bjoernager/rust
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Explain the use of blocks around self.data accesses

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oli 2020-10-29 10:29:26 +00:00
parent 02131f4dcd
commit c478574786
1 changed files with 14 additions and 0 deletions
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14
compiler/rustc_middle/src/ty/consts/int.rs
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@ -131,6 +131,10 @@ pub struct ScalarInt {
// can't take references to fields of packed structs. // can't take references to fields of packed structs.
impl<CTX> crate::ty::HashStable<CTX> for ScalarInt { impl<CTX> crate::ty::HashStable<CTX> for ScalarInt {
fn hash_stable(&self, hcx: &mut CTX, hasher: &mut crate::ty::StableHasher) { 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.data }.hash_stable(hcx, hasher);
self.size.hash_stable(hcx, hasher); self.size.hash_stable(hcx, hasher);
} }
@ -167,6 +171,11 @@ impl ScalarInt {
/// construct `Scalar`s). /// construct `Scalar`s).
#[inline(always)] #[inline(always)]
fn check_data(self) { 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!( debug_assert_eq!(
truncate(self.data, self.size()), truncate(self.data, self.size()),
{ self.data }, { self.data },
@ -336,6 +345,11 @@ impl fmt::LowerHex for ScalarInt {
self.check_data(); self.check_data();
// Format as hex number wide enough to fit any value of the given `size`. // 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". // 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$x}", { self.data }, self.size as usize * 2) write!(f, "{:01$x}", { self.data }, self.size as usize * 2)
} }
} }