add: duration_checked_sub RFC

text/0000-duration-checked-sub.md

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+- Feature Name: duration_checked_sub
+- Start Date: 2016-06-04
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+This RFC adds the `checked_*` methods already known from primitives like
+`usize` to `Duration`.
+
+# Motivation
+[motivation]: #motivation
+
+Generally this helps when subtracting `Duration`s which can be the case quite
+often.
+
+One abstract example would be executing a specific piece of code repeatedly
+after a constant amount of time.
+
+Specific examples would be a network service or a rendering process emitting a
+constant amount of frames per second.
+
+Example code would be as follows:
+
+```rust
+
+// This function is called repeatedly
+fn render() {
+    // 10ms delay results in 100 frames per second
+    let wait_time = Duration::from_millis(10);
+
+    // `Instant` for elapsed time
+    let start = Instant::now();
+
+    // execute code here
+    render_and_output_frame();
+
+    // there are no negative `Duration`s so this does nothing if the elapsed
+    // time is longer than the defined `wait_time`
+    start.elapsed().checked_sub(wait_time).and_then(std::thread::sleep);
+}
+```
+
+Of course it is also suitable to not introduce `panic!()`s when adding
+`Duration`s.
+
+# Detailed design
+[design]: #detailed-design
+
+The detailed design would be exactly as the current `sub()` method, just
+returning an `Option<Duration>` and passing possible `None` values from the
+underlying primitive types:
+
+```rust
+impl Duration {
+    fn checked_sub(self, rhs: Duration) -> Duration {
+        if let Some(mut secs) = self.secs.checked_sub(rhs.secs) {
+            let nanos = if self.nanos >= rhs.nanos {
+                self.nanos - rhs.nanos
+            } else {
+                if let Some(secs) = secs.checked_sub(1) {
+                    self.nanos + NANOS_PER_SEC - rhs.nanos
+                }
+                else {
+                    return None;
+                }
+            };
+            debug_assert!(nanos < NANOS_PER_SEC);
+            Duration { secs: secs, nanos: nanos }
+        }
+        else {
+            None
+        }
+    }
+}
+```
+
+The same accounts for all other added methods, namely:
+
+- `checked_add()`
+- `checked_sub()`
+- `checked_mul()`
+- `checked_div()`
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+`None`.
+
+# Alternatives
+[alternatives]: #alternatives
+
+The alternatives are simply not doing this and forcing the programmer to code
+the check on their behalf.
+This is not what you want.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+`None`.
+