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A proc-macro to break Rust IDEs
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A proc-macro to apply Caesar cipher to Rust source code.


caesar! {

sa pnrfne_qrpbqr(f: &fge) -> Fgevat {
    erghea f.punef().znc(qrpbqr_pune).pbyyrpg();

    sa qrpbqr_pune(p: pune) -> pune {
        zngpu p {
            'a'...'z' => ebg(p, 'a'),
            'A'...'Z' => ebg(p, 'A'),
            _ => p,

    sa ebg(p: pune, onfr: pune) -> pune {
        fgq::pune::sebz_h32(((p nf h32 - onfr nf h32) + 13) % 26 + onfr nf h32).hajenc()


fn decoding_works() {
        &caesar_decode("Ornhgvshy vf orggre guna htyl."),
        "Beautiful is better than ugly."


To break code completion. Seriously, the single reason this crate exists is to break code completion in IDEs.

More specifically, this is a simple demonstration that it is in general impossible to provide IDE features for code inside macro invocations.

Consider this snippet of code:

use std::collections::HashMap;

lazy_static! {
    static ref HASHMAP: HashMap<u32, &'static str> = {
        let mut m = HashMap::new();
        m.insert(0, "foo");
        m.insert(1, "bar");
        m.insert(2, "baz");

fn main() {
    let entry = HASHMAP.get(&0).unwrap();
    println!("The entry for `0` is \"{}\".", entry);

"IDE support for macros" here means two things:

  • inside main, IDE should complete HASHMAP
  • inside lazy_static!, IDE should complete HashMap

The first case is "easy": IDE needs to "just" expand the macro and process the resulting code as usual.

The second case, as proven by proc-caesar macro, is impossible to handle in general: connection between the tokens after the bang and the expanded code might be arbitrary complex. To provide correct completions for code inside caesar! macro, IDE would have to figure out the inverse transformation of identifiers!

Future Work

Currently the crate uses a particularly weak cipher, so a sufficiently smart AI-powered IDE could in theory figure out the inverse transformation required for completions. To make this macro AI-proof, we need to apply public-key cryptography, to make sure that computing the inverse transformation is computationally infeasible.

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