                         param.name,
+                        impl_self_ty,
+                        impl_subject,
+                        other_span,
+                        Vec::new(),
                     );
                 }
             }
@@ -143,6 +179,10 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
                         tcx.def_span(param.def_id),
                         "const",
                         param_ct.name,
+                        impl_self_ty,
+                        impl_subject,
+                        other_span,
+                        Vec::new(),
                     );
                 }
             }
@@ -169,7 +209,26 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
     // used elsewhere are not projected back out.
 }
 
-fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {
+fn report_unused_parameter(
+    tcx: TyCtxt<'_>,
+    span: Span,
+    kind: &str,
+    name: Symbol,
+    ty: Ty<'_>,
+    impl_subject: ImplSubject<'_>,
+    impl_subject_span: Span,
+    _extra_spans: Vec<Span>,
+) {
+    let raw_ty = format!("{ty}");
+    let ty_using_param = if raw_ty.contains("<") {
+        let mut tmp = raw_ty.clone();
+        let end_idx = raw_ty.rfind(">").unwrap();
+        tmp.insert_str(end_idx, &format!(", {name}"));
+        tmp
+    } else {
+        format!("{raw_ty}<{name}>")
+    };
+
     let mut err = struct_span_err!(
         tcx.sess,
         span,
@@ -180,6 +239,33 @@ fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol
         name
     );
     err.span_label(span, format!("unconstrained {kind} parameter"));
+
+    let param_span =
+        tcx.sess.source_map().span_extend_while(span, |tmp| tmp != '>' && tmp != ',').unwrap();
+
+    err.multipart_suggestion(
+        format!("consider using the Parameter like {ty_using_param}"),
+        core::iter::once((impl_subject_span, ty_using_param)).collect(),
+        rustc_lint_defs::Applicability::Unspecified,
+    );
+
+    match impl_subject {
+        ImplSubject::Trait(_) => {
+            // err.note(format!("Use the {kind} paramter `{name}` on an associated type"));
+        }
+        ImplSubject::Inherent(_) => {
+            err.multipart_suggestion(
+                format!(
+                    "consider moving the {kind} paramter `{name}` to one of the inner functions"
+                ),
+                core::iter::once((param_span, String::new()))
+                    // .chain(extra_spans.iter().map(|span| (span.to_owned(), String::new())))
+                    .collect(),
+                rustc_lint_defs::Applicability::Unspecified,
+            );
+        }
+    };
+
     if kind == "const" {
         err.note(
             "expressions using a const parameter must map each value to a distinct output value",

tests/ui/async-await/issues/issue-78654.full.stderr

@@ -10,6 +10,7 @@ error[E0207]: the const parameter `H` is not constrained by the impl trait, self
 LL | impl<const H: feature> Foo {
    |      ^^^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `H` in the impl on `Foo` or otherwise constrain `H`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 

tests/ui/async-await/issues/issue-78654.min.stderr

@@ -10,6 +10,7 @@ error[E0207]: the const parameter `H` is not constrained by the impl trait, self
 LL | impl<const H: feature> Foo {
    |      ^^^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `H` in the impl on `Foo` or otherwise constrain `H`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 

tests/ui/const-generics/issues/issue-68366.full.stderr

@@ -4,6 +4,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl <const N: usize> Collatz<{Some(N)}> {}
    |       ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `N` in the impl on `Collatz<{Some(N)}>` or otherwise constrain `N`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -13,6 +14,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl<const N: usize> Foo {}
    |      ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `N` in the impl on `Foo` or otherwise constrain `N`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 

tests/ui/const-generics/issues/issue-68366.min.stderr

@@ -13,6 +13,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl <const N: usize> Collatz<{Some(N)}> {}
    |       ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `N` in the impl on `Collatz<{Some(N)}>` or otherwise constrain `N`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 
@@ -22,6 +23,7 @@ error[E0207]: the const parameter `N` is not constrained by the impl trait, self
 LL | impl<const N: usize> Foo {}
    |      ^^^^^^^^^^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `N` in the impl on `Foo` or otherwise constrain `N`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 

tests/ui/consts/rustc-impl-const-stability.stderr

@@ -13,6 +13,7 @@ error[E0207]: the const parameter `host` is not constrained by the impl trait, s
 LL | impl const Default for Data {
    |      ^^^^^ unconstrained const parameter
    |
+   = note: consider using const parameter `host` in the impl on `Data` or otherwise constrain `host`
    = note: expressions using a const parameter must map each value to a distinct output value
    = note: proving the result of expressions other than the parameter are unique is not supported
 

tests/ui/duplicate/duplicate-type-parameter.stderr

@@ -59,6 +59,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T,T> Qux<T,T> for Option<T> {}
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `Option<T>` or otherwise constrain `T`
 
 error: aborting due to 8 previous errors
 

tests/ui/error-codes/E0207.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: Default> Foo {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `Foo` or otherwise constrain `T`
 
 error: aborting due to previous error
 

tests/ui/generic-associated-types/bugs/issue-87735.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `U` is not constrained by the impl trait, self
    |
 LL | impl<'b, T, U> AsRef2 for Foo<T>
    |             ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `Foo<T>` or otherwise constrain `U`
 
 error: aborting due to previous error
 

tests/ui/generic-associated-types/bugs/issue-88526.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `I` is not constrained by the impl trait, self
    |
 LL | impl<'q, Q, I, F> A for TestB<Q, F>
    |             ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `I` in the impl on `TestB<Q, F>` or otherwise constrain `I`
 
 error: aborting due to previous error
 

tests/ui/generic-associated-types/gat-trait-path-generic-type-arg.stderr

@@ -18,6 +18,8 @@ error[E0207]: the type parameter `T1` is not constrained by the impl trait, self
    |
 LL | impl <T, T1> Foo for T {
    |          ^^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T1` in the impl on `T` or otherwise constrain `T1`
 
 error: aborting due to 3 previous errors
 

tests/ui/impl-trait/in-trait/unconstrained-lt.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, T> Foo for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `T` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/impl-trait/issues/issue-87340.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> X for () {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `()` or otherwise constrain `T`
 
 error: aborting due to previous error
 

tests/ui/impl-unused-rps-in-assoc-type.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Fun for Holder {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `Holder` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/impl-unused-tps-inherent.stderr

@@ -3,12 +3,16 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> MyType {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `MyType` or otherwise constrain `T`
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps-inherent.rs:17:8
    |
 LL | impl<T,U> MyType1<T> {
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `MyType1<T>` or otherwise constrain `U`
 
 error: aborting due to 2 previous errors
 

tests/ui/impl-unused-tps.stderr

@@ -3,30 +3,40 @@ error[E0207]: the type parameter `U` is not constrained by the impl trait, self
    |
 LL | impl<T,U> Foo<T> for [isize;1] {
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `[isize; 1]` or otherwise constrain `U`
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:30:8
    |
 LL | impl<T,U> Bar for T {
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `T` or otherwise constrain `U`
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:38:8
    |
 LL | impl<T,U> Bar for T
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `T` or otherwise constrain `U`
 
 error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:46:8
    |
 LL | impl<T,U,V> Foo<T> for T
    |        ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `U` in the impl on `T` or otherwise constrain `U`
 
 error[E0207]: the type parameter `V` is not constrained by the impl trait, self type, or predicates
   --> $DIR/impl-unused-tps.rs:46:10
    |
 LL | impl<T,U,V> Foo<T> for T
    |          ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `V` in the impl on `T` or otherwise constrain `V`
 
 error: aborting due to 5 previous errors
 

tests/ui/issues/issue-16562.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T, M: MatrixShape> Collection for Col<M, usize> {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `Col<M, usize>` or otherwise constrain `T`
 
 error: aborting due to previous error
 

tests/ui/issues/issue-22886.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> Iterator for Newtype {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `Newtype` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/issues/issue-26262.stderr

@@ -3,12 +3,16 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T: Tr> S<T::Assoc> {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `S<<T as Tr>::Assoc>` or otherwise constrain `T`
 
 error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
   --> $DIR/issue-26262.rs:17:6
    |
 LL | impl<'a,T: Trait2<'a>> Trait1<<T as Trait2<'a>>::Foo> for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `T` or otherwise constrain `'a`
 
 error: aborting due to 2 previous errors
 

tests/ui/issues/issue-29861.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, T: 'a> MakeRef2 for T {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `T` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/issues/issue-35139.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a> MethodType for MTFn {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `MTFn` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/type-alias-impl-trait/assoc-type-lifetime-unconstrained.stderr

@@ -3,6 +3,8 @@ error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait,
    |
 LL | impl<'a, I> UnwrapItemsExt for I {
    |      ^^ unconstrained lifetime parameter
+   |
+   = note: consider using lifetime parameter `'a` in the impl on `I` or otherwise constrain `'a`
 
 error: aborting due to previous error
 

tests/ui/type-alias-impl-trait/impl-with-unconstrained-param.stderr

@@ -3,6 +3,8 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
    |
 LL | impl<T> X for () {
    |      ^ unconstrained type parameter
+   |
+   = note: consider using type parameter `T` in the impl on `()` or otherwise constrain `T`
 
 error: aborting due to previous error