rcchars

book.toml

@@ -5,6 +5,9 @@ multilingual = false
 src = "src"
 title = "Exercises"
 
+[rust]
+edition = "2021"
+
 [build]
 build-dir = "book"
 create-missing = false

src/SUMMARY.md

@@ -7,3 +7,6 @@
   - [test1.rs (refbind)](./exercises/refbind.md)
   - [A/A1/A2](./exercises/multiple_blanket.md)
   - [BoolStream](./exercises/bool_stream.md)
+  - [RcChars](./exercises/rcchars.md)
+- [Chapter 2](./chapter_2.md)
+  - [AnyStr](./exercises/anystr.md)

src/chapter_2.md

@@ -0,0 +1 @@
+# Chapter 2

src/exercises/anystr.md

@@ -0,0 +1,3 @@
+Create something similar to [Python's `typing.AnyStr`].
+
+[Python's `typing.AnyStr`]: https://docs.python.org/3/library/typing.html#typing.AnyStr

src/exercises/bool_stream.md

@@ -1,6 +1,29 @@
 Make `test_covariance` compile by making `BoolStream<'a>` covariant `'a`. Restrictions:
 - Can only change implementation details of `BoolStream` and its methods and add extra items outside of what's given, i.e. no signature/test change.
 - Changed version must behave the same way.
+
+Consider the following code:
+```rust,compile_fail
+pub struct BoolStream<'a>(
+    Box<dyn 'a + FnOnce() -> (bool, BoolStream<'a>)>,
+);
+
+impl<'a> BoolStream<'a> {
+    pub fn new(f: impl 'a + FnOnce() -> (bool, Self)) -> Self {
+        Self(Box::new(f))
+    }
+
+    pub fn next(self) -> (bool, Self) {
+        self.0()
+    }
+}
+
+fn test_covariance<'a: 'b, 'b>(e: BoolStream<'a>) -> BoolStream<'b> {
+    e
+}
+```
+Why does it fail to compile?
+
 ```rust
 pub struct BoolStream<'a>(
     // change this

src/exercises/rcchars.md

@@ -0,0 +1,50 @@
+[`Chars`] but keeping a strong reference ([`Rc`]) to the string.
+
+[`Chars`]: https://doc.rust-lang.org/std/str/struct.Chars.html
+[`Rc`]: https://doc.rust-lang.org/std/rc/struct.Rc.html
+
+```rust
+# mod rcchars {
+# pub struct RcChars { _rc: std::rc::Rc<String>, chars: std::mem::MaybeUninit<std::str::Chars<'static>> }
+# impl Iterator for RcChars {
+# type Item = char;
+# fn next(&mut self) -> Option<Self::Item> { unsafe { self.chars.assume_init_mut() }.next() }
+# }
+# impl RcChars {
+# pub fn from_rc(rc: std::rc::Rc<String>) -> Self {
+# let mut new = Self { _rc: rc, chars: std::mem::MaybeUninit::uninit() };
+# new.chars .write(unsafe { &*std::rc::Rc::as_ptr(&new._rc) }.chars());
+# new
+# }
+# }
+# impl From<std::rc::Rc<String>> for RcChars {
+# fn from(value: std::rc::Rc<String>) -> Self { Self::from_rc(value) }
+# }
+# impl Drop for RcChars {
+# fn drop(&mut self) { unsafe { self.chars.assume_init_drop() } }
+# }
+# }
+# use std::rc::Rc;
+# use rcchars::RcChars;
+{
+    let rc = Rc::new("abc".to_string());
+    let rcc = RcChars::from(rc.clone());
+    drop(rc);
+    assert_eq!(rcc.collect::<Vec<_>>(), ['a', 'b', 'c']);
+}
+{
+    let rc = Rc::new("abc".to_string());
+    let rcc = RcChars::from(rc);
+    let rcc = Box::new(rcc);
+    assert_eq!(rcc.collect::<Vec<_>>(), ['a', 'b', 'c']);
+}
+```
+
+## Solutions
+- [Solution] used in rattlescript.
+- [Same solution] but with some extra comments.
+- [Another solution].
+
+[Solution]: https://github.com/HavenSelph/rattlescript/blob/f8bafb8b063b9bf056efb1ea14188db0624d981c/src/interpreter/value.rs#L21-L50
+[Same solution]: https://gist.github.com/timotheyca/7e46c9734653b2fcbe826ea4d13b9aa0
+[Another solution]: https://gist.github.com/timotheyca/bc419e0997d6f7ea5722fd2823a78c62

src/overview.md

@@ -2,9 +2,11 @@
 
 Random Rust exercises.
 
+**Warning: this is a live-edited website, errors and accidental spoilers are to be expected.**
+
 # Solutions
 
-Currently solutions are only provided in a non-human-readable form in the page source.
+Currently solutions are mostly provided in a non-human-readable form in the page source.
 Those are mostly intended for testing that the exercise is even possible to solve.
 ```rust
 # /*