Some places use the local `tcx` variable, some use `self.tcx()`. This
commit removes the local variable so that all places use `self.tcx()`,
for consistency.

This wins 6% on `unicode_normalization`, by avoiding a call to
`lub_concrete_regions()` and a `Region` equality test.

The function only has one call site, so we don't need a tag argument.

And make it more uniform with other macros.
By merging placeholders for future derives' outputs into the derive container's output fragment early.

It means an allocation is required to create an empty `TokenStream`, but
all other operations are simpler and marginally faster due to not having
to check for `None`. Overall it simplifies the code for a negligible
performance effect.

The commit also removes `TokenStream::empty` by implementing `Default`,
which is now possible.

This avoids some unnecessary creation of empty token streams.

…=eddyb

Compute the layout of uninhabited structs

fixes rust-lang#64506

r? @eddyb

…hewjasper

expand: Simplify expansion of derives

And make it more uniform with other macros.
This is done by merging placeholders for future derives' outputs into the derive container's output fragment early (addressing FIXMEs from rust-lang#63667).

Also, macros with names starting with `_` are no longer reported as unused, in accordance with the usual behavior of `unused` lints.

r? @matthewjasper or @mark-i-m

…-expansion, r=nikomatsakis

Optimize `LexicalResolve::expansion`.

A win for `unicode_normalization`.

r? @nikomatsakis

…eam, r=petrochenkov

Remove `Option` from `TokenStream`

A code simplification.

r? @petrochenkov