Use the niche optimization if other variant are small enough

src/librustc_middle/ty/sty.rs

@@ -2204,7 +2204,7 @@ pub struct Const<'tcx> {
 }
 
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(Const<'_>, 48);
+static_assert_size!(Const<'_>, if cfg!(bootstrap) { 48 } else { 40 });
 
 impl<'tcx> Const<'tcx> {
     /// Literals and const generic parameters are eagerly converted to a constant, everything else
@@ -2432,7 +2432,7 @@ pub enum ConstKind<'tcx> {
 }
 
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(ConstKind<'_>, 40);
+static_assert_size!(ConstKind<'_>, if cfg!(bootstrap) { 40 } else { 32 });
 
 impl<'tcx> ConstKind<'tcx> {
     #[inline]

src/librustc_mir_build/hair/mod.rs

@@ -95,7 +95,7 @@ crate enum StmtKind<'tcx> {
 
 // `Expr` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-rustc_data_structures::static_assert_size!(Expr<'_>, 168);
+rustc_data_structures::static_assert_size!(Expr<'_>, if cfg!(bootstrap) { 168 } else { 160 });
 
 /// The Hair trait implementor lowers their expressions (`&'tcx H::Expr`)
 /// into instances of this `Expr` enum. This lowering can be done

src/test/ui/consts/const-eval/const_transmute.rs

@@ -33,8 +33,10 @@ impl Drop for Foo {
 }
 
 #[derive(Copy, Clone)]
+#[repr(C)]
 struct Fat<'a>(&'a Foo, &'static VTable);
 
+#[repr(C)]
 struct VTable {
     drop: Option<for<'a> fn(&'a mut Foo)>,
     size: usize,

src/test/ui/print_type_sizes/niche-filling.stdout

@@ -8,12 +8,12 @@ print-type-size     variant `Some`: 12 bytes
 print-type-size         field `.0`: 12 bytes
 print-type-size     variant `None`: 0 bytes
 print-type-size type: `EmbeddedDiscr`: 8 bytes, alignment: 4 bytes
+print-type-size     discriminant: 1 bytes
 print-type-size     variant `Record`: 7 bytes
-print-type-size         field `.val`: 4 bytes
-print-type-size         field `.post`: 2 bytes
 print-type-size         field `.pre`: 1 bytes
+print-type-size         field `.post`: 2 bytes
+print-type-size         field `.val`: 4 bytes
 print-type-size     variant `None`: 0 bytes
-print-type-size     end padding: 1 bytes
 print-type-size type: `MyOption<Union1<std::num::NonZeroU32>>`: 8 bytes, alignment: 4 bytes
 print-type-size     discriminant: 4 bytes
 print-type-size     variant `Some`: 4 bytes

src/test/ui/print_type_sizes/padding.stdout

@@ -1,23 +1,21 @@
-print-type-size type: `E1`: 12 bytes, alignment: 4 bytes
-print-type-size     discriminant: 1 bytes
-print-type-size     variant `B`: 11 bytes
-print-type-size         padding: 3 bytes
-print-type-size         field `.0`: 8 bytes, alignment: 4 bytes
-print-type-size     variant `A`: 7 bytes
-print-type-size         field `.1`: 1 bytes
+print-type-size type: `E1`: 8 bytes, alignment: 4 bytes
+print-type-size     variant `A`: 8 bytes
+print-type-size         padding: 1 bytes
+print-type-size         field `.1`: 1 bytes, alignment: 1 bytes
 print-type-size         padding: 2 bytes
 print-type-size         field `.0`: 4 bytes, alignment: 4 bytes
-print-type-size type: `E2`: 12 bytes, alignment: 4 bytes
-print-type-size     discriminant: 1 bytes
-print-type-size     variant `B`: 11 bytes
-print-type-size         padding: 3 bytes
-print-type-size         field `.0`: 8 bytes, alignment: 4 bytes
-print-type-size     variant `A`: 7 bytes
-print-type-size         field `.0`: 1 bytes
+print-type-size     variant `B`: 8 bytes
+print-type-size         field `.0`: 8 bytes
+print-type-size type: `E2`: 8 bytes, alignment: 4 bytes
+print-type-size     variant `A`: 8 bytes
+print-type-size         padding: 1 bytes
+print-type-size         field `.0`: 1 bytes, alignment: 1 bytes
 print-type-size         padding: 2 bytes
 print-type-size         field `.1`: 4 bytes, alignment: 4 bytes
+print-type-size     variant `B`: 8 bytes
+print-type-size         field `.0`: 8 bytes
 print-type-size type: `S`: 8 bytes, alignment: 4 bytes
-print-type-size     field `.g`: 4 bytes
-print-type-size     field `.a`: 1 bytes
 print-type-size     field `.b`: 1 bytes
-print-type-size     end padding: 2 bytes
+print-type-size     field `.a`: 1 bytes
+print-type-size     padding: 2 bytes
+print-type-size     field `.g`: 4 bytes, alignment: 4 bytes

src/test/ui/type-sizes.rs

@@ -102,6 +102,44 @@ enum Option2<A, B> {
     None
 }
 
+struct BoolInTheMiddle(std::num::NonZeroU16, bool, u8);
+
+enum NicheWithData {
+    A,
+    B([u16; 5]),
+    Largest { a1: u32, a2: BoolInTheMiddle, a3: u32 },
+    C,
+    D(u32, u32),
+}
+
+// A type with almost 2^16 invalid values.
+#[repr(u16)]
+pub enum NicheU16 {
+    _0,
+}
+
+pub enum EnumManyVariant<X> {
+    Dataful(u8, X),
+
+    // 0x100 niche variants.
+    _00, _01, _02, _03, _04, _05, _06, _07, _08, _09, _0A, _0B, _0C, _0D, _0E, _0F,
+    _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _1A, _1B, _1C, _1D, _1E, _1F,
+    _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _2A, _2B, _2C, _2D, _2E, _2F,
+    _30, _31, _32, _33, _34, _35, _36, _37, _38, _39, _3A, _3B, _3C, _3D, _3E, _3F,
+    _40, _41, _42, _43, _44, _45, _46, _47, _48, _49, _4A, _4B, _4C, _4D, _4E, _4F,
+    _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _5A, _5B, _5C, _5D, _5E, _5F,
+    _60, _61, _62, _63, _64, _65, _66, _67, _68, _69, _6A, _6B, _6C, _6D, _6E, _6F,
+    _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _7A, _7B, _7C, _7D, _7E, _7F,
+    _80, _81, _82, _83, _84, _85, _86, _87, _88, _89, _8A, _8B, _8C, _8D, _8E, _8F,
+    _90, _91, _92, _93, _94, _95, _96, _97, _98, _99, _9A, _9B, _9C, _9D, _9E, _9F,
+    _A0, _A1, _A2, _A3, _A4, _A5, _A6, _A7, _A8, _A9, _AA, _AB, _AC, _AD, _AE, _AF,
+    _B0, _B1, _B2, _B3, _B4, _B5, _B6, _B7, _B8, _B9, _BA, _BB, _BC, _BD, _BE, _BF,
+    _C0, _C1, _C2, _C3, _C4, _C5, _C6, _C7, _C8, _C9, _CA, _CB, _CC, _CD, _CE, _CF,
+    _D0, _D1, _D2, _D3, _D4, _D5, _D6, _D7, _D8, _D9, _DA, _DB, _DC, _DD, _DE, _DF,
+    _E0, _E1, _E2, _E3, _E4, _E5, _E6, _E7, _E8, _E9, _EA, _EB, _EC, _ED, _EE, _EF,
+    _F0, _F1, _F2, _F3, _F4, _F5, _F6, _F7, _F8, _F9, _FA, _FB, _FC, _FD, _FE, _FF,
+}
+
 pub fn main() {
     assert_eq!(size_of::<u8>(), 1 as usize);
     assert_eq!(size_of::<u32>(), 4 as usize);
@@ -145,4 +183,31 @@ pub fn main() {
     assert_eq!(size_of::<Option<Option<(&(), bool)>>>(), size_of::<(bool, &())>());
     assert_eq!(size_of::<Option<Option2<bool, &()>>>(), size_of::<(bool, &())>());
     assert_eq!(size_of::<Option<Option2<&(), bool>>>(), size_of::<(bool, &())>());
+
+    struct S1{ a: u16, b: std::num::NonZeroU16, c: u16, d: u8, e: u32, f: u64, g:[u8;2] }
+    assert_eq!(size_of::<S1>(), 24);
+    assert_eq!(size_of::<Option<S1>>(), 24);
+
+    assert_eq!(size_of::<NicheWithData>(), 12);
+    assert_eq!(size_of::<Option<NicheWithData>>(), 12);
+    assert_eq!(size_of::<Option<Option<NicheWithData>>>(), 12);
+    assert_eq!(
+        size_of::<Option<Option2<&(), Option<NicheWithData>>>>(),
+        size_of::<(&(), NicheWithData)>()
+    );
+
+    pub enum FillPadding { A(std::num::NonZeroU8, u32), B }
+    assert_eq!(size_of::<FillPadding>(), 8);
+    assert_eq!(size_of::<Option<FillPadding>>(), 8);
+    assert_eq!(size_of::<Option<Option<FillPadding>>>(), 8);
+
+    assert_eq!(size_of::<Result<(std::num::NonZeroU8, u8, u8), u16>>(), 4);
+    assert_eq!(size_of::<Option<Result<(std::num::NonZeroU8, u8, u8), u16>>>(), 4);
+    assert_eq!(size_of::<Result<(std::num::NonZeroU8, u8, u8, u8), u16>>(), 4);
+
+    assert_eq!(size_of::<EnumManyVariant<u16>>(), 6);
+    assert_eq!(size_of::<EnumManyVariant<NicheU16>>(), 4);
+    assert_eq!(size_of::<EnumManyVariant<Option<NicheU16>>>(), 4);
+    assert_eq!(size_of::<EnumManyVariant<Option2<NicheU16,u8>>>(), 6);
+    assert_eq!(size_of::<EnumManyVariant<Option<(NicheU16,u8)>>>(), 6);
 }