make libm an optional dependency

It still needs to be turned on whenever building for no_std.

Cargo.toml

@@ -19,15 +19,21 @@ specialized-div-rem = { version = "0.0.5", optional = true }
 rand = { version = "0.7", default-features = false, optional = true }
 serde = { version = "1.0", default-features = false, optional = true, features = ["derive"] }
 # Currently needed because of: https://github.com/rust-lang/rfcs/issues/2505
-libm = { version = "0.1", default-features = false }
+libm = { version = "0.1", default-features = false, optional = true }
 
 [dev-dependencies]
 serde_test = "1.0"
 itertools = "0.8"
 rand_xorshift = "0.2"
 
 [features]
-default = ["rand_support", "serde_support", "specialized-div-rem", "std"]
+default = [
+    "rand_support",
+    "serde_support",
+    "specialized-div-rem",
+    "std",
+    "libm_0",
+]
 std = [
     "smallvec/std",
     "rand/std",
@@ -39,6 +45,8 @@ rand_support = [
     "rand/getrandom",
 ]
 serde_support = ["serde"]
+# Usage of libm is required for `no_std` builds.
+libm_0 = ["libm"]
 
 [badges]
 travis-ci = { repository = "Robbepop/apint" }

src/apint/serialization.rs

@@ -259,6 +259,7 @@ impl ApInt {
             Digit,
             DigitRepr,
         };
+        #[cfg(feature = "libm_0")]
         use libm::F64Ext as _;
 
         debug_assert!(!v.is_empty() && !radix.is_power_of_two());