Improvements to the README item on `-Zmiri-track-raw-pointers`.
[Rendered](https://github.com/jrvanwhy/miri/tree/track-raw-pointers-doc)
Minor change: I changed the quotes around `<untagged>` into backticks, so they render correctly in markdown.
~~Significant change: I documented that `-Zmiri-track-raw-pointers` is a strictly more restrictive model that "normal" Stacked Borrows. **I am not confident this change is correct, please verify it.** If this change is not correct, let me know, and I'll update this PR to document that :-)~~
EDIT: I was wrong, `-Zmiri-track-raw-pointers` may not be strictly more restrictive. I added the following sentence to prevent others from making the same assumption that I did:
> Note that it is not currently guaranteed that code that works with `-Zmiri-track-raw-pointers` also works without `-Zmiri-track-raw-pointers`.
1. The double quotes around <untagged> are changed to backspaces, so <untagged>
will render correctly in markdown.
2. Clarify that -Zmiri-track-raw-pointers will never accept code that Miri
would not have accepted without -Zmiri-track-raw-pointers.
Improve error message of calling unsupported non-"C"/"system"-ABI foreign function
Miri currently reports the following `foo()` call has ABI-mismatch UB:
```rust
#[cfg(unix)]
extern "Rust" { // or any non-"C" ABI
fn foo();
}
#[cfg(windows)]
extern "C" { // or any non-"system" ABI
fn foo();
}
fn main() {
unsafe {
foo();
}
}
```
[Output when targeting Linux](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=72afc3bd4d9fdab962422cfc2c5a2166) (and maybe also macOS):
```
error: Undefined Behavior: calling a function with ABI C using caller ABI Rust
--> src/main.rs:13:9
|
13 | foo();
| ^^^^^ calling a function with ABI C using caller ABI Rust
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavio
```
Output when targeting Windows:
```
error: Undefined Behavior: calling a function with ABI system using caller ABI C
--> <anon>:13:9
|
13 | foo();
| ^^^^^ calling a function with ABI system using caller ABI C
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
```
However, to my knowledge, that's not UB -- it's just unsupported by Miri (and Miri can't assume the function has `"C"` or `"system"` ABI since Miri doesn't know about it). I believe that is because of the overzealous `check_abi()` call before the long `match` in `src/shims/{posix,windows}/foreign_items.rs`. The ABI is checked to match the system one (`"system"` on Windows, `"C"` otherwise) no matter the callee is recognized as a shim or an unsupported foreign function.
Therefore, this PR removes the `check_abi()` call before the `match` and inserts a `check_abi()` call to each non-wildcard match.
