Rollup merge of #76489 - GuillaumeGomez:add-explanation-e0756, r=jyn514

Add explanation for E0756

r? @pickfire
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ecstatic-morse 2020-09-21 20:40:46 -07:00 committed by GitHub
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3 changed files with 31 additions and 1 deletions

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@ -441,6 +441,7 @@
E0753: include_str!("./error_codes/E0753.md"),
E0754: include_str!("./error_codes/E0754.md"),
E0755: include_str!("./error_codes/E0755.md"),
E0756: include_str!("./error_codes/E0756.md"),
E0758: include_str!("./error_codes/E0758.md"),
E0759: include_str!("./error_codes/E0759.md"),
E0760: include_str!("./error_codes/E0760.md"),
@ -633,7 +634,6 @@
E0722, // Malformed `#[optimize]` attribute
E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
// E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
E0756, // `#[ffi_const]` is only allowed on foreign functions
E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`
E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
}

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@ -0,0 +1,29 @@
The `ffi_const` attribute was used on something other than a foreign function
declaration.
Erroneous code example:
```compile_fail,E0756
#![feature(ffi_const)]
#[ffi_const] // error!
pub fn foo() {}
# fn main() {}
```
The `ffi_const` attribute can only be used on foreign function declarations
which have no side effects except for their return value:
```
#![feature(ffi_const)]
extern "C" {
#[ffi_const] // ok!
pub fn strlen(s: *const i8) -> i32;
}
# fn main() {}
```
You can get more information about it in the [unstable Rust Book].
[unstable Rust Book]: https://doc.rust-lang.org/nightly/unstable-book/language-features/ffi-const.html

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@ -6,3 +6,4 @@ LL | #[ffi_const]
error: aborting due to previous error
For more information about this error, try `rustc --explain E0756`.