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: debug.rs
    path of file: ./repos/tfs/conc/src
    
    the code of the file until where you have to start completion: //! Runtime debugging tools.

#[cfg(feature = "debug-tools")]
extern crate backtrace;

/// Execute closure when the environment variable, `CONC_DEBUG_MODE`, is set.
///
/// When compiled in release mode, this is a NOP.
#[cfg(feature = "debug-tools")]
pub fn exec<F: FnOnce()>(f: F) {
    use self::backtrace::Backtrace;
    use std::env;

    thread_local! {
        /// Is `CONC_DEBUG_MODE` set?
        ///
        /// This is cached to avoid expensive repe
    
pub fn exec<F: FnOnce()>(f: F) {
    use self::backtrace::Backtrace;
    use std::env;

    thread_local! {
        /// Is `CONC_DEBUG_MODE` set?
        ///
        /// This is cached to avoid expensive repeated syscalls or similar things.
        static DEBUG_MODE_ENABLED: bool = env::var("CONC_DEBUG_MODE").is_ok();
        /// Is `CONC_DEBUG_STACKTRACE` set?
        ///
        /// This is cached to avoid expensive repeated syscalls or similar things.
        static STACK_TRACE_ENABLED: bool = env::var("CONC_DEBUG_STACKTRACE").is_ok();
    }

    // If enabled, run the closure.
    if DEBUG_MODE_ENABLED.with(|&x| x) {
        f();
        if STACK_TRACE_ENABLED.with(|&x| x) {
            println!("{:?}", Backtrace::new());
        }
    }
}