Auto merge of rust-lang#81113 - m-ou-se:rollup-a1unz4x, r=m-ou-se

Rollup of 13 pull requests

Successful merges:

 - rust-lang#79298 (correctly deal with late-bound lifetimes in anon consts)
 - rust-lang#80031 (resolve: Reject ambiguity built-in attr vs different built-in attr)
 - rust-lang#80201 (Add benchmark and fast path for BufReader::read_exact)
 - rust-lang#80635 (Improve diagnostics when closure doesn't meet trait bound)
 - rust-lang#80765 (resolve: Simplify collection of traits in scope)
 - rust-lang#80932 (Allow downloading LLVM on Windows and MacOS)
 - rust-lang#80983 (Remove is_dllimport_foreign_item definition from cg_ssa)
 - rust-lang#81064 (Support non-stage0 check)
 - rust-lang#81080 (Force vec![] to expression position only)
 - rust-lang#81082 (BTreeMap: clean up a few more comments)
 - rust-lang#81084 (Use Option::map instead of open-coding it)
 - rust-lang#81095 (Use Option::unwrap_or instead of open-coding it)
 - rust-lang#81107 (Add NonZeroUn::is_power_of_two)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

alloc/src/collections/btree/node.rs

@@ -184,7 +184,7 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
 
     /// Removes the internal root node, using its first child as the new root node.
     /// As it is intended only to be called when the root node has only one child,
-    /// no cleanup is done on any of the other children.
+    /// no cleanup is done on any of the keys, values and other children.
     /// This decreases the height by 1 and is the opposite of `push_internal_level`.
     ///
     /// Requires exclusive access to the `Root` object but not to the root node;
@@ -225,7 +225,7 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
 ///    - When this is `Owned`, the `NodeRef` acts roughly like `Box<Node>`,
 ///      but does not have a destructor, and must be cleaned up manually.
 ///   Since any `NodeRef` allows navigating through the tree, `BorrowType`
-///   effectively applies to the entire tree, not just the node itself.
+///   effectively applies to the entire tree, not just to the node itself.
 /// - `K` and `V`: These are the types of keys and values stored in the nodes.
 /// - `Type`: This can be `Leaf`, `Internal`, or `LeafOrInternal`. When this is
 ///   `Leaf`, the `NodeRef` points to a leaf node, when this is `Internal` the
@@ -425,7 +425,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
 
 impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
     /// Similar to `ascend`, gets a reference to a node's parent node, but also
-    /// deallocate the current node in the process. This is unsafe because the
+    /// deallocates the current node in the process. This is unsafe because the
     /// current node will still be accessible despite being deallocated.
     pub unsafe fn deallocate_and_ascend(
         self,
@@ -661,7 +661,10 @@ impl<'a, K: 'a, V: 'a> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
     /// Removes a key-value pair from the end of the node and returns the pair.
     /// Also removes the edge that was to the right of that pair and, if the node
     /// is internal, returns the orphaned subtree that this edge owned.
-    fn pop(&mut self) -> (K, V, Option<Root<K, V>>) {
+    ///
+    /// # Safety
+    /// The node must not be empty.
+    unsafe fn pop(&mut self) -> (K, V, Option<Root<K, V>>) {
         debug_assert!(self.len() > 0);
 
         let idx = self.len() - 1;

alloc/src/collections/btree/search.rs

@@ -12,8 +12,7 @@ pub enum SearchResult<BorrowType, K, V, FoundType, GoDownType> {
 
 /// Looks up a given key in a (sub)tree headed by the given node, recursively.
 /// Returns a `Found` with the handle of the matching KV, if any. Otherwise,
-/// returns a `GoDown` with the handle of the possible leaf edge where the key
-/// belongs.
+/// returns a `GoDown` with the handle of the leaf edge where the key belongs.
 ///
 /// The result is meaningful only if the tree is ordered by key, like the tree
 /// in a `BTreeMap` is.

alloc/src/lib.rs

@@ -140,6 +140,7 @@
 #![feature(type_alias_impl_trait)]
 #![feature(associated_type_bounds)]
 #![feature(slice_group_by)]
+#![feature(decl_macro)]
 // Allow testing this library
 
 #[cfg(test)]
@@ -193,4 +194,11 @@ mod std {
 #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
 pub mod __export {
     pub use core::format_args;
+
+    /// Force AST node to an expression to improve diagnostics in pattern position.
+    #[rustc_macro_transparency = "semitransparent"]
+    #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
+    pub macro force_expr($e:expr) {
+        $e
+    }
 }

alloc/src/macros.rs

@@ -37,16 +37,16 @@
 #[cfg(not(test))]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[allow_internal_unstable(box_syntax)]
+#[allow_internal_unstable(box_syntax, liballoc_internals)]
 macro_rules! vec {
     () => (
-        $crate::vec::Vec::new()
+        $crate::__export::force_expr!($crate::vec::Vec::new())
     );
     ($elem:expr; $n:expr) => (
-        $crate::vec::from_elem($elem, $n)
+        $crate::__export::force_expr!($crate::vec::from_elem($elem, $n))
     );
     ($($x:expr),+ $(,)?) => (
-        <[_]>::into_vec(box [$($x),+])
+        $crate::__export::force_expr!(<[_]>::into_vec(box [$($x),+]))
     );
 }
 

core/src/num/nonzero.rs

@@ -286,3 +286,43 @@ nonzero_integers_div! {
     NonZeroU128(u128);
     NonZeroUsize(usize);
 }
+
+macro_rules! nonzero_unsigned_is_power_of_two {
+    ( $( $Ty: ident )+ ) => {
+        $(
+            impl $Ty {
+
+                /// Returns `true` if and only if `self == (1 << k)` for some `k`.
+                ///
+                /// On many architectures, this function can perform better than `is_power_of_two()`
+                /// on the underlying integer type, as special handling of zero can be avoided.
+                ///
+                /// # Examples
+                ///
+                /// Basic usage:
+                ///
+                /// ```
+                /// #![feature(nonzero_is_power_of_two)]
+                ///
+                #[doc = concat!("let eight = std::num::", stringify!($Ty), "::new(8).unwrap();")]
+                /// assert!(eight.is_power_of_two());
+                #[doc = concat!("let ten = std::num::", stringify!($Ty), "::new(10).unwrap();")]
+                /// assert!(!ten.is_power_of_two());
+                /// ```
+                #[unstable(feature = "nonzero_is_power_of_two", issue = "81106")]
+                #[inline]
+                pub const fn is_power_of_two(self) -> bool {
+                    // LLVM 11 normalizes `unchecked_sub(x, 1) & x == 0` to the implementation seen here.
+                    // On the basic x86-64 target, this saves 3 instructions for the zero check.
+                    // On x86_64 with BMI1, being nonzero lets it codegen to `BLSR`, which saves an instruction
+                    // compared to the `POPCNT` implementation on the underlying integer type.
+
+                    intrinsics::ctpop(self.get()) < 2
+                }
+
+            }
+        )+
+    }
+}
+
+nonzero_unsigned_is_power_of_two! { NonZeroU8 NonZeroU16 NonZeroU32 NonZeroU64 NonZeroU128 NonZeroUsize }

std/src/io/buffered/bufreader.rs

@@ -271,6 +271,20 @@ impl<R: Read> Read for BufReader<R> {
         Ok(nread)
     }
 
+    // Small read_exacts from a BufReader are extremely common when used with a deserializer.
+    // The default implementation calls read in a loop, which results in surprisingly poor code
+    // generation for the common path where the buffer has enough bytes to fill the passed-in
+    // buffer.
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        if self.buffer().len() >= buf.len() {
+            buf.copy_from_slice(&self.buffer()[..buf.len()]);
+            self.consume(buf.len());
+            return Ok(());
+        }
+
+        crate::io::default_read_exact(self, buf)
+    }
+
     fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
         if self.pos == self.cap && total_len >= self.buf.len() {

std/src/io/buffered/tests.rs

@@ -443,6 +443,18 @@ fn bench_buffered_reader(b: &mut test::Bencher) {
     b.iter(|| BufReader::new(io::empty()));
 }
 
+#[bench]
+fn bench_buffered_reader_small_reads(b: &mut test::Bencher) {
+    let data = (0..u8::MAX).cycle().take(1024 * 4).collect::<Vec<_>>();
+    b.iter(|| {
+        let mut reader = BufReader::new(&data[..]);
+        let mut buf = [0u8; 4];
+        for _ in 0..1024 {
+            reader.read_exact(&mut buf).unwrap();
+        }
+    });
+}
+
 #[bench]
 fn bench_buffered_writer(b: &mut test::Bencher) {
     b.iter(|| BufWriter::new(io::sink()));

std/src/io/mod.rs

@@ -416,6 +416,25 @@ where
     write(buf)
 }
 
+pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
+    while !buf.is_empty() {
+        match this.read(buf) {
+            Ok(0) => break,
+            Ok(n) => {
+                let tmp = buf;
+                buf = &mut tmp[n..];
+            }
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+            Err(e) => return Err(e),
+        }
+    }
+    if !buf.is_empty() {
+        Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
+    } else {
+        Ok(())
+    }
+}
+
 /// The `Read` trait allows for reading bytes from a source.
 ///
 /// Implementors of the `Read` trait are called 'readers'.
@@ -766,23 +785,8 @@ pub trait Read {
     /// }
     /// ```
     #[stable(feature = "read_exact", since = "1.6.0")]
-    fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
-        while !buf.is_empty() {
-            match self.read(buf) {
-                Ok(0) => break,
-                Ok(n) => {
-                    let tmp = buf;
-                    buf = &mut tmp[n..];
-                }
-                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
-                Err(e) => return Err(e),
-            }
-        }
-        if !buf.is_empty() {
-            Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
-        } else {
-            Ok(())
-        }
+    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+        default_read_exact(self, buf)
     }
 
     /// Creates a "by reference" adaptor for this instance of `Read`.