fromref

src/SUMMARY.md

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

src/exercises/fromref.md

@@ -0,0 +1,29 @@
+Make this compile and pass tests:
+```rust
+fn with_slice<T>(f: impl FnOnce(&str) -> T) -> T {
+    f("test")
+}
+
+# pub trait FromRef<T: ?Sized>: for<'a> From<&'a T> { fn from_ref(value: &T) -> Self { Self::from(value) } }
+# impl<T: ?Sized, U: for<'a> From<&'a T>> FromRef<T> for U {}
+let string = with_slice(
+# /*
+    String::from
+# */ String::from_ref
+);
+assert_eq!(string, "test".to_string());
+```
+
+Try solving it in the playground:
+```rust,editable,compile_fail
+fn with_slice<T>(f: impl FnOnce(&str) -> T) -> T {
+    f("test")
+}
+
+fn main() {
+    let string = with_slice(
+        String::from // Change this
+    );
+    assert_eq!(string, "test".to_string());
+}
+```

src/topics.md

@@ -5,9 +5,9 @@ Rows are ordered based on estimated difficulty.
 | lifetimes\[-adjacent\] | `trait`s           | `struct`s     |
 |------------------------|--------------------|---------------|
 | [explicit lifetimes]   | [`Fn`?]            | [`Duration`]  |
-| [extracting lifetimes] | [exclusive traits] | [All Errors?] |
+| [from reference]       | [exclusive traits] | [All Errors?] |
-| [composition]          | [merging traits]   | [`RcChars`]   |
+| [extracting lifetimes] | [merging traits]   | [`RcChars`]   |
-|                        |                    | [`AnyStr`]    |
+| [composition]          |                    | [`AnyStr`]    |
 
 p.s. most of the exercises are actually about `trait`s, this categorisation isn't strict.
 
@@ -22,3 +22,4 @@ p.s. most of the exercises are actually about `trait`s, this categorisation isn'
 [composition]: ./exercises/composition.md
 [`Duration`]: ./exercises/duration.md
 [All Errors?]: ./exercises/all_errors.md
+[from reference]: ./exercises/fromref.md