Don't emit non-asm contents error for naked function composed of errors
## Motivation
For naked functions an error is emitted when they are composed of anything other than a single asm!() block. However, this error triggers in a couple situations in which it adds no additional information or is actively misleading.
One example is if you do have an asm!() block but simply one with a syntax error:
```rust
#[naked]
unsafe extern "C" fn compiler_errors() {
asm!(invalid_syntax)
}
```
This results in two errors, one for the syntax error itself and another telling you that you need an asm block in your function:
```rust
error[E0787]: naked functions must contain a single asm block
--> src/main.rs:6:1
|
6 | / unsafe extern "C" fn naked_compile_error() {
7 | | asm!(blah)
8 | | }
| |_^
```
This issue also comes up when [utilizing `compile_error!()` for improving your diagnostics](https://twitter.com/steveklabnik/status/1509538243020218372), such as raising a compiler error when compiling for an unsupported target.
## Implementation
The rules this PR implements are as follows:
1. If any non-erroneous non-asm statement is included, an error will still occur
2. If multiple asm statements are included, an error will still occur
3. If 0 or 1 asm statements are present, as well as any non-zero number of erroneous statements, then this error will *not* be raised as it is likely either redundant or incorrect
The rule of thumb is effectively "if an error is present and its correction could change things, don't raise an error".
Transition unsupported naked functions future incompatibility lint into
an error:
* Naked functions must contain a single inline assembly block.
Introduced as future incompatibility lint in 1.50 #79653.
Change into an error fixes a soundness issue described in #32489.
* Naked functions must not use any forms of inline attribute.
Introduced as future incompatibility lint in 1.56 #87652.
Replace `NestedVisitorMap` with generic `NestedFilter`
This is an attempt to make the `intravisit::Visitor` API simpler and "more const" with regard to nested visiting.
With this change, `intravisit::Visitor` does not visit nested things by default, unless you specify `type NestedFilter = nested_filter::OnlyBodies` (or `All`). `nested_visit_map` returns `Self::Map` instead of `NestedVisitorMap<Self::Map>`. It panics by default (unreachable if `type NestedFilter` is omitted).
One somewhat trixty thing here is that `nested_filter::{OnlyBodies, All}` live in `rustc_middle` so that they may have `type Map = map::Map` and so that `impl Visitor`s never need to specify `type Map` - it has a default of `Self::NestedFilter::Map`.
This check was previously categorized under the lint named
`UNSUPPORTED_NAKED_FUNCTIONS`. That lint is future incompatible and will
be turned into an error in a future release. However, as defined in the
Constrained Naked Functions RFC, this check should only be a warning.
This is because it is possible for a naked function to be implemented in
such a way that it does not break even the undefined ABI. For example, a
`jmp` to a `const`.
Therefore, this patch defines a new lint named
`UNDEFINED_NAKED_FUNCTION_ABI` which contains just this single check.
Unlike `UNSUPPORTED_NAKED_FUNCTIONS`, `UNDEFINED_NAKED_FUNCTION_ABI`
will not be converted to an error in the future.
rust-lang/rfcs#2774rust-lang/rfcs#2972
* Reject use of parameters inside naked function body.
* Reject use of patterns inside function parameters, to emphasize role
of parameters a signature declaration (mirroring existing behaviour
for function declarations) and avoid generating code introducing
specified bindings.
