Auto merge of #120080 - cuviper:128-align-packed, r=nikic

Pack u128 in the compiler to mitigate new alignment

This is based on #116672, adding a new `#[repr(packed(8))]` wrapper on `u128` to avoid changing any of the compiler's size assertions. This is needed in two places:

* `SwitchTargets`, otherwise its `SmallVec<[u128; 1]>` gets padded up to 32 bytes.
* `LitKind::Int`, so that entire `enum` can stay 24 bytes.
  * This change definitely has far-reaching effects though, since it's public.

2 files changed, 72 insertions, 0 deletions

diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 93b4032c310..077e90350a3 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -93,6 +93,7 @@ pub mod aligned;
 pub mod frozen;
 mod hashes;
 pub mod owned_slice;
+pub mod packed;
 pub mod sso;
 pub mod steal;
 pub mod tagged_ptr;
diff --git a/compiler/rustc_data_structures/src/packed.rs b/compiler/rustc_data_structures/src/packed.rs
new file mode 100644
index 00000000000..b8d4b295dfa
--- /dev/null
+++ b/compiler/rustc_data_structures/src/packed.rs
@@ -0,0 +1,71 @@
+use crate::stable_hasher::{HashStable, StableHasher};
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use std::cmp::Ordering;
+use std::fmt;
+
+#[repr(packed(8))]
+#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Pu128(pub u128);
+
+impl Pu128 {
+    #[inline]
+    pub fn get(self) -> u128 {
+        self.0
+    }
+}
+
+impl From<u128> for Pu128 {
+    #[inline]
+    fn from(value: u128) -> Self {
+        Self(value)
+    }
+}
+
+impl PartialEq<u128> for Pu128 {
+    #[inline]
+    fn eq(&self, other: &u128) -> bool {
+        ({ self.0 }) == *other
+    }
+}
+
+impl PartialOrd<u128> for Pu128 {
+    #[inline]
+    fn partial_cmp(&self, other: &u128) -> Option<Ordering> {
+        { self.0 }.partial_cmp(other)
+    }
+}
+
+impl fmt::Display for Pu128 {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        { self.0 }.fmt(f)
+    }
+}
+
+impl fmt::UpperHex for Pu128 {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        { self.0 }.fmt(f)
+    }
+}
+
+impl<CTX> HashStable<CTX> for Pu128 {
+    #[inline]
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
+        { self.0 }.hash_stable(ctx, hasher)
+    }
+}
+
+impl<S: Encoder> Encodable<S> for Pu128 {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        { self.0 }.encode(s);
+    }
+}
+
+impl<D: Decoder> Decodable<D> for Pu128 {
+    #[inline]
+    fn decode(d: &mut D) -> Self {
+        Self(u128::decode(d))
+    }
+}