Rollup of 9 pull requests

src/liballoc/borrow.rs

@@ -59,6 +59,7 @@ pub trait ToOwned {
     /// let vv: Vec<i32> = v.to_owned();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "cloning is often expensive and is not expected to have side effects"]
     fn to_owned(&self) -> Self::Owned;
 
     /// Uses borrowed data to replace owned data, usually by cloning.

src/liballoc/vec.rs

@@ -1567,40 +1567,69 @@ impl SpecFromElem for u8 {
     }
 }
 
-macro_rules! impl_spec_from_elem {
+impl<T: Clone + IsZero> SpecFromElem for T {
+    #[inline]
+    fn from_elem(elem: T, n: usize) -> Vec<T> {
+        if elem.is_zero() {
+            return Vec {
+                buf: RawVec::with_capacity_zeroed(n),
+                len: n,
+            }
+        }
+        let mut v = Vec::with_capacity(n);
+        v.extend_with(n, ExtendElement(elem));
+        v
+    }
+}
+
+unsafe trait IsZero {
+    /// Whether this value is zero
+    fn is_zero(&self) -> bool;
+}
+
+macro_rules! impl_is_zero {
     ($t: ty, $is_zero: expr) => {
-        impl SpecFromElem for $t {
+        unsafe impl IsZero for $t {
             #[inline]
-            fn from_elem(elem: $t, n: usize) -> Vec<$t> {
-                if $is_zero(elem) {
-                    return Vec {
-                        buf: RawVec::with_capacity_zeroed(n),
-                        len: n,
-                    }
-                }
-                let mut v = Vec::with_capacity(n);
-                v.extend_with(n, ExtendElement(elem));
-                v
+            fn is_zero(&self) -> bool {
+                $is_zero(*self)
             }
         }
-    };
+    }
 }
 
-impl_spec_from_elem!(i8, |x| x == 0);
-impl_spec_from_elem!(i16, |x| x == 0);
-impl_spec_from_elem!(i32, |x| x == 0);
-impl_spec_from_elem!(i64, |x| x == 0);
-impl_spec_from_elem!(i128, |x| x == 0);
-impl_spec_from_elem!(isize, |x| x == 0);
+impl_is_zero!(i8, |x| x == 0);
+impl_is_zero!(i16, |x| x == 0);
+impl_is_zero!(i32, |x| x == 0);
+impl_is_zero!(i64, |x| x == 0);
+impl_is_zero!(i128, |x| x == 0);
+impl_is_zero!(isize, |x| x == 0);
+
+impl_is_zero!(u16, |x| x == 0);
+impl_is_zero!(u32, |x| x == 0);
+impl_is_zero!(u64, |x| x == 0);
+impl_is_zero!(u128, |x| x == 0);
+impl_is_zero!(usize, |x| x == 0);
+
+impl_is_zero!(char, |x| x == '\0');
+
+impl_is_zero!(f32, |x: f32| x.to_bits() == 0);
+impl_is_zero!(f64, |x: f64| x.to_bits() == 0);
 
-impl_spec_from_elem!(u16, |x| x == 0);
-impl_spec_from_elem!(u32, |x| x == 0);
-impl_spec_from_elem!(u64, |x| x == 0);
-impl_spec_from_elem!(u128, |x| x == 0);
-impl_spec_from_elem!(usize, |x| x == 0);
+unsafe impl<T: ?Sized> IsZero for *const T {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        (*self).is_null()
+    }
+}
+
+unsafe impl<T: ?Sized> IsZero for *mut T {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        (*self).is_null()
+    }
+}
 
-impl_spec_from_elem!(f32, |x: f32| x.to_bits() == 0);
-impl_spec_from_elem!(f64, |x: f64| x.to_bits() == 0);
 
 ////////////////////////////////////////////////////////////////////////////////
 // Common trait implementations for Vec

src/libcore/clone.rs

@@ -105,6 +105,7 @@ pub trait Clone : Sized {
     /// assert_eq!("Hello", hello.clone());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "cloning is often expensive and is not expected to have side effects"]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.

src/libcore/iter/iterator.rs

@@ -1368,6 +1368,7 @@ pub trait Iterator {
     /// [`Result`]: ../../std/result/enum.Result.html
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
     fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
         FromIterator::from_iter(self)
     }

src/librustc_data_structures/indexed_set.rs

@@ -121,7 +121,9 @@ impl<T: Idx> IdxSetBuf<T> {
 
     /// Creates set holding every element whose index falls in range 0..universe_size.
     pub fn new_filled(universe_size: usize) -> Self {
-        Self::new(!0, universe_size)
+        let mut result = Self::new(!0, universe_size);
+        result.trim_to(universe_size);
+        result
     }
 
     /// Creates set holding no elements.
@@ -168,6 +170,36 @@ impl<T: Idx> IdxSet<T> {
         }
     }
 
+    /// Sets all elements up to `universe_size`
+    pub fn set_up_to(&mut self, universe_size: usize) {
+        for b in &mut self.bits {
+            *b = !0;
+        }
+        self.trim_to(universe_size);
+    }
+
+    /// Clear all elements above `universe_size`.
+    fn trim_to(&mut self, universe_size: usize) {
+        let word_bits = mem::size_of::<Word>() * 8;
+
+        // `trim_block` is the first block where some bits have
+        // to be cleared.
+        let trim_block = universe_size / word_bits;
+
+        // all the blocks above it have to be completely cleared.
+        if trim_block < self.bits.len() {
+            for b in &mut self.bits[trim_block+1..] {
+                *b = 0;
+            }
+
+            // at that block, the `universe_size % word_bits` lsbs
+            // should remain.
+            let remaining_bits = universe_size % word_bits;
+            let mask = (1<<remaining_bits)-1;
+            self.bits[trim_block] &= mask;
+        }
+    }
+
     /// Removes `elem` from the set `self`; returns true iff this changed `self`.
     pub fn remove(&mut self, elem: &T) -> bool {
         self.bits.clear_bit(elem.index())
@@ -252,3 +284,43 @@ impl<'a, T: Idx> Iterator for Iter<'a, T> {
         }
     }
 }
+
+#[test]
+fn test_trim_to() {
+    use std::cmp;
+
+    for i in 0..256 {
+        let mut idx_buf: IdxSetBuf<usize> = IdxSetBuf::new_filled(128);
+        idx_buf.trim_to(i);
+
+        let elems: Vec<usize> = idx_buf.iter().collect();
+        let expected: Vec<usize> = (0..cmp::min(i, 128)).collect();
+        assert_eq!(elems, expected);
+    }
+}
+
+#[test]
+fn test_set_up_to() {
+    for i in 0..128 {
+        for mut idx_buf in
+            vec![IdxSetBuf::new_empty(128), IdxSetBuf::new_filled(128)]
+            .into_iter()
+        {
+            idx_buf.set_up_to(i);
+
+            let elems: Vec<usize> = idx_buf.iter().collect();
+            let expected: Vec<usize> = (0..i).collect();
+            assert_eq!(elems, expected);
+        }
+    }
+}
+
+#[test]
+fn test_new_filled() {
+    for i in 0..128 {
+        let mut idx_buf = IdxSetBuf::new_filled(i);
+        let elems: Vec<usize> = idx_buf.iter().collect();
+        let expected: Vec<usize> = (0..i).collect();
+        assert_eq!(elems, expected);
+    }
+}

src/librustc_mir/build/mod.rs

@@ -317,7 +317,7 @@ newtype_index!(ScopeId);
 /// macro (and methods below) makes working with `BlockAnd` much more
 /// convenient.
 
-#[must_use] // if you don't use one of these results, you're leaving a dangling edge
+#[must_use = "if you don't use one of these results, you're leaving a dangling edge"]
 struct BlockAnd<T>(BasicBlock, T);
 
 trait BlockAndExtension {

src/librustc_mir/dataflow/impls/mod.rs

@@ -389,7 +389,7 @@ impl<'a, 'gcx, 'tcx> BitDenotation for MaybeUninitializedPlaces<'a, 'gcx, 'tcx>
     // sets on_entry bits for Arg places
     fn start_block_effect(&self, entry_set: &mut IdxSet<MovePathIndex>) {
         // set all bits to 1 (uninit) before gathering counterevidence
-        for e in entry_set.words_mut() { *e = !0; }
+        entry_set.set_up_to(self.bits_per_block());
 
         drop_flag_effects_for_function_entry(
             self.tcx, self.mir, self.mdpe,
@@ -443,7 +443,7 @@ impl<'a, 'gcx, 'tcx> BitDenotation for DefinitelyInitializedPlaces<'a, 'gcx, 'tc
 
     // sets on_entry bits for Arg places
     fn start_block_effect(&self, entry_set: &mut IdxSet<MovePathIndex>) {
-        for e in entry_set.words_mut() { *e = 0; }
+        entry_set.clear();
 
         drop_flag_effects_for_function_entry(
             self.tcx, self.mir, self.mdpe,