Split backtrace and std features up.

.travis.yml

@@ -8,3 +8,5 @@ cache: cargo
 script:
 - cargo test
 - cargo test --no-default-features
+- cargo test --no-default-features --features std
+- cargo test --no-default-features --features backtrace

Cargo.toml

@@ -17,8 +17,8 @@ optional = true
 version = "0.3.3"
 
 [features]
-default = ["std", "derive"]
-std = ["backtrace"]
+default = ["std", "derive", "backtrace"]
+std = []
 derive = ["failure_derive"]
 
 [[example]]

src/backtrace/mod.rs

@@ -1,6 +1,6 @@
 use core::fmt::{self, Debug, Display};
 
-without_std! {
+without_backtrace! {
     /// A `Backtrace`.
     ///
     /// This is an opaque wrapper around the backtrace provided by
@@ -39,6 +39,16 @@ without_std! {
         pub fn new() -> Backtrace {
             Backtrace { _secret: () }
         }
+
+        #[cfg(feature = "std")]
+        pub(crate) fn none() -> Backtrace {
+            Backtrace { _secret: () }
+        }
+
+        #[cfg(feature = "std")]
+        pub(crate) fn is_none(&self) -> bool {
+            true
+        }
     }
 
     impl Default for Backtrace {
@@ -60,7 +70,7 @@ without_std! {
     }
 }
 
-with_std! {
+with_backtrace! {
     extern crate backtrace;
 
     mod internal;

src/lib.rs

@@ -16,6 +16,12 @@
 
 macro_rules! with_std { ($($i:item)*) => ($(#[cfg(feature = "std")]$i)*) }
 macro_rules! without_std { ($($i:item)*) => ($(#[cfg(not(feature = "std"))]$i)*) }
+macro_rules! with_backtrace {
+    ($($i:item)*) => ($(#[cfg(all(feature = "backtrace", feature = "std"))]$i)*)
+}
+macro_rules! without_backtrace {
+    ($($i:item)*) => ($(#[cfg(any(not(feature = "backtrace"), not(feature = "std")))]$i)*)
+}
 
 mod backtrace;
 mod compat;