draft: cargo-teach

Author: killercup

_drafts/2017-08-14-converting-rustfix-to-cargo-teach.md

@@ -0,0 +1,75 @@
+---
+title: Converting Rustfix to cargo teach
+category:
+- rust
+---
+
+## User story examples
+
+### Binary tree
+
+Inspired by [this post][mre-tree] by [Matthias Endler][mre].
+
+```rust
+struct Tree {
+  root: i64,
+  left: Tree,
+  right: Tree,
+}
+```
+
+```sh
+cargo check
+```
+
+```
+error[E0072]: recursive type `Tree` has infinite size
+ --> src/main.rs:1:1
+  |
+1 | struct Tree {
+  | ^^^^^^^^^^^ recursive type has infinite size
+  |
+  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Tree` representable
+```
+
+```sh
+cargo teach
+```
+
+```
+==> Hi there! Let me have a look at that nice `$CRATE_NAME` code!
+  > ...done!
+  > This looks pretty good! You'll need to tweak a few things to make it work, though.
+  > Let's get started!
+
+==> Have a look at `src/main.rs`:
+  |
+1 | struct Tree {
+2 |     root: i64,
+3 |     left: Tree,
+4 |     right: Tree,
+5 | }
+  |
+
+==> Sadly, this doesn't work, because: Recursive type `Tree` has infinite size.
+  > When defining a recursive struct or enum, any use of the type being defined
+  > from inside the definition must occur behind a pointer (like `Box` or `&`).
+  > This is because structs and enums must have a well-defined size, and without
+  > the pointer, the size of the type would need to be unbounded.
+
+  ( ) I don't get it. Explain some more?
+  (x) I see! What should I do?
+
+==> Thanks for asking!
+  > I'd suggest you insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Tree` representable
+```
+
+This uses:
+
+- Quoted code showing the whole item (not just the header here)
+- Error messages, transformed to work in sentence
+- First paragraph of `rustc --explain E0072`
+- Suggestion
+
+[mre]: https://matthias-endler.de/
+[mre-tree]: https://matthias-endler.de/2017/boxes-and-trees/