explain how to turn integers into fn ptrs

(with an intermediate raw ptr, not a direct transmute)
Direct int2ptr transmute, under the semantics I am imagining, will produce a ptr with "invalid" provenance that is invalid to deref or call. We cannot give it the same semantics as int2ptr casts since those do [something complicated](https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html).

To my great surprise, that is already what the example in the `transmute` docs does. :)  I still added a comment to say that that part is important, and I added a section explicitly talking about this to the `fn()` type docs.

With rust-lang/miri#2151, Miri will start complaining about direct int-to-fnptr transmutes (in the sense that it is UB to dereference the resulting pointer).

explain how to turn integers into fn ptrs

(with an intermediate raw ptr, not a direct transmute)
Direct int2ptr transmute, under the semantics I am imagining, will produce a ptr with "invalid" provenance that is invalid to deref or call. We cannot give it the same semantics as int2ptr casts since those do [something complicated](https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html).

To my great surprise, that is already what the example in the `transmute` docs does. :)  I still added a comment to say that that part is important, and I added a section explicitly talking about this to the `fn()` type docs.

With rust-lang/miri#2151, Miri will start complaining about direct int-to-fnptr transmutes (in the sense that it is UB to call the resulting pointer).

Rollup of 4 pull requests

Successful merges:

 - rust-lang#97288 (Lifetime variance fixes for rustdoc)
 - rust-lang#97298 (Parse expression after `else` as a condition if followed by `{`)
 - rust-lang#97308 (Stabilize `cell_filter_map`)
 - rust-lang#97321 (explain how to turn integers into fn ptrs)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

enable number validity checking and ptr::invalid checking by default

This removes the `-Zmiri-check-number-validity` flag, enabling its effects by default. (We don't error when the flag is passed, for backwards compatibility.) We also enable by default that transmuting an integer to a pointer now creates a pointer with `None` provenance, which is invalid to dereference (and, in the case of a function pointer, invalid to call). I did this together since it is all related to ptr2int/int2ptr transmutation.

Two new flags are added to optionally take back these stricter checks:
- `-Zmiri-allow-uninit-numbers` makes Miri accept uninit data in integers and floats
- `-Zmiri-allow-ptr-int-transmute` makes Miri accept pointers (provenance data) in integers and floats, *and* makes Miri treat int2ptr transmutes as equivalent to a cast.

The flag names make sense IMO, but they are somewhat inconsistent with our existing flags since we usually call things `-Zmiri-disable-$CHECK` rather than `-Zmiri-allow-$THING`. But `-Zmiri-disable-uninit-number-check` sounds silly?

(Whenever I say "transmute" this includes union and pointer based type punning.)
Cc `@saethlin` I hope this won't break everything?^^ I think the most risky part is the int2ptr transmute aspect, in particular around function pointers where no `as` casts are possible. The correct pattern is to first cast to a raw ptr and then transmute that to a fn ptr. We should probably document this better, in the `transmute` documentation and maybe in the documentation for the `fn()` type. I should run this PR against the std test suite before we land it.
r? `@oli-obk`

- [x] Ensure stdlib docs recommend "usize -> raw ptr -> fn ptr" for int-to-fnptr casts: rust-lang/rust#97321
- [x] Run the stdlib test suite

enable number validity checking and ptr::invalid checking by default

This removes the `-Zmiri-check-number-validity` flag, enabling its effects by default. (We don't error when the flag is passed, for backwards compatibility.) We also enable by default that transmuting an integer to a pointer now creates a pointer with `None` provenance, which is invalid to dereference (and, in the case of a function pointer, invalid to call). I did this together since it is all related to ptr2int/int2ptr transmutation.

Two new flags are added to optionally take back these stricter checks:
- `-Zmiri-allow-uninit-numbers` makes Miri accept uninit data in integers and floats
- `-Zmiri-allow-ptr-int-transmute` makes Miri accept pointers (provenance data) in integers and floats, *and* makes Miri treat int2ptr transmutes as equivalent to a cast.

The flag names make sense IMO, but they are somewhat inconsistent with our existing flags since we usually call things `-Zmiri-disable-$CHECK` rather than `-Zmiri-allow-$THING`. But `-Zmiri-disable-uninit-number-check` sounds silly?

(Whenever I say "transmute" this includes union and pointer based type punning.)
Cc `@saethlin` I hope this won't break everything?^^ I think the most risky part is the int2ptr transmute aspect, in particular around function pointers where no `as` casts are possible. The correct pattern is to first cast to a raw ptr and then transmute that to a fn ptr. We should probably document this better, in the `transmute` documentation and maybe in the documentation for the `fn()` type. I should run this PR against the std test suite before we land it.
r? `@oli-obk`

- [x] Ensure stdlib docs recommend "usize -> raw ptr -> fn ptr" for int-to-fnptr casts: rust-lang/rust#97321
- [x] Run the stdlib test suite