[tialaramex]

Sure, but Rust can have a runtime, and presumably Rust on WASM does.

If you don't have a runtime in Rust you only get core.

https://doc.rust-lang.org/core/

This has obvious effects (you don't have an allocator so you can't have String) and more subtle effects (your slices don't have a sort() method! However they do have a sort_unstable() method) but while it's probably a reasonable environment for the firmware inside a custom $25 gizmo it's not a very comfortable one for general purpose programming.

To deliver a bit more, for example an allocator, you're bringing in a bunch of platform specific code, which, just like the Garbage Collector for Go, you did not write.

[kreetx]

IIRC every higher level language has some runtime, even C. Question is, how trivial/substantial is it?

[tialaramex]

Like Rust, C defines what happens if you don't have its rich standard runtime, you get "free standing" C.

C's free standing mode is even thinner than Rust's core, because it doesn't supply a library of code, you just get the primitive types, the operators and the language features that don't involve any libraries. To give a very concrete example, Rust's core depends on memcmp() existing, Rust assumes your toolchain knows how to memcmp() on your target architecture inherently, but in C you could write memcmp() if you had to. You won't these days, because your C compiler invariably provides this feature, but in principle you could.

In principle C++ also defines a "free standing" mode, but it's a mess and so people don't write for C++ free standing mode in the real world. C++ in an environment where you can't have the actual standard library is likely to be specified in terms of which features from the standard library you can have and which you cannot, for each such environment. For example maybe you can have threading but no filesystem APIs, or you can have a heap allocator but no threads.