The only case I think I've seen where a #![no_std] library ends up pulling in libc is if you haven't added a custom allocator and your platform's default allocator uses libc (and so you could switch to a libc-free allocator if you want). Are there other cases?
The good news is that every problem I found had an actively discussed GitHub issue and the community is active, so there will be progress.
Yeah, I've run into that, but IIRC this got solved super recently/is being solved. Probably https://github.com/rust-lang/cargo/pull/7820 ?
"Could be useful for" (FTFY).
Unfortunately, for the reasons mentioned above, most #![no_std] libraries aren't useful for that, because they link libc and other libraries.
Most people don't do this intentionally. The compiler doesn't complain about:
#![no_std]
extern "C" { fn malloc(...) -> ...; }
When you then go and build a #![no_std] binary, and run it in a platform when libc is not available, only then you get an error. And at that point, good luck figuring out which of your 200 dependencies has the error.
In particular, if you are running `#[test]`s, doing that links standard, so if your program implicitly depends on libc somewhere, tests won't reveal that, because while testing, libc will be linked.
It's actually hard to go out of your way and call malloc directly, because FFI calls are unsafe. It's a lot easier to use Box/Vec/String/etc., all of which are defined in liballoc and use the pluggable allocator interface.
I know this because I've successfully used #![no_std] libraries in places where libc doesn't exist and no function named malloc exists, and they do work. If you're having a linker issue it's almost certainly because you haven't changed the default allocator - if you have an example of this I'd be happy to take a look at debugging it.
Maybe you are just using different no_std libraries that I am using, but pretty much all of the no_std libraries that I use have `libc` as a direct dependency.
Not only for malloc, many of them just needs to write something to stdout or a file, generate random numbers, ..., and that's hard to do without libc. Why they advertise themselves as no_std escapes my comprehension.
The two main issues I see with #![no_std] are:
* its a flag for "doesn't link the standard library" but the standard library is often too high-level for bare metal apps that want to be in precise control about everything that gets linked
* it isn't a contract of any kind, so you can't rely on it for anything. In fact, this code is correct and works as intended:
#![no_std]
extern crate std;
So I agree with you that #![no_std] isn't a silver bullet. I think it is still useful in that it lets you avoid linking the standard library by default, which is necessary condition for embedded development. It is not a sufficient condition, in that in practice you actually want to forbid the standard library and other libraries from being linked, and not just "don't link them by default, but do so by accident".
Project I'm working on right now is `#[no_std]`, but my own sysroot covers 90% of real `std`. Which is why I even export it as `std`, so I can use pretty much all crates I would usually use. Because if your platform has an allocator, doesn't matter where it came from, then you can add `liballoc`. `libstd` = `libcore` + `liballoc` + locks + allocator + threads + a few other things that usually depend liballoc. Which means that you have nearly entire libstd available to you if your target platform has an allocator.
Most `#[no_std]` crates that depend on allocator clearly state that they depend on `liballoc` and that you gotta provide one. What I'm trying to say `#[no_std]` doesn't mean embed, it just means sysroot isnt' available for target.
To me, Go is the systems programming language for a world untethered by existing platform compatibility (and so, for instance, writing Go libraries to be called from C is awkward, calling C libraries from Go incurs overhead, Go has its own concurrency model, etc.) and Rust is the systems programming language for use cases where you'd otherwise want to use C (really "the platform's native systems language," but that's C on all the major platforms) but you want a better language. I appreciate that they both exist and target these different use cases.
Over the years I’ve gathered that it’s more of a competitor for C# & Java rather than Rust & C(++).
You can write it any language that runs on OS with linux's kernel. There are similar things written and damn bash. Does that make bash a system language?
Worth noting however that `writeln` only needs `core`, and as long you can provide an implementation of writing to standard output (pretty much create a struct and implement `core::fmt::Write` trait for it), creating `println` macro is trivial.
How did you come to this conclusion?
People using Rust rely on libc a lot.
For example, #![no_std] means "no standard-library", but it doesn't mean "no libc, no libunwind, etc.".
So a lot of people like to advertise their crates as "#![no_std]" compatible, because they compile with #![no_std], but then the first thing the crate does is linking against libc, against liballoc, against libm, .... or even the standard library itself...
So... if you are trying to build a binary that does not depend on libc or libstd, then there is no language feature to help you there.
#[no_std] binaries are not only unstable, but also probably not what you want, since that won't prevent you from linking any library that links libc, or the standard library, etc.
If you want to avoid libc, you have to enforce that, e.g., by checking in your build system that your binary doesn't contain any libc, libstd, etc. symbols (I just use nm for this). #![no_std] helps a bit here, but making sure you don't link any library that violates this is up to you.