Implement uDebug for str

[hnj2]

This implementation of uDebug shouldn't contain any panicking branches.
To do this I've used the unsafe str::get_unchecked function.
The necessary safety requirements result form the use of the str::char_indices and char::len_utf8 functions.

[japaric]

Thanks for the PR but this fails the nopanic check if you uncomment this line:

ufmt/nopanic/examples/str.rs

Line 29 in 7edbb12

// uwrite!(&mut W, "{:?}", a).unwrap();

The problem is not your implementation but rather that something in the core API you are using (maybe escape_debug or even char_indices?) brings in a panicking branch. The fix would require changes in libcore itself.

---

To have a better idea of where the panicking branch comes from you can use cargo-call-stack:

$ cargo new --bin app
$ cd app

change src/main.rs to this

#![no_main]
#![no_std]

use core::{convert::Infallible, panic::PanicInfo, ptr};

use ufmt::{uWrite, uwrite};

#[no_mangle]
fn _start() -> usize {
    write as usize
}

fn write(s: &str) {
    uwrite!(&mut W, "{:?}", s).unwrap();
}

struct W;

impl uWrite for W {
    type Error = Infallible;

    fn write_str(&mut self, s: &str) -> Result<(), Infallible> {
        s.as_bytes().iter().for_each(|b| unsafe {
            let _ = ptr::read_volatile(b);
        });

        Ok(())
    }
}

#[panic_handler]
fn panic(_: &PanicInfo) -> ! {
    loop {}
}

add this to the Cargo.toml (apart from the ufmt dependency)

[profile.release]
codegen-units = 1
incremental = false
lto = 'fat'
opt-level = 'z'

run this to build a call graph

$ rustup default nightly
$ rustup component add rust-src
$ rustc -V
rustc 1.65.0-nightly (f03ce3096 2022-08-08)

$ cargo call-stack --target thumbv7m-none-eabi --bin app > cg.dot
$ dot -Tsvg cg.dot -o cg.svg

if you insert some #[inline(never)] (I didn't try) you may be able to get more resolution. cargo-call-stack doesn't show inlined function calls