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: highlight.rs
    path of file: ./repos/llvm-cov-pretty/src
    
    the code of the file until where you have to start completion: //! Logic for turning plain text source code into HTML spans for code highlighting.
//!
//! Each line of source code is processed and turned into a set of HTML spans with CSS classes.
    
    fn run_highlighter() {
        let output = super::Highlighter::new()
            .source_to_spans(
                indoc! {r#"
                let a = "
                    test
                ";
            "#},
                false,
            )
            .unwrap();
        let expect = vec![
            "<span class=\"syntect-rust syntect-type syntect-storage\">let</span><span \
             class=\"syntect-rust syntect-source\"> a </span><span class=\"syntect-rust \
             syntect-operator syntect-keyword\">=</span> <span class=\"syntect-rust syntect-begin \
             syntect-string syntect-definition syntect-punctuation\">&quot;</span>"
                .to_owned(),
            "<span class=\"syntect-rust syntect-double syntect-quoted syntect-string\">    \
             test</span>"
                .to_owned(),
            "<span class=\"syntect-rust syntect-end syntect-string syntect-definition \
             syntect-punctuation\">&quot;</span><span class=\"syntect-rust syntect-terminator \
             syntect-punctuation\">;</span>"
                .to_owned(),
        ];

        assert_eq!(expect, output);
    }