combine Rust nested Result match arms with the same Error case - rust

Question

I am attempting to work with nested match arms, both handling the Err case with the same code.
This seems quite repetitive, and if there were further parsing and unwrapping to be done it would get even worse.
Is there a way to let the Err case from the inner arm "bubble up" to the outer one, or otherwise apply the same error handling to those multiple different, possibly nested Err cases?
pub fn get_content_length(
headers: axum::http::HeaderMap,
) -> Result<usize, (axum::http::StatusCode, String)> {
let content_length = match headers.get("content-length") {
Some(header_val) => match header_val.to_str() {
Ok(header_val) => match header_val.parse::<usize>() {
Ok(content_length) => content_length,
Err(e) => {
return Err((
axum::http::StatusCode::BAD_REQUEST,
format!("error parsing header content-length: {}", e.to_string()),
))
}
},
Err(e) => {
return Err((
axum::http::StatusCode::BAD_REQUEST,
format!("error parsing header content-length: {}", e.to_string()),
))
}
},
None => {
return Err((
axum::http::StatusCode::BAD_REQUEST,
String::from("missing content-length header"),
))
}
};
Ok(content_length)
}

Answer 1

The errors are of different type, so you cannot do that. But since all you want is to display them, you can convert them to strings:
pub fn get_content_length(
headers: axum::http::HeaderMap,
) -> Result<usize, (axum::http::StatusCode, String)> {
let content_length = match headers.get("content-length") {
Some(header_val) => match header_val
.to_str()
.map_err(|e| e.to_string())
.and_then(|header_val| header_val.parse::<usize>().map_err(|e| e.to_string()))
{
Ok(content_length) => content_length,
Err(e) => {
return Err((
axum::http::StatusCode::BAD_REQUEST,
format!("error parsing header content-length: {}", e.to_string()),
))
}
},
None => {
return Err((
axum::http::StatusCode::BAD_REQUEST,
String::from("missing content-length header"),
))
}
};
Ok(content_length)
}

Answer 2

You can simplify your code fith if-let or let-else constructions.
Your example is too big for me to show the use of constructs on it, sorry. In the future for MRE use playground.
pub fn get_content_length_ref(
headers: axum::http::HeaderMap,
) -> Result<usize, (axum::http::StatusCode, String)> {
let Some(header_val) = headers.get("content-length") else {
return Err((
axum::http::StatusCode::BAD_REQUEST,
"missing content-length header".to_owned(),
));
};
let Ok(header_str) = header_val.to_str() else {
return Err((
axum::http::StatusCode::BAD_REQUEST,
"error parsing header to str".to_owned(),
));
};
let Ok(header_val) = header_str.parse::<usize>() else {
return Err((
axum::http::StatusCode::BAD_REQUEST,
"error parsing header to int".to_owned(),
));
};
Ok(header_val)
}
But simplest way for errors - anyhow crate.
use anyhow::Context;
pub fn get_content_length(headers: axum::http::HeaderMap) -> anyhow::Result<usize> {
let header = headers
.get("content-length")
.context("missing content-length header")?;
let header = header.to_str()?;
Ok(header.parse::<usize>()?)
}