Rollup merge of #41892 - Gankro:needs_drop, r=alexcrichton

expose needs_drop under mem

Note that I purposefully didn't update the intrinsic's documentation, because I think it makes sense for it be more... "honest" about its semantics.

Author: Mark-Simulacrum

src/doc/unstable-book/src/library-features/needs-drop.md

@@ -0,0 +1,7 @@
+# `needs_drop`
+
+The tracking issue for this feature is: [#41890]
+
+[#41890]: https://github.com/rust-lang/rust/issues/41890
+
+------------------------

src/libarena/lib.rs

@@ -32,6 +32,7 @@
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
 #![feature(generic_param_attrs)]
+#![feature(needs_drop)]
 #![cfg_attr(stage0, feature(staged_api))]
 #![cfg_attr(test, feature(test))]
 
@@ -82,7 +83,7 @@ impl<T> TypedArenaChunk<T> {
     unsafe fn destroy(&mut self, len: usize) {
         // The branch on needs_drop() is an -O1 performance optimization.
         // Without the branch, dropping TypedArena<u8> takes linear time.
-        if intrinsics::needs_drop::<T>() {
+        if mem::needs_drop::<T>() {
             let mut start = self.start();
             // Destroy all allocated objects.
             for _ in 0..len {
@@ -350,7 +351,7 @@ impl DroplessArena {
     #[inline]
     pub fn alloc<T>(&self, object: T) -> &mut T {
         unsafe {
-            assert!(!intrinsics::needs_drop::<T>());
+            assert!(!mem::needs_drop::<T>());
             assert!(mem::size_of::<T>() != 0);
 
             self.align_for::<T>();
@@ -379,9 +380,7 @@ impl DroplessArena {
     #[inline]
     pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
         where T: Copy {
-        unsafe {
-            assert!(!intrinsics::needs_drop::<T>());
-        }
+        assert!(!mem::needs_drop::<T>());
         assert!(mem::size_of::<T>() != 0);
         assert!(slice.len() != 0);
         self.align_for::<T>();

src/libcore/mem.rs

@@ -302,6 +302,58 @@ pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
     unsafe { intrinsics::min_align_of_val(val) }
 }
 
+/// Returns whether dropping values of type `T` matters.
+///
+/// This is purely an optimization hint, and may be implemented conservatively.
+/// For instance, always returning `true` would be a valid implementation of
+/// this function.
+///
+/// Low level implementations of things like collections, which need to manually
+/// drop their data, should use this function to avoid unnecessarily
+/// trying to drop all their contents when they are destroyed. This might not
+/// make a difference in release builds (where a loop that has no side-effects
+/// is easily detected and eliminated), but is often a big win for debug builds.
+///
+/// Note that `ptr::drop_in_place` already performs this check, so if your workload
+/// can be reduced to some small number of drop_in_place calls, using this is
+/// unnecessary. In particular note that you can drop_in_place a slice, and that
+/// will do a single needs_drop check for all the values.
+///
+/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
+/// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
+/// values one at a time and should use this API.
+///
+///
+/// # Examples
+///
+/// Here's an example of how a collection might make use of needs_drop:
+///
+/// ```ignore
+/// #![feature(needs_drop)]
+/// use std::{mem, ptr};
+///
+/// pub struct MyCollection<T> { /* ... */ }
+///
+/// impl<T> Drop for MyCollection<T> {
+///     fn drop(&mut self) {
+///         unsafe {
+///             // drop the data
+///             if mem::needs_drop::<T>() {
+///                 for x in self.iter_mut() {
+///                     ptr::drop_in_place(x);
+///                 }
+///             }
+///             self.free_buffer();
+///         }
+///     }
+/// }
+/// ```
+#[inline]
+#[unstable(feature = "needs_drop", issue = "41890")]
+pub fn needs_drop<T>() -> bool {
+    unsafe { intrinsics::needs_drop::<T>() }
+}
+
 /// Creates a value whose bytes are all zero.
 ///
 /// This has the same effect as allocating space with

src/libstd/collections/hash/table.rs

@@ -12,9 +12,8 @@ use alloc::heap::{allocate, deallocate};
 
 use cmp;
 use hash::{BuildHasher, Hash, Hasher};
-use intrinsics::needs_drop;
 use marker;
-use mem::{align_of, size_of};
+use mem::{align_of, size_of, needs_drop};
 use mem;
 use ops::{Deref, DerefMut};
 use ptr::{self, Unique, Shared};

src/libstd/lib.rs

@@ -281,6 +281,7 @@
 #![feature(linkage)]
 #![feature(macro_reexport)]
 #![feature(needs_panic_runtime)]
+#![feature(needs_drop)]
 #![feature(never_type)]
 #![feature(num_bits_bytes)]
 #![feature(old_wrapping)]

src/libstd/sys/redox/fast_thread_local.rs

@@ -12,9 +12,10 @@
 #![unstable(feature = "thread_local_internals", issue = "0")]
 
 use cell::{Cell, UnsafeCell};
-use intrinsics;
+use mem;
 use ptr;
 
+
 pub struct Key<T> {
     inner: UnsafeCell<Option<T>>,
 
@@ -37,7 +38,7 @@ impl<T> Key<T> {
 
     pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
         unsafe {
-            if intrinsics::needs_drop::<T>() && self.dtor_running.get() {
+            if mem::needs_drop::<T>() && self.dtor_running.get() {
                 return None
             }
             self.register_dtor();
@@ -46,7 +47,7 @@ impl<T> Key<T> {
     }
 
     unsafe fn register_dtor(&self) {
-        if !intrinsics::needs_drop::<T>() || self.dtor_registered.get() {
+        if !mem::needs_drop::<T>() || self.dtor_registered.get() {
             return
         }
 

src/libstd/sys/unix/fast_thread_local.rs

@@ -13,7 +13,7 @@
 
 use cell::{Cell, UnsafeCell};
 use fmt;
-use intrinsics;
+use mem;
 use ptr;
 
 pub struct Key<T> {
@@ -44,7 +44,7 @@ impl<T> Key<T> {
 
     pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
         unsafe {
-            if intrinsics::needs_drop::<T>() && self.dtor_running.get() {
+            if mem::needs_drop::<T>() && self.dtor_running.get() {
                 return None
             }
             self.register_dtor();
@@ -53,7 +53,7 @@ impl<T> Key<T> {
     }
 
     unsafe fn register_dtor(&self) {
-        if !intrinsics::needs_drop::<T>() || self.dtor_registered.get() {
+        if !mem::needs_drop::<T>() || self.dtor_registered.get() {
             return
         }