Merge pull request diesel-rs#1429 from weiznich/bugfix/fix_pg_metadata_lookup

Do not use `mem::transmute` on non `#[repr(C)]` structs

Author: weiznich

diesel/src/pg/metadata_lookup.rs

@@ -1,5 +1,3 @@
-use std::mem;
-
 use prelude::*;
 use super::{PgConnection, PgTypeMetadata};
 
@@ -11,7 +9,7 @@ pub struct PgMetadataLookup {
 
 impl PgMetadataLookup {
     pub(crate) fn new(conn: &PgConnection) -> &Self {
-        unsafe { mem::transmute(conn) }
+        unsafe { &*(conn as *const PgConnection as *const PgMetadataLookup) }
     }
 
     /// Determine the type metadata for the given `type_name`