1 files changed, 33 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0617.md b/compiler/rustc_error_codes/src/error_codes/E0617.md
new file mode 100644
index 000000000..eed384b49
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0617.md
@@ -0,0 +1,33 @@
+Attempted to pass an invalid type of variable into a variadic function.
+
+Erroneous code example:
+
+```compile_fail,E0617
+# use std::os::raw::{c_char, c_int};
+extern "C" {
+    fn printf(format: *const c_char, ...) -> c_int;
+}
+
+unsafe {
+    printf("%f\n\0".as_ptr() as _, 0f32);
+    // error: cannot pass an `f32` to variadic function, cast to `c_double`
+}
+```
+
+Certain Rust types must be cast before passing them to a variadic function,
+because of arcane ABI rules dictated by the C standard. To fix the error,
+cast the value to the type specified by the error message (which you may need
+to import from `std::os::raw`).
+
+In this case, `c_double` has the same size as `f64` so we can use it directly:
+
+```no_run
+# use std::os::raw::{c_char, c_int};
+# extern "C" {
+#     fn printf(format: *const c_char, ...) -> c_int;
+# }
+
+unsafe {
+    printf("%f\n\0".as_ptr() as _, 0f64); // ok!
+}
+```