Match Ergonomics

In their fundamental operation, patterns must have the same type as the place they're matching on, e.g. Some(_) applies to places of type Option<..>. "Match ergonomics" is the feature that allows some mismatches here, specifically this allows patterns to transparently match through references.

The exact details are given in RFC 2005 "Match ergonomics" and the edition guide. In terms of desugaring, this step transforms patterns that involve match ergonomics into patterns that don't, i.e. that have exact types.

#![allow(unused)]
fn main() {
let opt: &&Option<u32> = ..;
if let Some(x) = opt {
    ..
}

// becomes:
if let &&Some(ref x) = opt {
    ..
}
}

After this step, patterns have exact types and explicit binding modes (i.e. x vs ref x vs ref mut x).