Auto merge of #50354 - varkor:initial-field-alignment-c-int, r=eddyb

Correct initial field alignment for repr(C)/repr(int)

Fixes #50098 following https://github.com/rust-lang/rust/issues/50098#issuecomment-385497333.

(I wasn't sure which kind of test was best suited here — I picked run-pass simply because that was convenient, but if codegen is more appropriate, let me know and I'll change it.)

r? @eddyb

src/test/ui/print_type_sizes/repr_int_c.rs

@@ -0,0 +1,35 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z print-type-sizes
+// compile-pass
+
+// This test makes sure that the tag is not grown for `repr(C)` or `repr(u8)`
+// variants (see https://github.com/rust-lang/rust/issues/50098 for the original bug).
+
+#![feature(start)]
+#![allow(dead_code)]
+
+#[repr(C, u8)]
+enum ReprCu8 {
+    A(u16),
+    B,
+}
+
+#[repr(u8)]
+enum Repru8 {
+    A(u16),
+    B,
+}
+
+#[start]
+fn start(_: isize, _: *const *const u8) -> isize {
+    0
+}

src/test/ui/print_type_sizes/repr_int_c.stdout

@@ -0,0 +1,12 @@
+print-type-size type: `ReprCu8`: 4 bytes, alignment: 2 bytes
+print-type-size     discriminant: 1 bytes
+print-type-size     variant `A`: 3 bytes
+print-type-size         padding: 1 bytes
+print-type-size         field `.0`: 2 bytes, alignment: 2 bytes
+print-type-size     variant `B`: 1 bytes
+print-type-size type: `Repru8`: 4 bytes, alignment: 2 bytes
+print-type-size     discriminant: 1 bytes
+print-type-size     variant `A`: 3 bytes
+print-type-size         padding: 1 bytes
+print-type-size         field `.0`: 2 bytes, alignment: 2 bytes
+print-type-size     variant `B`: 0 bytes