Explain the use of blocks around self.data accesses

Author: oli-obk

compiler/rustc_middle/src/ty/consts/int.rs

@@ -131,6 +131,10 @@ pub struct ScalarInt {
 // can't take references to fields of packed structs.
 impl<CTX> crate::ty::HashStable<CTX> for ScalarInt {
     fn hash_stable(&self, hcx: &mut CTX, hasher: &mut crate::ty::StableHasher) {
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `hash_stable` takes `&self` and would thus borrow `self.data`.
+        // Since `Self` is a packed struct, that would create a possibly unaligned reference,
+        // which is UB on a lot of platforms.
         { self.data }.hash_stable(hcx, hasher);
         self.size.hash_stable(hcx, hasher);
     }
@@ -167,6 +171,11 @@ impl ScalarInt {
     /// construct `Scalar`s).
     #[inline(always)]
     fn check_data(self) {
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `assert_eq` takes references to its arguments and formatting
+        // arguments and would thus borrow `self.data`. Since `Self`
+        // is a packed struct, that would create a possibly unaligned reference, which
+        // is UB on a lot of platforms.
         debug_assert_eq!(
             truncate(self.data, self.size()),
             { self.data },
@@ -336,6 +345,11 @@ impl fmt::LowerHex for ScalarInt {
         self.check_data();
         // Format as hex number wide enough to fit any value of the given `size`.
         // So data=20, size=1 will be "0x14", but with size=4 it'll be "0x00000014".
+        // Using a block `{self.data}` here to force a copy instead of using `self.data`
+        // directly, because `write!` takes references to its formatting arguments and
+        // would thus borrow `self.data`. Since `Self`
+        // is a packed struct, that would create a possibly unaligned reference, which
+        // is UB on a lot of platforms.
         write!(f, "{:01$x}", { self.data }, self.size as usize * 2)
     }
 }