Rollup merge of #87342 - midgleyc:add-E0757-long, r=GuillaumeGomez

Add long explanation for E0757

Helps with #61137

compiler/rustc_error_codes/src/error_codes/E0757.md

@@ -0,0 +1,33 @@
+A function was given both the `ffi_const` and `ffi_pure` attributes.
+
+Erroneous code example:
+
+```compile_fail,E0757
+#![feature(ffi_const, ffi_pure)]
+
+extern "C" {
+    #[ffi_const]
+    #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]`
+    pub fn square(num: i32) -> i32;
+}
+```
+
+As `ffi_const` provides stronger guarantees than `ffi_pure`, remove the
+`ffi_pure` attribute:
+
+```
+#![feature(ffi_const)]
+
+extern "C" {
+    #[ffi_const]
+    pub fn square(num: i32) -> i32;
+}
+```
+
+You can get more information about `const` and `pure` in the [GCC documentation
+on Common Function Attributes]. The unstable Rust Book has more information
+about [`ffi_const`] and [`ffi_pure`].
+
+[GCC documentation on Common Function Attributes]: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
+[`ffi_const`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/ffi-const.html
+[`ffi_pure`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/ffi-pure.html