Rollup merge of #113560 - fmease:assoc-tys-in-traits-depr-wc-loc, r=compiler-errors

Lint against misplaced where-clauses on associated types in traits

Extends the scope of the lint `deprecated_where_clause_location` (#89122) from associated types in impls to associated types in any location (impl or trait). This is only relevant for `#![feature(associated_type_defaults)]`. Previously we didn't warn on the following code for example:

```rs
#![feature(associated_type_defaults)]
trait Trait { type Assoc where u32: Copy = (); }
```

Personally I would've preferred to emit a *hard* error here instead of a lint warning since the feature is unstable but unfortunately we are constrained by back compat as associated type defaults won't necessarily trigger the feature-gate error if they are inside of a macro call (since they use a post-expansion feature-gate due to historical reasons, see also #66004).

I've renamed and moved related preexisting tests: 1. They test AST validation passes not the parser & thus shouldn't live in `parser/` (historical reasons?). 2. One test file was named after type aliases even though it tests assoc tys.

`@rustbot` label A-lint

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1300,14 +1300,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         });
                     }
                 }
-                AssocItemKind::Type(box TyAlias {
-                    generics,
-                    where_clauses,
-                    where_predicates_split,
-                    bounds,
-                    ty,
-                    ..
-                }) => {
+                AssocItemKind::Type(box TyAlias { bounds, ty, .. }) => {
                     if ty.is_none() {
                         self.session.emit_err(errors::AssocTypeWithoutBody {
                             span: item.span,
@@ -1315,18 +1308,26 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         });
                     }
                     self.check_type_no_bounds(bounds, "`impl`s");
-                    if ty.is_some() {
-                        self.check_gat_where(
-                            item.id,
-                            generics.where_clause.predicates.split_at(*where_predicates_split).0,
-                            *where_clauses,
-                        );
-                    }
                 }
                 _ => {}
             }
         }
 
+        if let AssocItemKind::Type(box TyAlias {
+            generics,
+            where_clauses,
+            where_predicates_split,
+            ty: Some(_),
+            ..
+        }) = &item.kind
+        {
+            self.check_gat_where(
+                item.id,
+                generics.where_clause.predicates.split_at(*where_predicates_split).0,
+                *where_clauses,
+            );
+        }
+
         if ctxt == AssocCtxt::Trait || self.in_trait_impl {
             self.visibility_not_permitted(&item.vis, errors::VisibilityNotPermittedNote::TraitImpl);
             if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {

compiler/rustc_lint_defs/src/builtin.rs

@@ -4084,7 +4084,7 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The preferred location for where clauses on associated types in impls
+    /// The preferred location for where clauses on associated types
     /// is after the type. However, for most of generic associated types development,
     /// it was only accepted before the equals. To provide a transition period and
     /// further evaluate this change, both are currently accepted. At some point in

tests/ui/where-clauses/where-clause-placement-assoc-type-in-impl.stderr

@@ -1,5 +1,5 @@
 warning: where clause not allowed here
-  --> $DIR/type-alias-where-fixable.rs:13:16
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:13:16
    |
 LL |     type Assoc where u32: Copy = ();
    |                ^^^^^^^^^^^^^^^
@@ -13,7 +13,7 @@ LL +     type Assoc  = () where u32: Copy;
    |
 
 warning: where clause not allowed here
-  --> $DIR/type-alias-where-fixable.rs:16:17
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:16:17
    |
 LL |     type Assoc2 where u32: Copy = () where i32: Copy;
    |                 ^^^^^^^^^^^^^^^
@@ -26,7 +26,7 @@ LL +     type Assoc2  = () where i32: Copy, u32: Copy;
    |
 
 warning: where clause not allowed here
-  --> $DIR/type-alias-where-fixable.rs:24:17
+  --> $DIR/where-clause-placement-assoc-type-in-impl.rs:24:17
    |
 LL |     type Assoc2 where u32: Copy, i32: Copy = ();
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/ui/where-clauses/where-clause-placement-assoc-type-in-trait.fixed

@@ -0,0 +1,15 @@
+// check-pass
+// run-rustfix
+
+#![feature(associated_type_defaults)]
+
+trait Trait {
+    // Not fine, suggests moving.
+    type Assoc  = () where u32: Copy;
+    //~^ WARNING where clause not allowed here
+    // Not fine, suggests moving `u32: Copy`
+    type Assoc2  = () where i32: Copy, u32: Copy;
+    //~^ WARNING where clause not allowed here
+}
+
+fn main() {}

tests/ui/where-clauses/where-clause-placement-assoc-type-in-trait.rs

@@ -0,0 +1,15 @@
+// check-pass
+// run-rustfix
+
+#![feature(associated_type_defaults)]
+
+trait Trait {
+    // Not fine, suggests moving.
+    type Assoc where u32: Copy = ();
+    //~^ WARNING where clause not allowed here
+    // Not fine, suggests moving `u32: Copy`
+    type Assoc2 where u32: Copy = () where i32: Copy;
+    //~^ WARNING where clause not allowed here
+}
+
+fn main() {}

tests/ui/where-clauses/where-clause-placement-assoc-type-in-trait.stderr

@@ -0,0 +1,29 @@
+warning: where clause not allowed here
+  --> $DIR/where-clause-placement-assoc-type-in-trait.rs:8:16
+   |
+LL |     type Assoc where u32: Copy = ();
+   |                ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+   = note: `#[warn(deprecated_where_clause_location)]` on by default
+help: move it to the end of the type declaration
+   |
+LL -     type Assoc where u32: Copy = ();
+LL +     type Assoc  = () where u32: Copy;
+   |
+
+warning: where clause not allowed here
+  --> $DIR/where-clause-placement-assoc-type-in-trait.rs:11:17
+   |
+LL |     type Assoc2 where u32: Copy = () where i32: Copy;
+   |                 ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
+help: move it to the end of the type declaration
+   |
+LL -     type Assoc2 where u32: Copy = () where i32: Copy;
+LL +     type Assoc2  = () where i32: Copy, u32: Copy;
+   |
+
+warning: 2 warnings emitted
+

tests/ui/where-clauses/where-clause-placement-type-alias.stderr

@@ -1,5 +1,5 @@
 error: where clauses are not allowed after the type for type aliases
-  --> $DIR/type-alias-where.rs:6:15
+  --> $DIR/where-clause-placement-type-alias.rs:6:15
    |
 LL | type Bar = () where u32: Copy;
    |               ^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL | type Bar = () where u32: Copy;
    = note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
 
 error: where clauses are not allowed after the type for type aliases
-  --> $DIR/type-alias-where.rs:8:15
+  --> $DIR/where-clause-placement-type-alias.rs:8:15
    |
 LL | type Baz = () where;
    |               ^^^^^