Add long explanation for E0757

compiler/rustc_error_codes/src/error_codes.rs

@@ -449,6 +449,7 @@
 E0754: include_str!("./error_codes/E0754.md"),
 E0755: include_str!("./error_codes/E0755.md"),
 E0756: include_str!("./error_codes/E0756.md"),
+E0757: include_str!("./error_codes/E0757.md"),
 E0758: include_str!("./error_codes/E0758.md"),
 E0759: include_str!("./error_codes/E0759.md"),
 E0760: include_str!("./error_codes/E0760.md"),
@@ -638,6 +639,5 @@
 //    E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`
     E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
 }

compiler/rustc_error_codes/src/error_codes/E0757.md

@@ -0,0 +1,25 @@
+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 `const` has a stricter set of requirements than `pure`, remove the `ffi_pure`
+attribute:
+
+```
+#![feature(ffi_const)]
+
+extern "C" {
+    #[ffi_const]
+    pub fn square(num: i32) -> i32;
+}
+```

src/test/ui/ffi_const2.stderr

@@ -6,3 +6,4 @@ LL |     #[ffi_pure]
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0757`.