Add long error explanation for E0634

src/librustc_error_codes/error_codes.rs

@@ -351,6 +351,7 @@ E0626: include_str!("./error_codes/E0626.md"),
 E0627: include_str!("./error_codes/E0627.md"),
 E0631: include_str!("./error_codes/E0631.md"),
 E0633: include_str!("./error_codes/E0633.md"),
+E0634: include_str!("./error_codes/E0634.md"),
 E0635: include_str!("./error_codes/E0635.md"),
 E0636: include_str!("./error_codes/E0636.md"),
 E0637: include_str!("./error_codes/E0637.md"),
@@ -587,7 +588,6 @@ E0748: include_str!("./error_codes/E0748.md"),
     E0630,
     E0632, // cannot provide explicit generic arguments when `impl Trait` is
            // used in argument position
-    E0634, // type has conflicting packed representaton hints
     E0640, // infer outlives requirements
 //  E0645, // trait aliases not finished
     E0657, // `impl Trait` can only capture lifetimes bound at the fn level

src/librustc_error_codes/error_codes/E0634.md

@@ -0,0 +1,20 @@
+A type has conflicting `packed` representation hints.
+
+Erroneous code examples:
+
+```compile_fail,E0634
+#[repr(packed, packed(2))] // error!
+struct Company(i32);
+
+#[repr(packed(2))] // error!
+#[repr(packed)]
+struct Company(i32);
+```
+
+You cannot use conflicting `packed` hints on a same type. If you want to pack a
+type to a given size, you should provide a size to packed:
+
+```
+#[repr(packed)] // ok!
+struct Company(i32);
+```

src/test/ui/conflicting-repr-hints.stderr

@@ -76,5 +76,5 @@ LL | | }
 
 error: aborting due to 10 previous errors
 
-Some errors have detailed explanations: E0566, E0587.
+Some errors have detailed explanations: E0566, E0587, E0634.
 For more information about an error, try `rustc --explain E0566`.