Account for incorrect where T::Assoc = Ty bound

Provide suggestoin to constrain trait bound for associated type.
Revert incorrect changes to `missing-bounds` test.

Address part of #20041.

Author: estebank

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -23,7 +23,7 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
 use rustc_target::spec::abi;
 use std::mem;
-use std::ops::DerefMut;
+use std::ops::{Deref, DerefMut};
 
 const MORE_EXTERN: &str =
     "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
@@ -1714,6 +1714,53 @@ 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, TraitBoundModifier::None) = bound
+                        {
+                            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.multipart_suggestion(
+                                    &format!(
+                                        "if `{}::{}` is an associated type you're trying to set, \
+                                        use the associated type binding syntax",
+                                        trait_segment.ident, potential_assoc.ident,
+                                    ),
+                                    vec![(span, args), (predicate.span, String::new())],
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
     err.note(
         "see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information",
     );

compiler/rustc_ast_passes/src/lib.rs

@@ -6,6 +6,7 @@
 
 #![feature(iter_is_partitioned)]
 #![feature(box_patterns)]
+#![feature(let_else)]
 #![recursion_limit = "256"]
 
 pub mod ast_validation;

src/test/ui/generic-associated-types/equality-bound.rs

@@ -0,0 +1,15 @@
+fn sum<I: Iterator<Item = ()>>(i: I) -> i32 where I::Item = i32 {
+//~^ ERROR equality constraints are not yet supported in `where` clauses
+    panic!()
+}
+fn sum2<I: Iterator>(i: I) -> i32 where I::Item = i32 {
+//~^ ERROR equality constraints are not yet supported in `where` clauses
+    panic!()
+}
+fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
+//~^ ERROR equality constraints are not yet supported in `where` clauses
+//~| ERROR failed to resolve: use of undeclared type `I`
+    panic!()
+}
+
+fn main() {}

src/test/ui/generic-associated-types/equality-bound.stderr

@@ -0,0 +1,43 @@
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:1:51
+   |
+LL | fn sum<I: Iterator<Item = ()>>(i: I) -> i32 where I::Item = i32 {
+   |                                                   ^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+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  {
+   | 
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:5:41
+   |
+LL | fn sum2<I: Iterator>(i: I) -> i32 where I::Item = i32 {
+   |                                         ^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+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  {
+   | 
+
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/equality-bound.rs:9:41
+   |
+LL | fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
+   |                                         ^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+
+error[E0433]: failed to resolve: use of undeclared type `I`
+  --> $DIR/equality-bound.rs:9:41
+   |
+LL | fn sum3<J: Iterator>(i: J) -> i32 where I::Item = i32 {
+   |                                         ^ use of undeclared type `I`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0433`.

src/test/ui/generic-associated-types/missing-bounds.fixed

@@ -34,11 +34,12 @@ impl<B: std::ops::Add<Output = B>> Add for D<B> {
 
 struct E<B>(B);
 
-impl<B: Add> Add for E<B> where B: Add<Output = B> {
+impl<B: Add> Add for E<B> where B: Add<Output = B>, B: Add<Output = B> {
+    //~^ ERROR equality constraints are not yet supported in `where` clauses
     type Output = Self;
 
     fn add(self, rhs: Self) -> Self {
-        Self(self.0 + rhs.0)
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
     }
 }
 

src/test/ui/generic-associated-types/missing-bounds.rs

@@ -34,11 +34,12 @@ impl<B> Add for D<B> {
 
 struct E<B>(B);
 
-impl<B: Add> Add for E<B> where B: Add<Output = B> {
+impl<B: Add> Add for E<B> where <B as Add>::Output = B {
+    //~^ ERROR equality constraints are not yet supported in `where` clauses
     type Output = Self;
 
     fn add(self, rhs: Self) -> Self {
-        Self(self.0 + rhs.0)
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
     }
 }
 

src/test/ui/generic-associated-types/missing-bounds.stderr

@@ -1,3 +1,15 @@
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/missing-bounds.rs:37:33
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `Output` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL | impl<B: Add> Add for E<B> where B: Add<Output = B> {
+   |                                 ~~~~~~~~~~~~~~~~~~
+
 error[E0308]: mismatched types
   --> $DIR/missing-bounds.rs:11:11
    |
@@ -43,7 +55,23 @@ 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
+error[E0308]: mismatched types
+  --> $DIR/missing-bounds.rs:42:14
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = 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 Add>::Output`
+help: consider further restricting type parameter `B`
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B, B: Add<Output = B> {
+   |                                                       ++++++++++++++++++++
+
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0308, E0369.
 For more information about an error, try `rustc --explain E0308`.