Adjust lint levels for using unsafe blocks in unsafe fns

If the `unsafe_op_in_unsafe_fn` lint is available (since Rust 1.52.0),
set it to "deny", requiring `unsafe { }` blocks in order to perform
unsafe operations even in unsafe functions; this silences the otherwise
default warning of the `unused_unsafe` lint as well.

If the `unsafe_op_in_unsafe_fn` lint is not available, then just supress
warnings from the `unused_unsafe` lint.

Author: clint-white

Cargo.toml

@@ -10,6 +10,9 @@ keywords = ["binary", "heap", "priority", "queue"]
 categories = ["data-structures", "algorithms", ]
 edition = "2018"
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 compare = "0.1.0"
 serde = { version = "1.0.116", optional = true, features = ["derive"] }

build.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let ac = autocfg::new();
+
+    // Required for stabilization of `unsafe_op_in_unsafe_fn` lint.
+    ac.emit_rustc_version(1, 52);
+
+    autocfg::rerun_path("build.rs");
+}

src/binary_heap.rs

@@ -155,7 +155,8 @@
 //! }
 //! ```
 
-#![deny(unsafe_op_in_unsafe_fn)]
+#![cfg_attr(has_rustc_1_52, deny(unsafe_op_in_unsafe_fn))]
+#![cfg_attr(not(has_rustc_1_52), allow(unused_unsafe))]
 #![allow(clippy::needless_doctest_main)]
 #![allow(missing_docs)]
 // #![stable(feature = "rust1", since = "1.0.0")]