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More clearly define the default abi of an extern block #2001

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More clearly define the default abi of an extern block #2001

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19 changes: 12 additions & 7 deletions src/items/external-blocks.md
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Expand Up @@ -98,14 +98,19 @@ r[items.extern.abi]
## ABI

r[items.extern.abi.intro]
By default external blocks assume that the library they are calling uses the
standard C ABI on the specific platform. Other ABIs may be specified using an
`abi` string, as shown here:
The `extern` keyword can be followed by an optional <abbr title="application binary interface">ABI</abbr> string. The ABI specifies the calling convention of the functions in the block. The calling convention defines a low-level interface for functions, such as how arguments are placed in registers or on the stack, how return values are passed, and who is responsible for cleaning up the stack.

```rust
// Interface to the Windows API
unsafe extern "system" { }
```
> [!EXAMPLE]
> ```rust
> // Interface to the Windows API
> unsafe extern "system" { }
> ```

r[items.extern.abi.default]
If the ABI string is not specified, it defaults to `"C"`.

> [!NOTE]
> It is recommended to always specify the ABI to make it clear which ABI is used. Future versions of Rust may make this a requirement, see [#134986](https://github.com/rust-lang/rust/issues/134986).

r[items.extern.abi.standard]
The following ABI strings are supported on all platforms:
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