extend extern tests to include FiveU16s

As described in the code, this extends just beyond a 64bit reg, but
isn't a round number, so it triggers some edge cases in the cast ABI.

tests/auxiliary/rust_test_helpers.c

@@ -118,6 +118,30 @@ rust_dbg_extern_identity_TwoDoubles(struct TwoDoubles u) {
     return u;
 }
 
+struct FiveU16s {
+    uint16_t one;
+    uint16_t two;
+    uint16_t three;
+    uint16_t four;
+    uint16_t five;
+};
+
+struct FiveU16s
+rust_dbg_extern_return_FiveU16s() {
+    struct FiveU16s s;
+    s.one = 10;
+    s.two = 20;
+    s.three = 30;
+    s.four = 40;
+    s.five = 50;
+    return s;
+}
+
+struct FiveU16s
+rust_dbg_extern_identity_FiveU16s(struct FiveU16s u) {
+    return u;
+}
+
 struct ManyInts {
     int8_t arg1;
     int16_t arg2;

tests/ui/abi/extern/extern-pass-FiveU16s.rs

@@ -0,0 +1,30 @@
+//@ run-pass
+#![allow(improper_ctypes)]
+
+// Test a foreign function that accepts and returns a struct by value.
+
+// FiveU16s in particular is interesting because it is larger than a single 64 bit or 32 bit
+// register, which are used as cast destinations on some targets, but does not evenly divide those
+// sizes, causing there to be padding in the last element.
+
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub struct FiveU16s {
+    one: u16,
+    two: u16,
+    three: u16,
+    four: u16,
+    five: u16,
+}
+
+#[link(name = "rust_test_helpers", kind = "static")]
+extern "C" {
+    pub fn rust_dbg_extern_identity_FiveU16s(v: FiveU16s) -> FiveU16s;
+}
+
+pub fn main() {
+    unsafe {
+        let x = FiveU16s { one: 22, two: 23, three: 24, four: 25, five: 26 };
+        let y = rust_dbg_extern_identity_FiveU16s(x);
+        assert_eq!(x, y);
+    }
+}

tests/ui/abi/extern/extern-return-FiveU16s.rs

@@ -0,0 +1,26 @@
+//@ run-pass
+#![allow(improper_ctypes)]
+
+pub struct FiveU16s {
+    one: u16,
+    two: u16,
+    three: u16,
+    four: u16,
+    five: u16,
+}
+
+#[link(name = "rust_test_helpers", kind = "static")]
+extern "C" {
+    pub fn rust_dbg_extern_return_FiveU16s() -> FiveU16s;
+}
+
+pub fn main() {
+    unsafe {
+        let y = rust_dbg_extern_return_FiveU16s();
+        assert_eq!(y.one, 10);
+        assert_eq!(y.two, 20);
+        assert_eq!(y.three, 30);
+        assert_eq!(y.four, 40);
+        assert_eq!(y.five, 50);
+    }
+}