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Minor: note why we can rely on Any trait for safety

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This commit is contained in:
Peter Todd 2020-01-28 20:53:45 -05:00
parent 3761dcd346
commit f722964d00
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1 changed files with 6 additions and 2 deletions
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8
src/libcore/any.rs
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@ -194,7 +194,9 @@ impl dyn Any {
#[inline]
pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
if self.is::<T>() {
// SAFETY: just checked whether we are pointing to the correct type
// SAFETY: just checked whether we are pointing to the correct type, and we can rely on
// that check for memory safety because we have implemented Any for all types; no other
// impls can exist as they would conflict with our impl.
unsafe { Some(&*(self as *const dyn Any as *const T)) }
} else {
None
@ -228,7 +230,9 @@ impl dyn Any {
#[inline]
pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
if self.is::<T>() {
// SAFETY: just checked whether we are pointing to the correct type
// SAFETY: just checked whether we are pointing to the correct type, and we can rely on
// that check for memory safety because we have implemented Any for all types; no other
// impls can exist as they would conflict with our impl.
unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
} else {
None