cargo/rustc should notify users when optimizations are disabled

[Centril]

Refiling rust-lang/rfcs#777 as it seems more appropriate here.

As the Rust community gets more developers coming from JIT'ed or interpreted languages like Java, Python, Ruby, etc., we are going to see more problems where they assume rustc my_file.rs will give fast results even though optimizations are off by default. This can be seen in several threads on Stack Overflow and other forums, including Rust's own (http://users.rust-lang.org/t/reading-numbers-from-a-file/171).

I suggest that when no optimization flags are enabled, rustc should print out a message similar to the following:

rustc my_file.rs
Note: Optimizations are disabled (debug mode). Turn them on by passing the -O flag.

[sfackler]

I think this has already been implemented:

$ cargo build
   Compiling openssl v0.10.12 (file:///home/sfackler/rust/rust-openssl/openssl)                                                       
    Finished dev [unoptimized + debuginfo] target(s) in 6.58s

[Centril]

@sfackler Yeah that seems good enough to satisfy the original issue's motivation; closing this as completed.