diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index 4af26e19b370c..eb42831daf87d 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -764,12 +764,28 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                         size = cmp::max(size, field.size);
                     }
 
+                    size = size.align_to(align.abi);
+
+                    if let Abi::ScalarPair(a, b) = &abi {
+                        // Only use `ScalarPair` if there is no padding between
+                        // `a` and `b`, or after `b`, to ensure that every byte
+                        // of a `#[repr(transparent)]` `union` is covered by
+                        // either a byte of `a` or one of `b`.
+                        let a_size = a.value.size(dl);
+                        let b_offset = a_size.align_to(b.value.align(dl).abi);
+                        let has_inner_padding = a_size < b_offset;
+                        let has_trailing_padding = b_offset + b.value.size(dl) < size;
+                        if has_inner_padding || has_trailing_padding {
+                            abi = Abi::Aggregate { sized: true };
+                        }
+                    }
+
                     return Ok(tcx.intern_layout(LayoutDetails {
                         variants: Variants::Single { index },
                         fields: FieldPlacement::Union(variants[index].len()),
                         abi,
                         align,
-                        size: size.align_to(align.abi)
+                        size,
                     }));
                 }
 
diff --git a/src/test/ui/union/union-padding-preserved.rs b/src/test/ui/union/union-padding-preserved.rs
new file mode 100644
index 0000000000000..eb45626a94e0d
--- /dev/null
+++ b/src/test/ui/union/union-padding-preserved.rs
@@ -0,0 +1,87 @@
+// run-pass
+
+// Test that unions don't lose padding bytes (i.e. not covered by any leaf field).
+
+#![feature(core_intrinsics, test, transparent_unions)]
+
+extern crate test;
+
+#[repr(transparent)]
+#[derive(Copy, Clone)]
+union U<T: Copy> { _x: T, y: () }
+
+impl<T: Copy> U<T> {
+    fn uninit() -> Self {
+        U { y: () }
+    }
+
+    unsafe fn write(&mut self, i: usize, v: u8) {
+        (self as *mut _ as *mut u8).add(i).write(v);
+    }
+
+    #[inline(never)]
+    unsafe fn read_rust(self, i: usize) -> u8 {
+        test::black_box((&self as *const _ as *const u8).add(i)).read()
+    }
+
+    #[inline(never)]
+    unsafe extern "C" fn read_c(self, i: usize) -> u8 {
+        test::black_box((&self as *const _ as *const u8).add(i)).read()
+    }
+}
+
+#[derive(Copy, Clone, Default)]
+struct Options {
+    demote_c_to_warning: bool,
+}
+
+unsafe fn check_at<T: Copy>(i: usize, v: u8, opts: Options) {
+    let mut u = U::<T>::uninit();
+    u.write(i, v);
+    let msg = |abi: &str| format!(
+        "check_at::<{}>: {} ABI failed at byte {}",
+        std::intrinsics::type_name::<T>(),
+        abi,
+        i,
+    );
+    if u.read_rust(i) != v {
+        panic!(msg("Rust"));
+    }
+    if u.read_c(i) != v {
+        if opts.demote_c_to_warning {
+            eprintln!("warning: {}", msg("C"));
+        } else {
+            panic!(msg("C"));
+        }
+    }
+}
+
+fn check_all<T: Copy>(opts: Options) {
+    for i in 0..std::mem::size_of::<T>() {
+        unsafe {
+            check_at::<T>(i, 100 + i as u8, opts);
+        }
+    }
+}
+
+#[repr(C, align(16))]
+#[derive(Copy, Clone)]
+struct Align16<T>(T);
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+struct Pair<A, B>(A, B);
+
+fn main() {
+    // NOTE(eddyb) we can't error for the `extern "C"` ABI in these cases, as
+    // the x86_64 SysV calling convention will drop padding bytes inside unions.
+    check_all::<Align16<u8>>(Options { demote_c_to_warning: true });
+    check_all::<Align16<f32>>(Options { demote_c_to_warning: true });
+    check_all::<Align16<f64>>(Options { demote_c_to_warning: true });
+
+    check_all::<(u8, u32)>(Options::default());
+    check_all::<(u8, u64)>(Options::default());
+
+    check_all::<Pair<u8, u32>>(Options::default());
+    check_all::<Pair<u8, u64>>(Options::default());
+}