refactor(instant): Make instant an optional dependency

Cargo.toml

@@ -19,7 +19,7 @@ travis-ci = { repository = "ihrwein/backoff" }
 [dependencies]
 async_std_1 = { package = "async-std", version = "1.9", optional = true }
 futures-core = { version = "0.3.8", default-features = false, optional = true }
-instant = "0.1"
+instant = { version = "0.1", optional = true }
 pin-project-lite = { version = "0.2.7", optional = true }
 rand = "0.8"
 getrandom = "0.2"
@@ -32,8 +32,9 @@ tokio_1 = { package = "tokio", version = "1.0", features = ["macros", "time", "r
 futures-executor = "0.3"
 
 [features]
-default = []
-wasm-bindgen = ["instant/wasm-bindgen", "getrandom/js"]
+default = ["instant"]
+instant = ["dep:instant"]
+wasm-bindgen = ["instant", "instant/wasm-bindgen", "getrandom/js"]
 futures = ["futures-core", "pin-project-lite"]
 tokio = ["futures", "tokio_1"]
 async-std = ["futures", "async_std_1"]

src/clock.rs

@@ -1,4 +1,4 @@
-use instant::Instant;
+use crate::instant::Instant;
 
 /// Clock returns the current time.
 pub trait Clock {

src/error.rs

@@ -1,7 +1,7 @@
 use std::error;
 use std::fmt;
 
-use instant::Duration;
+use std::time::Duration;
 
 /// Error is the error value in an operation's
 /// result.

src/exponential.rs

@@ -1,10 +1,10 @@
-use instant::Instant;
 use std::marker::PhantomData;
 use std::time::Duration;
 
 use crate::backoff::Backoff;
 use crate::clock::Clock;
 use crate::default;
+use crate::instant::Instant;
 
 #[derive(Debug)]
 pub struct ExponentialBackoff<C> {

src/instant.rs

@@ -0,0 +1,4 @@
+#[cfg(feature = "instant")]
+pub use instant::Instant;
+#[cfg(not(feature = "instant"))]
+pub use std::time::Instant;

src/lib.rs

@@ -217,6 +217,7 @@ mod clock;
 pub mod default;
 mod error;
 pub mod exponential;
+pub mod instant;
 
 #[cfg(feature = "futures")]
 #[cfg_attr(docsrs, doc(cfg(feature = "futures")))]

tests/exponential.rs

@@ -1,11 +1,10 @@
 extern crate backoff;
-extern crate instant;
 
 use backoff::backoff::Backoff;
 use backoff::exponential::ExponentialBackoff;
+use backoff::instant::Instant;
 use backoff::{Clock, SystemClock};
 
-use instant::Instant;
 use std::cell::RefCell;
 use std::time::Duration;