error[E0507]: cannot move out of borrowed content
   --> src/lib.rs:126:29
    |
126 |         if let Some(code) = diagnostic.code {
    |                     ----    ^^^^^^^^^^ cannot move out of borrowed content
    |                     |
    |                     hint: to prevent move, use `ref code` or `ref mut code`

Why?

Rust is all about compile-time checks

But also: Rust’s errors are pretty and helpful

Warnings, too

• Lints
• Prevent errors
• Examples
  • unused Results or Iterators
  • Missing docs

Clippy

It looks like you’re enjoying errors. Would you like some more?

• 208 lints written by the community
• Find common bugs
  • For example: if_same_then_else

iter.filter(|x| x == 0).next()

if !m.contains_key(&k) { m.insert(k, v) }

• Learn new APIs when you are almost using them
• Each clippy lint is documented with examples

Suggestions

error: this range is empty so this for loop will never run
   --> $DIR/for_loop.rs:148:14
    |
148 |     for i in 10..0 {
    |              ^^^^^
    |
    = note: `-D reverse-range-loop` implied by `-D warnings`
help: consider using the following if you are attempting to iterate over this range in reverse
    |
148 |     for i in (0..10).rev() {
    |              ^^^^^^^^^^^^^

Why fix it yourself?

I’m sold. These suggestions are great. But…

• don’t make me type
• don’t make me wait
• just fix my code already

It’s already there

No magic! Just:

• Take existing errors
• Parse the compiler output (JSON)
• Apply suggestions (search & replace)

Make it interactive (1)

rustfix as CLI tool is interactive

• for each error (that has a suggestion) it
  • shows you the error and the fix(es)
  • asks what you want to do
  • fixes your code

Make it interactive (2)

You need to manually call the CLI tool

You do that after working on the code

We can do better

RLS

• Rust Language Server exposes warnings and suggestions
• VSCode plugin can already apply fixes

Laptop didn’t work?

Totally saw that coming.

– Past Me

How does this work?

• Compiler plugin
• Find nodes and items of interest
• ‘Annotate’ spans of code
• Compiler generates human/JSON output

What is auto-fixable?

• A lot!
• Do we need to make a list?
• Rust PR #47540 adds flag for “approximate suggestions”
• rustfix-like tools will use this in the future
• UPDATE Merged! 🎉

You can contribute lints and suggestions!

Search for “Rust Clippy” or go to https://github.com/rust-lang-nursery/rust-clippy

There are a lot of issues labelled good first issue

You get to call yourself a compiler hacker!

Any questions?

Visit rust-lang.org

Follow me on Twitter: @killercup

Slides available at git.io/fosdem-rustfix