Missed optimization: layout optimized enums produce slow derived code

[oli-obk]

Changing

const FOO_A: u32 = 0xFFFF_FFFF;
const FOO_B: u32 = 0xFFFF_FFFE;
const BAR_X: u32 = 0;
const BAR_Y: u32 = 1;
const BAR_Z: u32 = 2;
struct Foo { u: u32 }

https://play.rust-lang.org/?gist=9d1ff0355fbfabbc0c47f15e78e94687&version=nightly&mode=debug&edition=2015

to

pub enum Bar {
    X, Y, Z
}
enum Foo {
    A,
    B,
    Other(Bar),
}

https://play.rust-lang.org/?gist=faf6db37cdc627b1c5f8d582ad5c6779&version=nightly&mode=release&edition=2015

While this will result in pretty much the same layout as before, any derived code on Foo will now generate less optimal code. Apparently llvm can't manage to clean that up.

The llvm IR for the first playground link is

define zeroext i1 @_ZN10playground3foo17h7604dbf314c89374E(i32, i32) unnamed_addr #0 {
start:
  %2 = icmp eq i32 %0, %1
  ret i1 %2
}

while the one for the second link is

define zeroext i1 @_ZN10playground3foo17ha662001f5519a11dE(i32, i32) unnamed_addr #0 {
start:
  %2 = add nsw i32 %0, -3
  %3 = icmp ult i32 %2, 2
  %narrow.i = select i1 %3, i32 %2, i32 2
  %4 = add nsw i32 %1, -3
  %5 = icmp ult i32 %4, 2
  %narrow8.i = select i1 %5, i32 %4, i32 2
  %6 = icmp eq i32 %narrow.i, %narrow8.i
  br i1 %6, label %bb6.i, label %"_ZN56_$LT$playground..Foo$u20$as$u20$core..cmp..PartialEq$GT$2eq17h647dd5d9c0e8f1fcE.exit"

bb6.i:                                            ; preds = %start
  %7 = icmp eq i32 %0, %1
  %not.or.cond.i = or i1 %3, %5
  %spec.select.i = or i1 %7, %not.or.cond.i
  br label %"_ZN56_$LT$playground..Foo$u20$as$u20$core..cmp..PartialEq$GT$2eq17h647dd5d9c0e8f1fcE.exit"

"_ZN56_$LT$playground..Foo$u20$as$u20$core..cmp..PartialEq$GT$2eq17h647dd5d9c0e8f1fcE.exit": ; preds = %start, %bb6.i
  %8 = phi i1 [ %spec.select.i, %bb6.i ], [ false, %start ]
  ret i1 %8
}

We can't improve the derives, because the derives on Foo can't see the definition of Bar.