rust 70 lines · 9 steps

Building a custom error type in Rust

A hand-rolled ConfigError enum implements Display, Error, and From so config parsing can use the `?` operator cleanly.

Explained by highlit
Intermediate error-handling enums trait-implementation the-question-mark-operator
1use std::fmt;
2 
3#[derive(Debug)]
4pub enum ConfigError {
5 Io(std::io::Error),
6 Parse { field: String, source: std::num::ParseIntError },
7 Missing(String),
8}
9 
10impl fmt::Display for ConfigError {
11 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
12 match self {
13 ConfigError::Io(e) => write!(f, "failed to read config file: {e}"),
14 ConfigError::Parse { field, source } => {
15 write!(f, "invalid value for `{field}`: {source}")
16 }
17 ConfigError::Missing(field) => write!(f, "missing required field `{field}`"),
18 }
19 }
20}
21 
22impl std::error::Error for ConfigError {
23 fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
24 match self {
25 ConfigError::Io(e) => Some(e),
26 ConfigError::Parse { source, .. } => Some(source),
27 ConfigError::Missing(_) => None,
28 }
29 }
30}
31 
32impl From<std::io::Error> for ConfigError {
33 fn from(e: std::io::Error) -> Self {
34 ConfigError::Io(e)
35 }
36}
37 
38pub struct Config {
39 pub port: u16,
40 pub workers: u32,
41}
42 
43pub fn load_config(path: &str) -> Result<Config, ConfigError> {
44 let contents = std::fs::read_to_string(path)?;
45 
46 let mut port = None;
47 let mut workers = None;
48 
49 for line in contents.lines() {
50 let Some((key, value)) = line.split_once('=') else {
51 continue;
52 };
53 let key = key.trim();
54 let value = value.trim();
55 let parsed = value.parse().map_err(|source| ConfigError::Parse {
56 field: key.to_string(),
57 source,
58 })?;
59 match key {
60 "port" => port = Some(parsed as u16),
61 "workers" => workers = Some(parsed),
62 _ => {}
63 }
64 }
65 
66 Ok(Config {
67 port: port.ok_or_else(|| ConfigError::Missing("port".into()))?,
68 workers: workers.ok_or_else(|| ConfigError::Missing("workers".into()))?,
69 })
70}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Modeling errors as an enum lets one type carry several distinct failure cases with their own data.
  2. 2Implementing Display, Error, and From turns a custom type into a first-class citizen of Rust's error ecosystem.
  3. 3A From impl is what makes the `?` operator automatically convert lower-level errors into your own type.

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