support stable rust

Signed-off-by: Yang Keao <keao.yang@yahoo.com>

Cargo.toml

@@ -14,6 +14,7 @@ default = ["cpp"]
 flamegraph = ["inferno"]
 protobuf = ["prost", "prost-derive", "prost-build"]
 cpp = ["symbolic-demangle/cpp"]
+heap = []
 
 [dependencies]
 backtrace = "0.3"
@@ -55,7 +56,7 @@ required-features = ["flamegraph"]
 
 [[example]]
 name = "heap_profiler"
-required-features = ["protobuf", "flamegraph"]
+required-features = ["protobuf", "flamegraph", "heap"]
 
 [[bench]]
 name = "collector"

src/lib.rs

@@ -21,7 +21,7 @@
 //!};
 //! ```
 
-#![feature(const_fn)]
+#![cfg_attr(feature = "heap", feature(const_fn))]
 
 /// Define the MAX supported stack depth. TODO: make this variable mutable.
 pub const MAX_DEPTH: usize = 32;
@@ -32,15 +32,20 @@ pub const MAX_THREAD_NAME: usize = 16;
 mod collector;
 mod error;
 mod frames;
+#[cfg(feature = "heap")]
 mod heap_profiler;
+
 mod profiler;
 mod report;
 mod timer;
 
 pub use self::collector::{Collector, StackHashCounter};
 pub use self::error::{Error, Result};
 pub use self::frames::{Frames, Symbol};
+
+#[cfg(feature = "heap")]
 pub use self::heap_profiler::{AllocRecorder, HeapProfilerGuard};
+
 pub use self::profiler::ProfilerGuard;
 pub use self::report::{Report, ReportBuilder};