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Rollup merge of #67722 - petertodd:2019-improve-any-comment, r=Mark-Simulacrum

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Minor: note how Any is an unsafe trait in SAFETY comments

Motivation: helpful to people like myself reading the standard library source to better understand how to use Any, especially if we do go ahead with https://github.com/rust-lang/rust/pull/67562 and make it an unsafe trait.
This commit is contained in:
Yuki Okushi 2020-01-29 18:56:22 +09:00 committed by GitHub
commit 36fd7b9a59
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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