Conditional compilation via if statements

nordlow · 2017-01-14 15:36:51 UTC

How does the Rust compiler analyze, diagnose and optimize if-expressions that are compile-time constants. For instance such as in

github.com

Wilfred/remacs/blob/master/rust_src/build.rs#L27




/// Exit with error $code after printing the $fmtstr to stderr
macro_rules! fail_with_msg {
($code:expr, $modname:expr, $lineno:expr, $($arg:expr),*) => {{
    eprintln!("In {} on line {}", $modname, $lineno);
    eprintln!($($arg),*);
    process::exit($code);
}};
}


struct LintMsg {
modname: String,
lineno: i32,
msg: String,
}


impl LintMsg {
fn new(modname: &str, lineno: i32, msg: String) -> Self {
    Self {
        modname: modname.to_string(),
        lineno: lineno,

?

In D, static if should have been used instead. And the D compilers warn when the if-expression is a compile-time constant (D enum).

DanielKeep · 2017-01-14 15:00:12 UTC

Insofar as I know, it hopes LLVM optimises the branch out.

If you want guaranteed conditional compilation, you need to resort to #[cfg(...)]-ing items, or a cfg-branch macro.