Add Read::size_hint, similar to Iterator::size_hint

Author: SimonSapin

src/libstd/fs.rs

@@ -449,6 +449,13 @@ impl Read for File {
         self.inner.read(buf)
     }
 
+    fn size_hint(&self) -> usize {
+        match self.metadata() {
+            Ok(meta) => meta.len() as usize,
+            Err(_) => 0,
+        }
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
@@ -473,6 +480,10 @@ impl<'a> Read for &'a File {
         self.inner.read(buf)
     }
 
+    fn size_hint(&self) -> usize {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()

src/libstd/io/buffered.rs

@@ -211,6 +211,12 @@ impl<R: Read> Read for BufReader<R> {
         Ok(nread)
     }
 
+    #[inline]
+    fn size_hint(&self) -> usize {
+        let buffered_len = self.cap - self.pos;
+        buffered_len.saturating_add(self.inner.size_hint())
+    }
+
     // we can't skip unconditionally because of the large buffer case in read.
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()

src/libstd/io/cursor.rs

@@ -230,6 +230,10 @@ impl<T> Read for Cursor<T> where T: AsRef<[u8]> {
         Ok(n)
     }
 
+    fn size_hint(&self) -> usize {
+        (self.inner.as_ref().len() as u64).saturating_sub(self.pos) as usize
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()

src/libstd/io/impls.rs

@@ -23,6 +23,11 @@ impl<'a, R: Read + ?Sized> Read for &'a mut R {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn size_hint(&self) -> usize {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -92,6 +97,11 @@ impl<R: Read + ?Sized> Read for Box<R> {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn size_hint(&self) -> usize {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -181,6 +191,11 @@ impl<'a> Read for &'a [u8] {
         Ok(amt)
     }
 
+    #[inline]
+    fn size_hint(&self) -> usize {
+        self.len()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()

src/libstd/io/mod.rs

@@ -544,6 +544,19 @@ pub trait Read {
         Initializer::zeroing()
     }
 
+    /// Return an estimate of how many bytes would be read from this source until EOF,
+    /// or zero if that is unknown.
+    ///
+    /// This is used by [`read_to_end`] and [`read_to_string`] to pre-allocate a memory buffer.
+    ///
+    /// [`read_to_end`]: #method.read_to_end
+    /// [`read_to_string`]: #method.read_to_string
+    #[unstable(feature = "read_size_hint", issue = /* FIXME */ "0")]
+    #[inline]
+    fn size_hint(&self) -> usize {
+        0
+    }
+
     /// Read all bytes until EOF in this source, placing them into `buf`.
     ///
     /// All bytes read from this source will be appended to the specified buffer
@@ -1720,6 +1733,14 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
         self.second.read(buf)
     }
 
+    fn size_hint(&self) -> usize {
+        if self.done_first {
+            self.second.size_hint()
+        } else {
+            self.first.size_hint().saturating_add(self.second.size_hint())
+        }
+    }
+
     unsafe fn initializer(&self) -> Initializer {
         let initializer = self.first.initializer();
         if initializer.should_initialize() {
@@ -1918,6 +1939,14 @@ impl<T: Read> Read for Take<T> {
         Ok(n)
     }
 
+    fn size_hint(&self) -> usize {
+        if self.limit == 0 {
+            0
+        } else {
+            cmp::min(self.limit, self.inner.size_hint() as u64) as usize
+        }
+    }
+
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
     }