Add precondition to Layout that the align fit in a u32.

This precondition takes the form of a behavorial change in
`Layout::from_size_align` (so it returns `None` if the `align` is too
large) and a new requirement for safe usage of
`Layout::from_size_align_unchecked`.

Fix rust-lang#30170.

src/liballoc/allocator.rs

@@ -65,11 +65,13 @@ pub struct Layout {
 
 impl Layout {
     /// Constructs a `Layout` from a given `size` and `align`,
-    /// or returns `None` if either of the following conditions
+    /// or returns `None` if any of the following conditions
     /// are not met:
     ///
     /// * `align` must be a power of two,
     ///
+    /// * `align` must not exceed 2^31 (i.e. `1 << 31`),
+    ///
     /// * `size`, when rounded up to the nearest multiple of `align`,
     ///    must not overflow (i.e. the rounded value must be less than
     ///    `usize::MAX`).
@@ -79,6 +81,10 @@ impl Layout {
             return None;
         }
 
+        if align > (1 << 31) {
+            return None;
+        }
+
         // (power-of-two implies align != 0.)
 
         // Rounded up size is:
@@ -106,8 +112,10 @@ impl Layout {
     ///
     /// # Unsafety
     ///
-    /// This function is unsafe as it does not verify that `align` is a power of
-    /// two nor that `size` aligned to `align` fits within the address space.
+    /// This function is unsafe as it does not verify that `align` is
+    /// a power-of-two that is also less than or equal to 2^31, nor
+    /// that `size` aligned to `align` fits within the address space
+    /// (i.e. the `Layout::from_size_align` preconditions).
     #[inline]
     pub unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Layout {
         Layout { size: size, align: align }
@@ -217,10 +225,10 @@ impl Layout {
             Some(alloc_size) => alloc_size,
         };
 
-        // We can assume that `self.align` is a power-of-two.
-        // Furthermore, `alloc_size` has alreayd been rounded up
-        // to a multiple of `self.align`; therefore, the call
-        // to `Layout::from_size_align` below should never panic.
+        // We can assume that `self.align` is a power-of-two that does
+        // not exceed 2^31. Furthermore, `alloc_size` has already been
+        // rounded up to a multiple of `self.align`; therefore, the
+        // call to `Layout::from_size_align` below should never panic.
         Some((Layout::from_size_align(alloc_size, self.align).unwrap(), padded_size))
     }