Add error explanation for E0755

2 files changed, 29 insertions, 1 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 789a1fc35a6..af9ca7e10fc 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -440,6 +440,7 @@ E0751: include_str!("./error_codes/E0751.md"),
 E0752: include_str!("./error_codes/E0752.md"),
 E0753: include_str!("./error_codes/E0753.md"),
 E0754: include_str!("./error_codes/E0754.md"),
+E0755: include_str!("./error_codes/E0755.md"),
 E0758: include_str!("./error_codes/E0758.md"),
 E0759: include_str!("./error_codes/E0759.md"),
 E0760: include_str!("./error_codes/E0760.md"),
@@ -631,7 +632,6 @@ E0773: include_str!("./error_codes/E0773.md"),
     E0722, // Malformed `#[optimize]` attribute
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0755, // `#[ffi_pure]` is only allowed on foreign functions
     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 {}`.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0755.md b/compiler/rustc_error_codes/src/error_codes/E0755.md
new file mode 100644
index 00000000000..88b7f484969
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0755.md
@@ -0,0 +1,28 @@
+The `ffi_pure` attribute was used on a non-foreign function.
+
+Erroneous code example:
+
+```compile_fail,E0755
+#![feature(ffi_pure)]
+
+#[ffi_pure] // error!
+pub fn foo() {}
+# fn main() {}
+```
+
+The `ffi_pure` attribute can only be used on foreign functions which do not have
+side effects or infinite loops:
+
+```
+#![feature(ffi_pure)]
+
+extern "C" {
+    #[ffi_pure] // ok!
+    pub fn strlen(s: *const i8) -> isize;
+}
+# fn main() {}
+```
+
+You can find more information about it in the [unstable Rust Book].
+
+[unstable Rust Book]: https://doc.rust-lang.org/unstable-book/language-features/ffi-pure.html