You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/pants/src/rust/engine/rule_graph/src
    
    the code of the file until where you have to start completion: // Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
// Licensed under the Apache License, Version 2.0 (see LICENSE).

mod builder;
mod rules;

use std::io;

use deepsize::DeepSizeOf;
use fnv::{FnvHashMap as HashMap, FnvHashSet as HashSet};
use indexmap::IndexSet;
u
    
    fn color_string(&self) -> String {
        // These color values are all in HSV. See https://www.graphviz.org/doc/info/colors.html for
        // other methods of specifying
        // colors. https://renenyffenegger.ch/notes/tools/Graphviz/attributes/_color/index may also be
        // useful.
        match self {
            Self::Olive => "0.2214,0.7179,0.8528".to_string(),
            Self::Gray => "0.576,0,0.6242".to_string(),
            Self::Orange => "0.08,0.5,0.976".to_string(),
            Self::Blue => "0.5,1,0.9".to_string(),
        }
    }