…ynchronizations

…scope`

Calls to `assemble_extension_candidates_for_traits_in_scope` with
`DUMMY_HIR_ID` as `expr_hir_id` are useless because the first thing
that this function does is to return `Ok(())` in this case.

A valid hir id should always be returned in this case.

Those usage ends up forwarded to a `ObligationClause` which uses
`CRATE_HIR_ID` for dummy value as seen in `ObligationClause::dummy`.

…crum

Explain how to work with subtree

cc rust-lang#70651

r? @Centril @RalfJung

This PR just contains the usage documentation, we'll do actual moves in later PRs

…=eddyb

Remove some usage of `DUMMY_HIR_ID`

Add test case for type aliasing `impl Sized`

Fixes rust-lang#71085

allow const generics in const fn

This was explicitly forbidden before. As we were unable to think of a reason
why this should still be the case, this check has been removed.

r? @eddyb
cc @varkor @Centril