Compiler bug when implementing core::fmt::Write - rust

Question

When implementing core::fmt::Write for an avr serial monitor, calling unwrap on write_str writes what looks like a compiler error to the output. Calling write_fmt in any capacity crashes, but I think these problems might be related. I'm using a custom target for avr: which is
{
"arch": "avr",
"cpu": "atmega328p",
"data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
"max-atomic-width": 0,
"env": "",
"executables": true,
"linker": "avr-gcc",
"linker-flavor": "gcc",
"linker-is-gnu": true,
"llvm-target": "avr-unknown-unknown",
"os": "unknown",
"position-independent-executables": false,
"exe-suffix": ".elf",
"eh-frame-header": false,
"pre-link-args": {
"gcc": ["-mmcu=atmega328p"]
},
"late-link-args": {
"gcc": ["-lgcc", "-lc"]
},
"target-c-int-width": "16",
"target-endian": "little",
"target-pointer-width": "16",
"vendor": "unknown"
}
When calling serial.write_str("hello world"); it prints "hello world" like normal, but if I call serial.write_str("hello world").unwrap(); it instead prints part of a longer string. The longer the message, the more of this string prints, and I think the final string is:
args.len()C:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\fmt\mod.rsunsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed 'isize::MAX'called 'Option::unwrap()' on a 'None' valueC:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\char\convert.rsC:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\str\iter.rsC:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\str\validations.rsErrorattempt to add with overflowC:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\iter\traits\accum.rsunsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed 'isize::MAX'attempt to add with overflowunsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed 'isize::MAX'C:\Users\Jett\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rust
Update
Here's the implementation:
impl Write for Serial {
fn write_str(&mut self, s: &str) -> core::fmt::Result {
for c in s.chars() {
self.transmit(c as u8);
}
Ok(())
}
}

Answer 1

After a bit more testing, it turns out that adding lto = true to [profile.dev] in Cargo.toml fixes this bug.