Support defining C compatible variadic functions
## Summary
Add support for defining C compatible variadic functions in unsafe rust with
`extern "C"` according to [RFC 2137].
## Details
### Parsing
When parsing a user defined function that is `unsafe` and `extern "C"` allow
variadic signatures and inject a "spoofed" `VaList` in the new functions
signature. This allows the user to interact with the variadic arguments via a
`VaList` instead of manually using `va_start` and `va_end` (See [RFC 2137] for
details).
### Codegen
When running codegen for a variadic function, remove the "spoofed" `VaList`
from the function signature and inject `va_start` when the arg local
references are created for the function and `va_end` on return.
## TODO
- [x] Get feedback on injecting `va_start/va_end` in MIR vs codegen
- [x] Properly inject `va_end` - It seems like it should be possible to inject
`va_end` on the `TerminatorKind::Return`. I just need to figure out how
to get the `LocalRef` here.
- [x] Properly call Rust defined C variadic functions in Rust - The spoofed
`VaList` causes problems here.
Related to: #44930
r? @ghost
[RFC 2137]: https://github.com/rust-lang/rfcs/blob/master/text/2137-variadic.md
Make `Unique::as_ptr` const without feature attribute as it's unstable
Make `NonNull::dangling` and `NonNull::cast` const with `feature = "const_ptr_nonnull"`
We'll use this as a temporary measure to get an LLVM update landed, but
we'll have to go through and update images later to make sure they've
got the right toolchains.
Function signatures with the `variadic` member set are actually
C-variadic functions. Make this a little more explicit by renaming the
`variadic` boolean value, `c_variadic`.
- Move closures defined in codegen_terminator into a separate helper
structure and implementation.
- Create helper functions for each of the complex match arms on the
terminators kind in codegen_terminator.
Normalize the type Self resolves to in an impl
This is required at the very least in order to evaluate associated
constants for arrays.
Fixes#57796Fixes#58212.
r? @oli-obk
cc @hellow554
update scoped_tls to 1.0
scoped_tls has been updated to version 1.0
This PR will hopefully merge flawlessly :)
This fixes, among others, https://github.com/alexcrichton/scoped-tls/issues/9
Note, that the nightly feature has been removed in 64bd7b84a1
std: docs: Disable running several Stdio doctests
* A number of `Stdio` related doc examples include running the "rev"
command to illustrate piping commands. The majority of these tests are
marked as `no_run` except for two tests which were not
* Not running these tests is unlikely to cause any negative impact, and
doing so also allows the test suite to pass in environments where the
"rev" command is unavailable
