Rust Roadmap 2021: Adding a useful dev mode and making it the default

In this blog entry following the Call for Rust 2021 Roadmap items, I shall layout my vision for giving Rust the ability to create debuggable binaries which will also fit in the flash of a microcontroller.


This request has multiple motivations but following the Call for Rust 2021 Roadmap items rules I'm notating them in the agile user story way:

So let me give you a bit more background about the status quo and what I mean by those two stories before giving you my ideas how to address them.

Debuggable applications which fit into flash

The "problem" with Rust applications in general is that once even just the bare-minimum optimisations are enabled, the compiler will completely restructure the whole application in a way that makes debugging hard or even impossible.

On the other hand, if optimisations are completely disabled, rustc will happily churn out code which is so highly inefficient and large that it either won't fit in the available flash space or be so slow that crucial deadlines can not be met (like USB handling) and the application will simply fail; quite often those two issues also rear their ugly heads at the same time. It also introduces panicing branches into code which is not even supposed to be anything more than a compiler hint, cf. Issue 68208.

Other languages do a lot better by creating more efficient code without any optimisations but they also offer optimisation levels which don't or only lightly effect the debugability of an application built with them.

In addition to the code generation problems it's also a big stumbling block and source of surprise and frustration that rustc uses dev mode by default, often creating unusable binaries or even just errors.

Imagine exploring Rust coming from C and seeing that the most basic program suddenly takes up many kB of precious flash instead of a few hundred Bytes when compiled with default flags (and including a fixed overhead of a few hundred bytes):

   text	   data	    bss	    dec	    hex	filename
    532	      0	      0	    532	    214	miniblink.elf


   text    data     bss     dec     hex filename
   9976       0       4    9980    26fc blinky

Implementation ideas

I think it should be possible to come up with a set of debugging friendly basic optimisations applied at opt-level=g which operate similar to clang's or gcc's -Og. Those are similar to opt-level=1 but disable debugging harming optimisations. Furthermore this level should be the default for dev mode to make Rust in embedded scenarios useful by default!

Thanks for your attention and hope to blog to y'all soon!