Inefficient usage of PathBuf

[sinkuu]

PathBuf comes with inherent mutating methods like push, set_extension, etc. along with methods come from Deref<Target = Path> like join, with_extension, etc.

Each usage of the latter group of methods throws current PathBuf away and creates a new one:

let mut p: PathBuf;

// bad (in terms of performance)
p
 .join("x")              // &PathBuf -> PathBuf
 .with_extension("txt"); // &PathBuf -> PathBuf

// good
p.push("x");
p.set_extension("txt");

The lint to detect this situation may be under perf, but we might as well make it pedantic since the former code looks more clean and idiomatic.

[tesuji]

Or AsRef<Path>::join.join with ::join.push