cmse: disallow `impl Trait` in `cmse-nonsecure-entry` return types
tracking issue: https://github.com/rust-lang/rust/issues/75835
fixes https://github.com/rust-lang/rust/issues/147242
Refactors some logic to be more robust in the future, and then disallows `impl Trait` as a return type for the cmse ABIs.
The `is_valid_cmse_output_layout` function disallows `union` values like before. That is not entirely correct, but preserves the current behavior. Some additional logic is needed for `union` values (and any types where parts may be uninitialized) that I'll tackle in a later PR.
can be reviewed commit-by-commit.
r? types
Split out the `extern_system_varargs` feature
After the stabilization PR was opened, `extern "system"` functions were added to `extended_varargs_abi_support`. This has a number of questions regarding it that were not discussed and were somewhat surprising. It deserves to be considered as its own feature, separate from `extended_varargs_abi_support`.
Tracking issue:
- https://github.com/rust-lang/rust/issues/136946
After the stabilization PR was opened, `extern "system"` functions were
added to `extended_varargs_abi_support`. This has a number of questions
regarding it that were not discussed and were somewhat surprising.
It deserves to be considered as its own feature, separate from
`extended_varargs_abi_support`.
```
error[E0610]: `{integer}` is a primitive type and therefore doesn't have fields
--> $DIR/attempted-access-non-fatal.rs:7:15
|
LL | let _ = 2.l;
| ^
|
help: if intended to be a floating point literal, consider adding a `0` after the period and a `f64` suffix
|
LL - let _ = 2.l;
LL + let _ = 2.0f64;
|
```
when the `C-cmse-nonsecure-call` ABI is used, arguments and return values must be passed via registers. Failing to do so (i.e. spilling to the stack) causes an LLVM error down the line, but now rustc will properly emit an error a bit earlier in the chain
