duration

src/SUMMARY.md

@@ -5,6 +5,7 @@
 - [Chapter 1](./chapter_1.md)
   - [test0.rs (bind)](./exercises/bind.md)
   - [test1.rs (refbind)](./exercises/refbind.md)
+  - [Duration](./exercises/duration.md)
   - [A/A1/A2](./exercises/multiple_blanket.md)
   - [BoolStream](./exercises/bool_stream.md)
   - [RcChars](./exercises/rcchars.md)

src/exercises/bind.md

@@ -1,4 +1,4 @@
-Make this compile and pass tests.
+Make this compile and pass tests:
 ```rust
 # mod __ {
 fn bind<T, F: Fn(T) -> Option<T>>(f: F, fa: Option<T>) -> Option<T> {

src/exercises/duration.md

@@ -0,0 +1,25 @@
+# `const` `Add`?
+
+Define `FIVE_SECONDS` as `const`:
+```rust
+# use std::time::Duration;
+# const fn unwrap_time(d: Option<Duration>) -> Duration { match d { Some(d) => d, None => panic!() } }
+const TWO_SECONDS: Duration = Duration::from_secs(2);
+# /*
+const FIVE_SECONDS: Duration = TWO_SECONDS + Duration::from_secs(3);
+# */
+# const FIVE_SECONDS: Duration = unwrap_time(TWO_SECONDS.checked_add(Duration::from_secs(3)));
+assert_eq!(FIVE_SECONDS, Duration::from_secs(5));
+```
+
+Try solving it in the playground:
+```rust,editable,compile_fail
+use std::time::Duration;
+
+const TWO_SECONDS: Duration = Duration::from_secs(2);
+const FIVE_SECONDS: Duration = TWO_SECONDS + Duration::from_secs(3);
+
+fn main() {
+    assert_eq!(FIVE_SECONDS, Duration::from_secs(5));
+}
+```

src/exercises/refbind.md

@@ -1,4 +1,4 @@
-Make this compile and pass tests.
+Make this compile and pass tests:
 ```rust
 # mod __ {
 fn refbind<T, F: Fn(&T) -> Option<&T>>(f: F, fa: Option<&T>) -> Option<&T> {

src/topics.md

@@ -3,8 +3,8 @@
 Rows are ordered based on estimated difficulty.
 
 | lifetimes\[-adjacent\] | `trait`s           | `struct`s    |
-|------------------------|--------------------|-------------|
+|------------------------|--------------------|--------------|
-| [explicit lifetimes]   | [`Fn`?]            |             |
+| [explicit lifetimes]   | [`Fn`?]            | [`Duration`] |
 | [extracting lifetimes] | [exclusive traits] | [`RcChars`]  |
 | [composition]          | [`AnyStr`]         |              |
 |                        | [merging traits]   |              |
@@ -18,3 +18,4 @@ Rows are ordered based on estimated difficulty.
 [merging traits]: ./exercises/modes.md
 [`AnyStr`]: ./exercises/anystr.md
 [composition]: ./exercises/composition.md
+[`Duration`]: ./exercises/duration.md