Rollup merge of rust-lang#131857 - WaffleLapkin:dyn-drop-principal-3, r=compiler-errors

Allow dropping dyn principal

Revival of rust-lang#126660, which was a revival of rust-lang#114679. Fixes rust-lang#126313.

Allows dropping principal when coercing trait objects, e.g. `dyn Debug + Send` -> `dyn Send`.

cc `@compiler-errors` `@Jules-Bertholet`
r? `@lcnr`

…l-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

rust-lang#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of rust-lang#114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in rust-lang#126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes rust-lang#139418

r? `@oli-obk` (or anyone)

…l-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

rust-lang#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of rust-lang#114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in rust-lang#126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes rust-lang#139418

r? ``@oli-obk`` (or anyone)

…l-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

rust-lang#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of rust-lang#114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in rust-lang#126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes rust-lang#139418

r? ```@oli-obk``` (or anyone)

…l-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

rust-lang#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of rust-lang#114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in rust-lang#126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes rust-lang#139418

r? ````@oli-obk```` (or anyone)

Rollup merge of rust-lang#139421 - compiler-errors:upcast-no-principal-with-proj, r=oli-obk

Fix trait upcasting to dyn type with no principal when there are projections

rust-lang#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of rust-lang#114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in rust-lang#126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes rust-lang#139418

r? ````@oli-obk```` (or anyone)