Emit suggestions when equality constraints are wronlgy used

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -15,11 +15,13 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::traits::FulfillmentError;
 use rustc_middle::query::Key;
-use rustc_middle::ty::{self, suggest_constraining_type_param, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{
+    self, suggest_constraining_type_param, GenericParamCount, Ty, TyCtxt, TypeVisitableExt,
+};
 use rustc_session::parse::feature_err;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{sym, Ident};
-use rustc_span::{Span, Symbol, DUMMY_SP};
+use rustc_span::{BytePos, Span, Symbol, DUMMY_SP};
 use rustc_trait_selection::traits::object_safety_violations_for_assoc_item;
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
@@ -1029,12 +1031,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 /// Emits an error regarding forbidden type binding associations
 pub fn prohibit_assoc_item_binding(
     tcx: TyCtxt<'_>,
-    span: Span,
-    segment: Option<(&hir::PathSegment<'_>, Span)>,
+    binding: &hir::TypeBinding<'_>,
+    segment: Option<(DefId, &hir::PathSegment<'_>, Span)>,
 ) {
-    tcx.dcx().emit_err(AssocTypeBindingNotAllowed {
-        span,
-        fn_trait_expansion: if let Some((segment, span)) = segment
+    let mut err = tcx.dcx().create_err(AssocTypeBindingNotAllowed {
+        span: binding.span,
+        fn_trait_expansion: if let Some((_, segment, span)) = segment
             && segment.args().parenthesized == hir::GenericArgsParentheses::ParenSugar
         {
             Some(ParenthesizedFnTraitExpansion {
@@ -1045,6 +1047,78 @@ pub fn prohibit_assoc_item_binding(
             None
         },
     });
+
+    // Emit a suggestion if possible
+    if let Some((def_id, segment, _)) = segment
+        && segment.args().parenthesized == hir::GenericArgsParentheses::No
+        && let hir::TypeBindingKind::Equality { term: binding_arg_ty } = binding.kind
+    {
+        // Suggests removal of the offending equality constraint
+        let suggest_removal = |e: &mut Diag<'_>| {
+            let mut suggestion_span = binding.span.with_hi(binding.span.hi() + BytePos(1)); // Include the comma or the angle bracket at the end
+            if segment.args().bindings.len() == 1 {
+                // If it is the only binding specified
+                // include the starting angle bracket as well
+                suggestion_span = suggestion_span.with_lo(suggestion_span.lo() - BytePos(1))
+            }
+            if let Ok(suggestion) = tcx.sess.source_map().span_to_snippet(suggestion_span) {
+                e.span_suggestion_verbose(
+                    suggestion_span,
+                    "try removing this type binding",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        };
+
+        // Suggests replacing the quality constraint with
+        // normal type argument
+        let suggest_direct_use = |e: &mut Diag<'_>, sp: Span, is_ty: bool| {
+            if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(sp) {
+                let ty_or_const = if is_ty { "generic" } else { "const" };
+                e.span_suggestion_verbose(
+                    binding.span,
+                    format!("to use `{snippet}` as a {ty_or_const} argument specify it directly"),
+                    snippet,
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        };
+
+        // Get a sense of what generic args the type expects
+        let generics = tcx.generics_of(def_id);
+        let GenericParamCount { mut types, consts, .. } = generics.own_counts();
+        if generics.has_self && types > 0 {
+            types -= 1 // Ignore the `Self` type
+        }
+
+        // Now emit suggestion
+        if types == 0 && consts == 0 {
+            err.note(format!("`{0}` is not a generic type", segment.ident));
+            suggest_removal(&mut err);
+        } else {
+            match binding_arg_ty {
+                hir::Term::Ty(ty) => {
+                    if types > 0 {
+                        suggest_direct_use(&mut err, ty.span, true);
+                    } else {
+                        suggest_removal(&mut err);
+                    }
+                }
+                hir::Term::Const(c) if consts > 0 => {
+                    if consts > 0 {
+                        let span = tcx.hir().span(c.hir_id);
+                        suggest_direct_use(&mut err, span, false);
+                    } else {
+                        suggest_removal(&mut err);
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+
+    err.emit();
 }
 
 pub(crate) fn fn_trait_to_string(

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -454,7 +454,7 @@ pub(crate) fn check_generic_arg_count(
     if gen_pos != GenericArgPosition::Type
         && let Some(b) = gen_args.bindings.first()
     {
-        prohibit_assoc_item_binding(tcx, b.span, None);
+        prohibit_assoc_item_binding(tcx, b, None);
     }
 
     let explicit_late_bound =

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -322,7 +322,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ty::BoundConstness::NotConst,
         );
         if let Some(b) = item_segment.args().bindings.first() {
-            prohibit_assoc_item_binding(self.tcx(), b.span, Some((item_segment, span)));
+            prohibit_assoc_item_binding(self.tcx(), b, Some((def_id, item_segment, span)));
         }
         args
     }
@@ -619,7 +619,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             ty::BoundConstness::NotConst,
         );
         if let Some(b) = item_segment.args().bindings.first() {
-            prohibit_assoc_item_binding(self.tcx(), b.span, Some((item_segment, span)));
+            prohibit_assoc_item_binding(self.tcx(), b, Some((item_def_id, item_segment, span)));
         }
         args
     }
@@ -758,7 +758,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             constness,
         );
         if let Some(b) = trait_segment.args().bindings.first() {
-            prohibit_assoc_item_binding(self.tcx(), b.span, Some((trait_segment, span)));
+            prohibit_assoc_item_binding(self.tcx(), b, Some((trait_def_id, trait_segment, span)));
         }
         ty::TraitRef::new(self.tcx(), trait_def_id, generic_args)
     }
@@ -1724,7 +1724,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         for segment in segments {
             // Only emit the first error to avoid overloading the user with error messages.
             if let Some(b) = segment.args().bindings.first() {
-                prohibit_assoc_item_binding(self.tcx(), b.span, None);
+                prohibit_assoc_item_binding(self.tcx(), b, None);
                 return true;
             }
         }

tests/ui/associated-consts/issue-102335-const.stderr

@@ -3,14 +3,25 @@ error[E0229]: associated type bindings are not allowed here
    |
 LL |     type A: S<C<X = 0i32> = 34>;
    |                 ^^^^^^^^ associated type not allowed here
+   |
+   = note: `C` is not a generic type
+help: try removing this type binding
+   |
+LL |     type A: S<C<X = 0i32> = 34>;
+   |                ~~~~~~~~~~
 
 error[E0229]: associated type bindings are not allowed here
   --> $DIR/issue-102335-const.rs:4:17
    |
 LL |     type A: S<C<X = 0i32> = 34>;
    |                 ^^^^^^^^ associated type not allowed here
    |
+   = note: `C` is not a generic type
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: try removing this type binding
+   |
+LL |     type A: S<C<X = 0i32> = 34>;
+   |                ~~~~~~~~~~
 
 error: aborting due to 2 previous errors
 

tests/ui/associated-type-bounds/issue-102335-ty.rs

@@ -1,5 +1,11 @@
 trait T {
-    type A: S<C<i32 = u32> = ()>;
+    type A: S<C<i32 = u32> = ()>; // Just one erroneous equality constraint
+    //~^ ERROR associated type bindings are not allowed here
+    //~| ERROR associated type bindings are not allowed here
+}
+
+trait T2 {
+    type A: S<C<i32 = u32, X = i32> = ()>; // More than one erroneous equality constraints
     //~^ ERROR associated type bindings are not allowed here
     //~| ERROR associated type bindings are not allowed here
 }

tests/ui/associated-type-bounds/issue-102335-ty.stderr

@@ -1,17 +1,53 @@
 error[E0229]: associated type bindings are not allowed here
   --> $DIR/issue-102335-ty.rs:2:17
    |
-LL |     type A: S<C<i32 = u32> = ()>;
+LL |     type A: S<C<i32 = u32> = ()>; // Just one erroneous equality constraint
    |                 ^^^^^^^^^ associated type not allowed here
+   |
+   = note: `C` is not a generic type
+help: try removing this type binding
+   |
+LL |     type A: S<C<i32 = u32> = ()>; // Just one erroneous equality constraint
+   |                ~~~~~~~~~~~
 
 error[E0229]: associated type bindings are not allowed here
   --> $DIR/issue-102335-ty.rs:2:17
    |
-LL |     type A: S<C<i32 = u32> = ()>;
+LL |     type A: S<C<i32 = u32> = ()>; // Just one erroneous equality constraint
+   |                 ^^^^^^^^^ associated type not allowed here
+   |
+   = note: `C` is not a generic type
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: try removing this type binding
+   |
+LL |     type A: S<C<i32 = u32> = ()>; // Just one erroneous equality constraint
+   |                ~~~~~~~~~~~
+
+error[E0229]: associated type bindings are not allowed here
+  --> $DIR/issue-102335-ty.rs:8:17
+   |
+LL |     type A: S<C<i32 = u32, X = i32> = ()>; // More than one erroneous equality constraints
+   |                 ^^^^^^^^^ associated type not allowed here
+   |
+   = note: `C` is not a generic type
+help: try removing this type binding
+   |
+LL |     type A: S<C<i32 = u32, X = i32> = ()>; // More than one erroneous equality constraints
+   |                 ~~~~~~~~~~
+
+error[E0229]: associated type bindings are not allowed here
+  --> $DIR/issue-102335-ty.rs:8:17
+   |
+LL |     type A: S<C<i32 = u32, X = i32> = ()>; // More than one erroneous equality constraints
    |                 ^^^^^^^^^ associated type not allowed here
    |
+   = note: `C` is not a generic type
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: try removing this type binding
+   |
+LL |     type A: S<C<i32 = u32, X = i32> = ()>; // More than one erroneous equality constraints
+   |                 ~~~~~~~~~~
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0229`.

tests/ui/associated-types/associated-types-eq-2.rs

@@ -1,19 +1,69 @@
 // Test equality constraints on associated types. Check we get an error when an
-// equality constraint is used in a qualified path.
+// equality constraint is used outside of type parameter declarations
 
 pub trait Foo {
     type A;
     fn boo(&self) -> <Self as Foo>::A;
 }
 
 struct Bar;
+struct Qux;
 
 impl Foo for isize {
     type A = usize;
     fn boo(&self) -> usize { 42 }
 }
 
-fn baz<I: Foo>(x: &<I as Foo<A=Bar>>::A) {}
+fn baz<I: Foo>(_x: &<I as Foo<A=Bar>>::A) {}
 //~^ ERROR associated type bindings are not allowed here
 
+
+trait Tr1<T1> {
+}
+
+impl Tr1<T1 = String> for Bar {
+//~^ ERROR associated type bindings are not allowed here
+//~| ERROR trait takes 1 generic argument but 0 generic arguments were supplied
+}
+
+
+trait Tr2<T1, T2, T3> {
+}
+
+// E0229 is emitted only for the first erroneous equality
+// constraint (T2) not for any subequent ones (e.g. T3)
+impl Tr2<i32, T2 = Qux, T3 = usize> for Bar {
+//~^ ERROR associated type bindings are not allowed here
+//~| ERROR trait takes 3 generic arguments but 1 generic argument was supplied
+}
+
+struct GenericStruct<T> { _t: T }
+
+impl Tr2<i32, Qux, T3 = GenericStruct<i32>> for Bar {
+//~^ ERROR associated type bindings are not allowed here
+//~| ERROR trait takes 3 generic arguments but 2 generic arguments were supplied
+}
+
+
+// Covers the case when the type has a const param
+trait Tr3<const N: i32, T2, T3> {
+}
+
+impl Tr3<N = 42, T2 = Qux, T3 = usize> for Bar {
+//~^ ERROR associated type bindings are not allowed here
+//~| ERROR associated const equality is incomplete
+//~| ERROR trait takes 3 generic arguments but 0 generic arguments were supplied
+}
+
+
+// Covers the case when the type
+// in question has no generic params
+trait Tr4 {
+}
+
+impl Tr4<T = Qux, T2 = usize> for Bar {
+//~^ ERROR associated type bindings are not allowed here
+}
+
+
 pub fn main() {}