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Macro Madness
Pascal Hertleif
2019-07-09
rust
presentation
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Hi, I'm Pascal Hertleif

  • Senior dev here at TTC
  • Dev tools team
  • CLI WG
  • {twitter,github}.com/killercup

What this talk will teach you

Just kidding!

This topic is way too large for this short a talk

I just don't want to write the same code over and over again

. . .

I also like to make up abstract solution

. . .

This is a dangerous combination…

Case Study 1: A repetitive impl

A derive macro for a specialized Debug

Code: github.com/killercup/snafu-cli-debug

Case Study 2: A byte pattern

Task: Find a pattern like fd fd ? ? df 1e in a slice of bytes

Idea: Use regex!

But how do turn that pattern format into a valid regular expression?

With a macro of course! And so we dove into the world of macro_rules! macros…

Token trees

Problem: Can't parse a string in pattern macros

. . .

Approach: Munch single tokens one after the other and concat to a string!

byte_pattern!(fd fd ? ? df 1e)

. . .

cf. The Little Book of Rust Macros

Limits of macro_rules!

Except that macros operate on Rust tokens…

error: expected at least one digit in exponent
 --> src/main.rs:6:34
  |
6 |     byte_pattern!(fd fd ? ? df 1e)
  |                                  ^

(code on playpen)

And a proc it was

Since function-like macros aren't stable yet, we went for a derive

#[derive(Debug, BytePattern)]
#[byte_pattern="fd fd (? ?) df 1e"]
struct MyMatch<'source> {
  capture: &'source [u8; 2],
}

In case of errors, add more crates

Crates that are proc-macro = true are a bit special

For example: Adding nom as a dependency to a proc-macro crate with tests results in link error

::: notes

Splitting implementations up into multiple crates makes most errors go away, except for what to call them.

Ideas for names:

  • -core
  • -common
  • -shared
  • -lib
  • -derive
  • -macros
  • -base
  • -helpers
  • -utils

:::

Case Study 3: This escalated quickly

'twas a fortnight ago, when I found myself in the peculiar situation of having written a rather complex PEG grammar, that I then had to parse into an AST of sorts…

::: notes

Ramble on and on about the joy of parsing stuff and building ASTs.

…for however much time is still available.

:::

Long story short:

  • pest is a parser using its own grammer language
  • pest-ast is a derive macro for building data types from pest token trees
  • pest-ast-generator is a derive macro that generates the structs that it calls pest-ast on by reading the original grammar

Thank you!

I'm Pascal and I want you to talk to me!

{twitter,github}.com/killercup

Slides: git.io/rust-macro-madness