Rollup merge of #120751 - estebank:issue-68982, r=nnethercote

Provide more suggestions on invalid equality where bounds

```
error: equality constraints are not yet supported in `where` clauses
  --> $DIR/equality-bound.rs:50:9
   |
LL |         IntoIterator::Item = A
   |         ^^^^^^^^^^^^^^^^^^^^^^ not supported
   |
   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
   |
LL ~     fn from_iter<T: IntoIterator<Item = A>>(_: T) -> Self
LL ~
   |

error: equality constraints are not yet supported in `where` clauses
  --> $DIR/equality-bound.rs:63:9
   |
LL |         T::Item = A
   |         ^^^^^^^^^^^ not supported
   |
   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
   |
LL ~     fn from_iter<T: IntoIterator<Item = A>>(_: T) -> Self
LL ~
   |
```

Fix #68982.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1593,44 +1593,98 @@ fn deny_equality_constraints(
             }
         }
     }
-    // Given `A: Foo, A::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
-    if let TyKind::Path(None, full_path) = &predicate.lhs_ty.kind {
-        if let [potential_param, potential_assoc] = &full_path.segments[..] {
-            for param in &generics.params {
-                if param.ident == potential_param.ident {
-                    for bound in &param.bounds {
-                        if let ast::GenericBound::Trait(trait_ref, TraitBoundModifiers::NONE) =
-                            bound
+
+    let mut suggest =
+        |poly: &PolyTraitRef, potential_assoc: &PathSegment, predicate: &WhereEqPredicate| {
+            if let [trait_segment] = &poly.trait_ref.path.segments[..] {
+                let assoc = pprust::path_to_string(&ast::Path::from_ident(potential_assoc.ident));
+                let ty = pprust::ty_to_string(&predicate.rhs_ty);
+                let (args, span) = match &trait_segment.args {
+                    Some(args) => match args.deref() {
+                        ast::GenericArgs::AngleBracketed(args) => {
+                            let Some(arg) = args.args.last() else {
+                                return;
+                            };
+                            (format!(", {assoc} = {ty}"), arg.span().shrink_to_hi())
+                        }
+                        _ => return,
+                    },
+                    None => (format!("<{assoc} = {ty}>"), trait_segment.span().shrink_to_hi()),
+                };
+                let removal_span = if generics.where_clause.predicates.len() == 1 {
+                    // We're removing th eonly where bound left, remove the whole thing.
+                    generics.where_clause.span
+                } else {
+                    let mut span = predicate.span;
+                    let mut prev: Option<Span> = None;
+                    let mut preds = generics.where_clause.predicates.iter().peekable();
+                    // Find the predicate that shouldn't have been in the where bound list.
+                    while let Some(pred) = preds.next() {
+                        if let WherePredicate::EqPredicate(pred) = pred
+                            && pred.span == predicate.span
                         {
-                            if let [trait_segment] = &trait_ref.trait_ref.path.segments[..] {
-                                let assoc = pprust::path_to_string(&ast::Path::from_ident(
-                                    potential_assoc.ident,
-                                ));
-                                let ty = pprust::ty_to_string(&predicate.rhs_ty);
-                                let (args, span) = match &trait_segment.args {
-                                    Some(args) => match args.deref() {
-                                        ast::GenericArgs::AngleBracketed(args) => {
-                                            let Some(arg) = args.args.last() else {
-                                                continue;
-                                            };
-                                            (format!(", {assoc} = {ty}"), arg.span().shrink_to_hi())
-                                        }
-                                        _ => continue,
-                                    },
-                                    None => (
-                                        format!("<{assoc} = {ty}>"),
-                                        trait_segment.span().shrink_to_hi(),
-                                    ),
-                                };
-                                err.assoc2 = Some(errors::AssociatedSuggestion2 {
-                                    span,
-                                    args,
-                                    predicate: predicate.span,
-                                    trait_segment: trait_segment.ident,
-                                    potential_assoc: potential_assoc.ident,
-                                });
+                            if let Some(next) = preds.peek() {
+                                // This is the first predicate, remove the trailing comma as well.
+                                span = span.with_hi(next.span().lo());
+                            } else if let Some(prev) = prev {
+                                // Remove the previous comma as well.
+                                span = span.with_lo(prev.hi());
                             }
                         }
+                        prev = Some(pred.span());
+                    }
+                    span
+                };
+                err.assoc2 = Some(errors::AssociatedSuggestion2 {
+                    span,
+                    args,
+                    predicate: removal_span,
+                    trait_segment: trait_segment.ident,
+                    potential_assoc: potential_assoc.ident,
+                });
+            }
+        };
+
+    if let TyKind::Path(None, full_path) = &predicate.lhs_ty.kind {
+        // Given `A: Foo, Foo::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
+        for bounds in generics.params.iter().map(|p| &p.bounds).chain(
+            generics.where_clause.predicates.iter().filter_map(|pred| match pred {
+                WherePredicate::BoundPredicate(p) => Some(&p.bounds),
+                _ => None,
+            }),
+        ) {
+            for bound in bounds {
+                if let GenericBound::Trait(poly, TraitBoundModifiers::NONE) = bound {
+                    if full_path.segments[..full_path.segments.len() - 1]
+                        .iter()
+                        .map(|segment| segment.ident.name)
+                        .zip(poly.trait_ref.path.segments.iter().map(|segment| segment.ident.name))
+                        .all(|(a, b)| a == b)
+                        && let Some(potential_assoc) = full_path.segments.iter().last()
+                    {
+                        suggest(poly, potential_assoc, predicate);
+                    }
+                }
+            }
+        }
+        // Given `A: Foo, A::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
+        if let [potential_param, potential_assoc] = &full_path.segments[..] {
+            for (ident, bounds) in generics.params.iter().map(|p| (p.ident, &p.bounds)).chain(
+                generics.where_clause.predicates.iter().filter_map(|pred| match pred {
+                    WherePredicate::BoundPredicate(p)
+                        if let ast::TyKind::Path(None, path) = &p.bounded_ty.kind
+                            && let [segment] = &path.segments[..] =>
+                    {
+                        Some((segment.ident, &p.bounds))
+                    }
+                    _ => None,
+                }),
+            ) {
+                if ident == potential_param.ident {
+                    for bound in bounds {
+                        if let ast::GenericBound::Trait(poly, TraitBoundModifiers::NONE) = bound {
+                            suggest(poly, potential_assoc, predicate);
+                        }
                     }
                 }
             }

tests/ui/generic-associated-types/equality-bound.rs

@@ -12,4 +12,71 @@ fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
     panic!()
 }
 
+use std::iter::FromIterator;
+
+struct X {}
+
+impl FromIterator<bool> for X {
+    fn from_iter<T>(_: T) -> Self where T: IntoIterator, IntoIterator::Item = A,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
+
+struct Y {}
+
+impl FromIterator<bool> for Y {
+    fn from_iter<T>(_: T) -> Self where T: IntoIterator, T::Item = A,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
+
+struct Z {}
+
+impl FromIterator<bool> for Z {
+    fn from_iter<T: IntoIterator>(_: T) -> Self where IntoIterator::Item = A,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
+
+struct K {}
+
+impl FromIterator<bool> for K {
+    fn from_iter<T: IntoIterator>(_: T) -> Self where T::Item = A,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
+
+struct L {}
+
+impl FromIterator<bool> for L {
+    fn from_iter<T>(_: T) -> Self where IntoIterator::Item = A, T: IntoIterator,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
+
+struct M {}
+
+impl FromIterator<bool> for M {
+    fn from_iter<T>(_: T) -> Self where T::Item = A, T: IntoIterator,
+        //~^ ERROR equality constraints are not yet supported in `where` clauses
+        //~| ERROR cannot find type `A` in this scope
+    {
+        todo!()
+    }
+}
 fn main() {}

tests/ui/generic-associated-types/equality-bound.stderr

@@ -8,7 +8,7 @@ LL | fn sum<I: Iterator<Item = ()>>(i: I) -> i32 where I::Item = i32 {
 help: if `Iterator::Item` is an associated type you're trying to set, use the associated type binding syntax
    |
 LL - fn sum<I: Iterator<Item = ()>>(i: I) -> i32 where I::Item = i32 {
-LL + fn sum<I: Iterator<Item = (), Item = i32>>(i: I) -> i32 where  {
+LL + fn sum<I: Iterator<Item = (), Item = i32>>(i: I) -> i32  {
    |
 
 error: equality constraints are not yet supported in `where` clauses
@@ -21,7 +21,7 @@ LL | fn sum2<I: Iterator>(i: I) -> i32 where I::Item = i32 {
 help: if `Iterator::Item` is an associated type you're trying to set, use the associated type binding syntax
    |
 LL - fn sum2<I: Iterator>(i: I) -> i32 where I::Item = i32 {
-LL + fn sum2<I: Iterator<Item = i32>>(i: I) -> i32 where  {
+LL + fn sum2<I: Iterator<Item = i32>>(i: I) -> i32  {
    |
 
 error: equality constraints are not yet supported in `where` clauses
@@ -32,6 +32,138 @@ LL | fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
    |
    = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
 
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:20:58
+   |
+LL |     fn from_iter<T>(_: T) -> Self where T: IntoIterator, IntoIterator::Item = A,
+   |                                                          ^^^^^^^^^^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T>(_: T) -> Self where T: IntoIterator, IntoIterator::Item = A,
+LL +     fn from_iter<T>(_: T) -> Self where T: IntoIterator<Item = A>,
+   |
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:31:58
+   |
+LL |     fn from_iter<T>(_: T) -> Self where T: IntoIterator, T::Item = A,
+   |                                                          ^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T>(_: T) -> Self where T: IntoIterator, T::Item = A,
+LL +     fn from_iter<T>(_: T) -> Self where T: IntoIterator<Item = A>,
+   |
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:42:55
+   |
+LL |     fn from_iter<T: IntoIterator>(_: T) -> Self where IntoIterator::Item = A,
+   |                                                       ^^^^^^^^^^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T: IntoIterator>(_: T) -> Self where IntoIterator::Item = A,
+LL +     fn from_iter<T: IntoIterator<Item = A>>(_: T) -> Self 
+   |
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:53:55
+   |
+LL |     fn from_iter<T: IntoIterator>(_: T) -> Self where T::Item = A,
+   |                                                       ^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T: IntoIterator>(_: T) -> Self where T::Item = A,
+LL +     fn from_iter<T: IntoIterator<Item = A>>(_: T) -> Self 
+   |
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:64:41
+   |
+LL |     fn from_iter<T>(_: T) -> Self where IntoIterator::Item = A, T: IntoIterator,
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T>(_: T) -> Self where IntoIterator::Item = A, T: IntoIterator,
+LL +     fn from_iter<T>(_: T) -> Self where T: IntoIterator<Item = A>,
+   |
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:75:41
+   |
+LL |     fn from_iter<T>(_: T) -> Self where T::Item = A, T: IntoIterator,
+   |                                         ^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `IntoIterator::Item` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL -     fn from_iter<T>(_: T) -> Self where T::Item = A, T: IntoIterator,
+LL +     fn from_iter<T>(_: T) -> Self where T: IntoIterator<Item = A>,
+   |
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:20:79
+   |
+LL |     fn from_iter<T>(_: T) -> Self where T: IntoIterator, IntoIterator::Item = A,
+   |                                                                               ^ help: a struct with a similar name exists: `K`
+...
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:31:68
+   |
+LL |     fn from_iter<T>(_: T) -> Self where T: IntoIterator, T::Item = A,
+   |                                                                    ^ help: a struct with a similar name exists: `K`
+...
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:42:76
+   |
+LL |     fn from_iter<T: IntoIterator>(_: T) -> Self where IntoIterator::Item = A,
+   |                                                                            ^ help: a struct with a similar name exists: `K`
+...
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:53:65
+   |
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+...
+LL |     fn from_iter<T: IntoIterator>(_: T) -> Self where T::Item = A,
+   |                                                                 ^ help: a struct with a similar name exists: `K`
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:64:62
+   |
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+...
+LL |     fn from_iter<T>(_: T) -> Self where IntoIterator::Item = A, T: IntoIterator,
+   |                                                              ^ help: a struct with a similar name exists: `K`
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/equality-bound.rs:75:51
+   |
+LL | struct K {}
+   | -------- similarly named struct `K` defined here
+...
+LL |     fn from_iter<T>(_: T) -> Self where T::Item = A, T: IntoIterator,
+   |                                                   ^ help: a struct with a similar name exists: `K`
+
 error[E0433]: failed to resolve: use of undeclared type `I`
   --> $DIR/equality-bound.rs:9:41
    |
@@ -41,6 +173,7 @@ LL | fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
    |                                         use of undeclared type `I`
    |                                         help: a type parameter with a similar name exists: `J`
 
-error: aborting due to 4 previous errors
+error: aborting due to 16 previous errors
 
-For more information about this error, try `rustc --explain E0433`.
+Some errors have detailed explanations: E0412, E0433.
+For more information about an error, try `rustc --explain E0412`.