Change how anonymous associated types are printed.

Give them their own symbol `anon_assoc`, as is done for all the other
anonymous `DefPathData` variants.

compiler/rustc_hir/src/definitions.rs

@@ -446,7 +446,7 @@ impl DefPathData {
             Ctor => DefPathDataName::Anon { namespace: sym::constructor },
             AnonConst => DefPathDataName::Anon { namespace: sym::constant },
             OpaqueTy => DefPathDataName::Anon { namespace: sym::opaque },
-            AnonAssocTy => DefPathDataName::Anon { namespace: sym::synthetic },
+            AnonAssocTy => DefPathDataName::Anon { namespace: sym::anon_assoc },
             SyntheticCoroutineBody => DefPathDataName::Anon { namespace: sym::synthetic },
         }
     }

compiler/rustc_span/src/symbol.rs

@@ -454,6 +454,7 @@ symbols! {
         and_then,
         anon,
         anon_adt,
+        anon_assoc,
         anonymous_lifetime_in_impl_trait,
         any,
         append_const_msg,

tests/ui-fulldeps/stable-mir/check_assoc_items.rs

@@ -51,7 +51,7 @@ fn test_assoc_items() -> ControlFlow<()> {
     check_items(
         &trait_assoc_item_defs,
         &[
-            "ATrait::{synthetic#0}",
+            "ATrait::{anon_assoc#0}",
             "ATrait::rpitit",
             "ATrait::Assoc",
             "ATrait::assoc_fn_no_self",
@@ -64,7 +64,7 @@ fn test_assoc_items() -> ControlFlow<()> {
     check_items(
         &impl_assoc_item_defs,
         &[
-            "<AStruct as ATrait>::{synthetic#0}",
+            "<AStruct as ATrait>::{anon_assoc#0}",
             "<AStruct as ATrait>::rpitit",
             "<AStruct as ATrait>::Assoc",
             "<AStruct as ATrait>::assoc_fn_no_self",

tests/ui/delegation/unsupported.stderr

@@ -1,4 +1,4 @@
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:21:5: 21:24>::{synthetic#0}`
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:21:5: 21:24>::{anon_assoc#0}`
   --> $DIR/unsupported.rs:22:25
    |
 LL |         reuse to_reuse::opaque_ret;
@@ -9,7 +9,7 @@ note: ...which requires comparing an impl and trait method signature, inferring
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:21:5: 21:24>::{synthetic#0}`, completing the cycle
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:21:5: 21:24>::{anon_assoc#0}`, completing the cycle
 note: cycle used when checking that `opaque::<impl at $DIR/unsupported.rs:21:5: 21:24>` is well-formed
   --> $DIR/unsupported.rs:21:5
    |
@@ -17,7 +17,7 @@ LL |     impl ToReuse for u8 {
    |     ^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:24:5: 24:25>::{synthetic#0}`
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/unsupported.rs:24:5: 24:25>::{anon_assoc#0}`
   --> $DIR/unsupported.rs:25:24
    |
 LL |         reuse ToReuse::opaque_ret;
@@ -28,7 +28,7 @@ note: ...which requires comparing an impl and trait method signature, inferring
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:24:5: 24:25>::{synthetic#0}`, completing the cycle
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/unsupported.rs:24:5: 24:25>::{anon_assoc#0}`, completing the cycle
 note: cycle used when checking that `opaque::<impl at $DIR/unsupported.rs:24:5: 24:25>` is well-formed
   --> $DIR/unsupported.rs:24:5
    |

tests/ui/impl-trait/in-trait/doesnt-satisfy.stderr

@@ -6,11 +6,11 @@ LL |     fn bar() -> () {}
    |
    = help: the trait `std::fmt::Display` is not implemented for `()`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-note: required by a bound in `Foo::{synthetic#0}`
+note: required by a bound in `Foo::{anon_assoc#0}`
   --> $DIR/doesnt-satisfy.rs:2:22
    |
 LL |     fn bar() -> impl std::fmt::Display;
-   |                      ^^^^^^^^^^^^^^^^^ required by this bound in `Foo::{synthetic#0}`
+   |                      ^^^^^^^^^^^^^^^^^ required by this bound in `Foo::{anon_assoc#0}`
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/in-trait/dump.rs

@@ -8,7 +8,7 @@ trait Foo {
 }
 
 fn hello<'s, T: Foo>(x: &'s T) -> impl Sized + use<'s, T> {
-//~^ ERROR <T as Foo>::{synthetic#0}<'s/#1>
+//~^ ERROR <T as Foo>::{anon_assoc#0}<'s/#1>
     x.hello()
 }
 

tests/ui/impl-trait/in-trait/dump.stderr

@@ -1,4 +1,4 @@
-error: <T as Foo>::{synthetic#0}<'s/#1>
+error: <T as Foo>::{anon_assoc#0}<'s/#1>
   --> $DIR/dump.rs:10:35
    |
 LL | fn hello<'s, T: Foo>(x: &'s T) -> impl Sized + use<'s, T> {

tests/ui/impl-trait/in-trait/return-dont-satisfy-bounds.stderr

@@ -14,11 +14,11 @@ LL |     fn foo<F2: Foo<u8>>(self) -> impl Foo<u8> {
    |                                  ^^^^^^^^^^^^ the trait `Foo<char>` is not implemented for `impl Foo<u8>`
    |
    = help: the trait `Foo<char>` is implemented for `Bar`
-note: required by a bound in `Foo::{synthetic#0}`
+note: required by a bound in `Foo::{anon_assoc#0}`
   --> $DIR/return-dont-satisfy-bounds.rs:2:30
    |
 LL |     fn foo<F2>(self) -> impl Foo<T>;
-   |                              ^^^^^^ required by this bound in `Foo::{synthetic#0}`
+   |                              ^^^^^^ required by this bound in `Foo::{anon_assoc#0}`
 
 error[E0277]: the trait bound `Bar: Foo<u8>` is not satisfied
   --> $DIR/return-dont-satisfy-bounds.rs:8:34

tests/ui/rfcs/rfc-1937-termination-trait/issue-103052-2.stderr

@@ -4,11 +4,11 @@ error[E0277]: the trait bound `Something: Termination` is not satisfied
 LL |         fn main() -> Something {
    |                      ^^^^^^^^^ the trait `Termination` is not implemented for `Something`
    |
-note: required by a bound in `Main::{synthetic#0}`
+note: required by a bound in `Main::{anon_assoc#0}`
   --> $DIR/issue-103052-2.rs:3:27
    |
 LL |         fn main() -> impl std::process::Termination;
-   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Main::{synthetic#0}`
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Main::{anon_assoc#0}`
 
 error: aborting due to 1 previous error