Use MaybeUninit when available

Fix deprecation warnings for mem::uninitialized.
(They also affected other crates, since it’s used in a public macro.)

Author: SimonSapin

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cssparser"
-version = "0.25.7"
+version = "0.25.8"
 authors = [ "Simon Sapin <simon.sapin@exyr.org>" ]
 
 description = "Rust implementation of CSS Syntax Level 3"
@@ -30,6 +30,7 @@ serde = {version = "1.0", optional = true}
 smallvec = "0.6"
 
 [build-dependencies]
+autocfg = "0.1.4"
 syn = { version = "0.15.12", features = ["extra-traits", "fold", "full"] }
 quote = "0.6"
 proc-macro2 = "0.4"

build.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+extern crate autocfg;
 #[macro_use]
 extern crate quote;
 #[macro_use]
@@ -49,5 +50,7 @@ fn main() {
         println!("cargo:rustc-cfg=rustc_has_pr45225")
     }
 
+    autocfg::new().emit_has_path("std::mem::MaybeUninit");
+
     codegen::main();
 }

src/macros.rs

@@ -110,13 +110,26 @@ macro_rules! ascii_case_insensitive_phf_map {
 #[doc(hidden)]
 macro_rules! cssparser_internal__to_lowercase {
     ($input: expr, $BUFFER_SIZE: expr => $output: ident) => {
-        // mem::uninitialized() is ok because `buffer` is only used in `_internal__to_lowercase`,
+        let mut buffer;
+        // Safety: `buffer` is only used in `_internal__to_lowercase`,
         // which initializes with `copy_from_slice` the part of the buffer it uses,
         // before it uses it.
         #[allow(unsafe_code)]
-        let mut buffer: [u8; $BUFFER_SIZE] = unsafe { ::std::mem::uninitialized() };
+        let buffer = unsafe {
+            // FIXME: remove this when we require Rust 1.36
+            #[cfg(not(has_std__mem__MaybeUninit))]
+            {
+                buffer = ::std::mem::uninitialized::<[u8; $BUFFER_SIZE]>();
+                &mut buffer
+            }
+            #[cfg(has_std__mem__MaybeUninit)]
+            {
+                buffer = ::std::mem::MaybeUninit::<[u8; $BUFFER_SIZE]>::uninit();
+                &mut *(buffer.as_mut_ptr())
+            }
+        };
         let input: &str = $input;
-        let $output = $crate::_internal__to_lowercase(&mut buffer, input);
+        let $output = $crate::_internal__to_lowercase(buffer, input);
     };
 }