Rollup merge of rust-lang#135971 - compiler-errors:self-projection, r=fmease

Properly report error when object type param default references self

I accidentally broke this error for cases where a type parameter references `Self` via a projection (i.e. `trait Foo<Arg = Self::Bar> {}`). This PR fixes that, and also makes the error a bit easier to understand.

Fixes rust-lang#135918

Author: matthiaskrgr

compiler/rustc_error_codes/src/error_codes/E0393.md

@@ -3,12 +3,10 @@ A type parameter which references `Self` in its default value was not specified.
 Erroneous code example:
 
 ```compile_fail,E0393
-trait A<T=Self> {}
+trait A<T = Self> {}
 
-fn together_we_will_rule_the_galaxy(son: &A) {}
-// error: the type parameter `T` must be explicitly specified in an
-//        object type because its default value `Self` references the
-//        type `Self`
+fn together_we_will_rule_the_galaxy(son: &dyn A) {}
+// error: the type parameter `T` must be explicitly specified
 ```
 
 A trait object is defined over a single, fully-defined trait. With a regular
@@ -23,7 +21,7 @@ disallowed. Making the trait concrete by explicitly specifying the value of the
 defaulted parameter will fix this issue. Fixed example:
 
 ```
-trait A<T=Self> {}
+trait A<T = Self> {}
 
-fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
+fn together_we_will_rule_the_galaxy(son: &dyn A<i32>) {} // Ok!
 ```

compiler/rustc_hir_analysis/messages.ftl

@@ -353,7 +353,13 @@ hir_analysis_missing_type_params =
         [one] reference
         *[other] references
     } to {$parameters}
-    .note = because of the default `Self` reference, type parameters must be specified on object types
+    .note = because the parameter {$parameterCount ->
+        [one] default references
+        *[other] defaults reference
+    } `Self`, the {$parameterCount ->
+        [one] parameter
+        *[other] parameters
+    } must be specified on the object type
 
 hir_analysis_multiple_relaxed_default_bounds =
     type parameter has more than one relaxed default bound, only one is supported

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -237,16 +237,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     // Skip `Self`
                     .skip(1)
                     .map(|(index, arg)| {
-                        if arg == dummy_self.into() {
+                        if arg.walk().any(|arg| arg == dummy_self.into()) {
                             let param = &generics.own_params[index];
                             missing_type_params.push(param.name);
                             Ty::new_misc_error(tcx).into()
-                        } else if arg.walk().any(|arg| arg == dummy_self.into()) {
-                            let guar = self.dcx().span_delayed_bug(
-                                span,
-                                "trait object trait bounds reference `Self`",
-                            );
-                            replace_dummy_self_with_error(tcx, arg, guar)
                         } else {
                             arg
                         }

tests/ui/dyn-compatibility/default-param-self-projection.rs

@@ -0,0 +1,17 @@
+trait A<C = <Self as D>::E> {}
+
+trait D {
+    type E;
+}
+
+impl A<()> for () {}
+impl D for () {
+    type E = ();
+}
+
+fn f() {
+    let B: &dyn A = &();
+    //~^ ERROR the type parameter `C` must be explicitly specified
+}
+
+fn main() {}

tests/ui/dyn-compatibility/default-param-self-projection.stderr

@@ -0,0 +1,18 @@
+error[E0393]: the type parameter `C` must be explicitly specified
+  --> $DIR/default-param-self-projection.rs:13:17
+   |
+LL | trait A<C = <Self as D>::E> {}
+   | --------------------------- type parameter `C` must be specified for this
+...
+LL |     let B: &dyn A = &();
+   |                 ^
+   |
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
+help: set the type parameter to the desired type
+   |
+LL |     let B: &dyn A<C> = &();
+   |                  +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0393`.

tests/ui/error-codes/E0393.stderr

@@ -7,7 +7,7 @@ LL |
 LL | fn together_we_will_rule_the_galaxy(son: &dyn A) {}
    |                                               ^
    |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
 help: set the type parameter to the desired type
    |
 LL | fn together_we_will_rule_the_galaxy(son: &dyn A<T>) {}

tests/ui/issues/issue-22370.stderr

@@ -7,7 +7,7 @@ LL |
 LL | fn f(a: &dyn A) {}
    |              ^
    |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
 help: set the type parameter to the desired type
    |
 LL | fn f(a: &dyn A<T>) {}

tests/ui/traits/object/canonicalize-fresh-infer-vars-issue-103626.rs

@@ -10,6 +10,7 @@ fn w<'a, T: 'a, F: Fn(&'a T)>() {
     let b: &dyn FromResidual = &();
     //~^ ERROR: the trait `FromResidual` is not dyn compatible
     //~| ERROR: the trait `FromResidual` is not dyn compatible
+    //~| ERROR the type parameter `R` must be explicitly specified
 }
 
 fn main() {}

tests/ui/traits/object/canonicalize-fresh-infer-vars-issue-103626.stderr

@@ -1,3 +1,18 @@
+error[E0393]: the type parameter `R` must be explicitly specified
+  --> $DIR/canonicalize-fresh-infer-vars-issue-103626.rs:10:17
+   |
+LL | trait FromResidual<R = <Self as Try>::Residual> {
+   | ----------------------------------------------- type parameter `R` must be specified for this
+...
+LL |     let b: &dyn FromResidual = &();
+   |                 ^^^^^^^^^^^^
+   |
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
+help: set the type parameter to the desired type
+   |
+LL |     let b: &dyn FromResidual<R> = &();
+   |                             +++
+
 error[E0038]: the trait `FromResidual` is not dyn compatible
   --> $DIR/canonicalize-fresh-infer-vars-issue-103626.rs:10:32
    |
@@ -45,6 +60,7 @@ help: alternatively, consider constraining `from_residual` so it does not apply
 LL |     fn from_residual(residual: R) -> Self where Self: Sized;
    |                                           +++++++++++++++++
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0038`.
+Some errors have detailed explanations: E0038, E0393.
+For more information about an error, try `rustc --explain E0038`.

tests/ui/traits/unspecified-self-in-trait-ref.stderr

@@ -97,7 +97,7 @@ LL | pub trait Bar<X=usize, A=Self> {
 LL |     let e = Bar::<usize>::lol();
    |             ^^^^^^^^^^^^ missing reference to `A`
    |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
 
 error: aborting due to 5 previous errors; 5 warnings emitted
 

tests/ui/type/type-parameter-defaults-referencing-Self.stderr

@@ -7,7 +7,7 @@ LL | trait Foo<T=Self> {
 LL | fn foo(x: &dyn Foo) { }
    |                ^^^
    |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
+   = note: because the parameter default references `Self`, the parameter must be specified on the object type
 help: set the type parameter to the desired type
    |
 LL | fn foo(x: &dyn Foo<T>) { }