Auto merge of #1836 - alexcrichton:fix-nightly, r=alexcrichton

The lints for C apis were tweaked recently and 0-size structs aren't liked.
These should be empty `enum`s anyway, however, to ensure they're never created.

Author: bors

src/cargo/util/sha256.rs

@@ -3,12 +3,13 @@ pub use self::imp::Sha256;
 // Someone upstream will link to OpenSSL, so we don't need to explicitly
 // link to it ourselves. Hence we pick up Sha256 digests from OpenSSL
 #[cfg(not(windows))]
+#[allow(bad_style)]
 mod imp {
     use libc;
 
-    #[repr(C)] struct EVP_MD_CTX;
-    #[repr(C)] struct EVP_MD;
-    #[repr(C)] struct ENGINE;
+    enum EVP_MD_CTX {}
+    enum EVP_MD {}
+    enum ENGINE {}
 
     extern {
         fn EVP_DigestInit_ex(ctx: *mut EVP_MD_CTX,