How to set &str that from enum A to struct B?

help
1

Hi guys.

I defined an error enum:

pub enum MyError<'a> {
    MissingField(&'a str),
}

and defined an struct:

pub struct SimpleMessageError<'a>(pub &'a str);

then, I implemented From trait for SimpleMessageError:

impl<'a: 'b, 'b> From<MyError<'b>> for SimpleMessageError<'a> {
    fn from(e: MyError<'b>) -> Self {
        match e {
            MyError::MissingField(f) => SimpleMessageError(f),
        }
    }
}

but rustc said:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
   --> src/err.rs:102:15
    |
102 |         match e {
    |               ^
    |
note: first, the lifetime cannot outlive the lifetime 'b as defined on the impl at 100:14...
   --> src/err.rs:100:14
    |
100 | impl<'a: 'b, 'b> From<MyError<'b>> for SimpleMessageError<'a> {
    |              ^^
    = note: ...so that the types are compatible:
            expected err::MyError<'_>
               found err::MyError<'b>
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 100:6...
   --> src/err.rs:100:6
    |
100 | impl<'a: 'b, 'b> From<MyError<'b>> for SimpleMessageError<'a> {
    |      ^^
    = note: ...so that the expression is assignable:
            expected err::SimpleMessageError<'a>
               found err::SimpleMessageError<'_>

although rustc gave some explaination, but I still can not figure out how to solve this problem?

or maybe this is a program design issue?

Once I changed lifetime 'a and 'b both to 'static, compile error was gone.

2

You messed up the order of lifetimes. Not 'a depends on 'b, but the other way around. So this should be

impl<'a, 'b: 'a> From<MyError<'b>> for SimpleMessageError<'a> {

or simplified

impl<'a> From<MyError<'a>> for SimpleMessageError<'a> {
3

Thank you hellow
simplified workaround is more clear than 2 lifetime params