Why Compiler?
The Rust compiler does the most significant job to prevent errors in Rust programs. It analyzes the code at compile-time and issues warnings, if the code does not follow memory management rules or lifetime annotations correctly.
For example,
#[allow(unused_variables)] //💡 A lint attribute used to suppress the warning; unused variable: `b`
fn main() {
let a = vec![1, 2, 3];
let b = a;
println!("{:?}", a);
}
// ------ Compile-time error ------
error[E0382]: use of moved value: `a`
--> src/main.rs:6:22
|
3 | let b = a;
| - value moved here
4 |
5 | println!("{:?}", a);
| ^ value used here after move
|
= note: move occurs because `a` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
error: aborting due to previous error
For more information about this error, try `rustc --explain E0382`.
// ⭐ instead using #[allow(unused_variables)], consider using "let _b = a;" in line 4.
// Also you can use "let _ =" to completely ignore return values
💭 In the previous sections, we have discussed memory management concepts like ownership, borrowing, lifetimes and etc.
Rust compiler checks not only issues related with lifetimes or memory management and also common coding mistakes, like the following code.
struct Color {
r: u8,
g: u8,
b: u8,
}
fn main() {
let yellow = Color {
r: 255,
g: 255,
d: 0,
};
println!("Yellow = rgb({},{},{})", yellow.r, yellow.g, yellow.b);
}
// ------------ Compile-time error ------------
error[E0560]: struct `Color` has no field named `d`
--> src/main.rs:11:9
|
11 | d: 0,
| ^ field does not exist - did you mean `b`?
error: aborting due to previous error
For more information about this error, try `rustc --explain E0560`.
Explain Error Codes
Above error messages are very descriptive and we can easily see where is the error. But while we can not identify the issue via the error message, rustc --explain commands help us to identify the error type and how to solve it, by showing simple code samples which express the same problem and the solution we have to use.
For example, rustc --explain E0571 shows the following output in the console.
A `break` statement with an argument appeared in a non-`loop` loop.
Example of erroneous code:
｀｀｀
let result = while true {
if satisfied(i) {
break 2*i; // error: `break` with value from a `while` loop
}
i += 1;
};
｀｀｀
The `break` statement can take an argument (which will be the value of the loop
expression if the `break` statement is executed) in `loop` loops, but not
`for`, `while`, or `while let` loops.
Make sure `break value;` statements only occur in `loop` loops:
｀｀｀
let result = loop { // ok!
if satisfied(i) {
break 2*i;
}
i += 1;
};
｀｀｀
💡 Also you can read the same explanations via Rust Compiler Error Index. For example to check the explanation of E0571 error, you can use https://doc.rust-lang.org/error-index.html#E0571.