Merge pull request #18 from bluss/needs-drop

nodrop: Add feature flag use_needs_drop; Tag version 0.1.5

.travis.yml

@@ -7,7 +7,10 @@ matrix:
     - rust: nightly
     - rust: nightly
       env:
-      - NODROP_FEATURES='no_drop_flag'
+      - NODROP_FEATURES='no_drop_flag use_needs_drop'
+branches:
+  only:
+    - master
 script:
   - |
       [ -z "$NODROP_FEATURES" ] && cargo build --verbose --features "$FEATURES"

README.rst

@@ -19,8 +19,8 @@ __ http://bluss.github.io/arrayvec
 .. |crates2| image:: http://meritbadge.herokuapp.com/nodrop
 .. _crates2: https://crates.io/crates/nodrop
 
-Recent Changes
---------------
+Recent Changes (arrayvec)
+-------------------------
 
 - 0.3.12
 
@@ -44,6 +44,16 @@ Recent Changes
   - Added method .into_inner()
   - Added unsafe method .set_len()
 
+Recent Changes (nodrop)
+-----------------------
+
+- 0.1.5
+
+  - Added crate feature ``use_needs_drop`` which is a nightly-only
+    optimization, which skips overwriting if the inner value does not need
+    drop.
+
+
 License
 =======
 

nodrop/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "nodrop"
-version = "0.1.4"
+version = "0.1.5"
 authors = ["bluss"]
 
 license = "MIT/Apache-2.0"
@@ -17,5 +17,9 @@ keywords = ["container", "drop"]
 # Use no drop flag. See API doc for more info.
 no_drop_flag = []
 
+# Optional, nightly channel
+# Use `needs_drop` to skip overwriting if not necessary
+use_needs_drop = []
+
 [dependencies.odds]
 version = "0.2"

nodrop/src/lib.rs

@@ -1,6 +1,10 @@
 //!
 //! The **nodrop** crate has the following cargo feature flags:
 //!
+//! - `use_needs_drop`
+//!   - Optional
+//!   - Requires nightly channel.
+//!   - Use `needs_drop` to skip overwriting if not necessary
 //! - `no_drop_flag`.
 //!   - Optional.
 //!   - Requires nightly channel.
@@ -11,6 +15,7 @@
 //!
 
 #![cfg_attr(feature="no_drop_flag", feature(unsafe_no_drop_flag))]
+#![cfg_attr(feature="use_needs_drop", feature(core_intrinsics))]
 
 extern crate odds;
 
@@ -52,12 +57,28 @@ impl<T> NoDrop<T> {
     }
 }
 
+#[cfg(not(feature = "use_needs_drop"))]
+#[inline]
+fn needs_drop<T>() -> bool {
+    true
+}
+
+#[cfg(feature = "use_needs_drop")]
+#[inline]
+fn needs_drop<T>() -> bool {
+    unsafe {
+        std::intrinsics::needs_drop::<T>()
+    }
+}
+
 impl<T> Drop for NoDrop<T> {
     fn drop(&mut self) {
-        // no drop flag info: writing repeatedly is idempotent
-        // inhibit drop
-        unsafe {
-            ptr::write(&mut self.0, Flag::Dropped);
+        if needs_drop::<T>() {
+            // no drop flag info: writing repeatedly is idempotent
+            // inhibit drop
+            unsafe {
+                ptr::write(&mut self.0, Flag::Dropped);
+            }
         }
     }
 }