I happened upon this problem where format!
creates a temporary value in a pattern that is not anchored to anything, as far as I understand it.
let x = 42;
let category = match x {
0...9 => "Between 0 and 9",
number @ 10 => format!("It's a {}!", number).as_str(),
_ if x < 0 => "Negative",
_ => "Something else",
};
println!("{}", category);
In this code, the type of category
is a &str
, which is satisfied by returning a literal like "Between 0 and 9"
. If I want to format the matched value to a slice using as_str()
, then I get an error:
error[E0716]: temporary value dropped while borrowed
--> src/main.rs:5:24
|
3 | let category = match x {
| -------- borrow later stored here
4 | 0...9 => "Between 0 and 9",
5 | number @ 10 => format!("It's a {}!", number).as_str(),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| |
| creates a temporary which is freed while still in use
|
= note: consider using a `let` binding to create a longer lived value
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
I have done some reading, and found people with similar problems, but I couldn't seem to find any solution.
A simple workaround would be to have category
be a String
instead of a &str
, but I don't like the idea of having to put .to_string()
on the end of every literal in the pattern, as it's not as clean.
Is there a way to solve the problem, or do I just need to work around it?
In my case How to overcome "temporary value dropped while borrowed" when converting an i32 to &str
I could solve it by moving the call inside the branches
Kind of what is suggested in the error message https://doc.rust-lang.org/error-index.html#E0597
format!
can't return&str
because it will always allocateString
. What is possible to do is to return a&str
from aString
, which is what you did in your code.As the compiler hinted, the created
String
is immediately dropped after its creation because it went out of the current scope and one way around could be an external variable that is not bounded to thematch
scope. E.g.:If you want an
[no_std]
environment or don't want to do any dynamic allocation, you can take a look at this limited code snippet:This is 90% a duplicate of Return local String as a slice (&str), see that for multiple other solutions.
There's one extra possibility since this is all in one function: You can declare a variable for the
String
and only set it when you need to allocate. The compiler (obliquely) suggests this:This is mostly the same as using a
Cow
, just handled by the compiler instead of a specific type.