# Compiler-Hint to assign to a mutably borrowed value **URL:** \ **Category:** compiler\ **Created:** [January 22, 2025, 7:20pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209 "2025-01-22T19:20:18Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ValonPrime](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/valonprime/32/12924_2.png) [@ValonPrime](https://internals.rust-lang.org/u/ValonPrime)\ **Post date:** [January 22, 2025, 7:20pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209/1 "2025-01-22T19:20:18Z")
A beginner might try the following code: ```rust fn main() { let mut some_val = 5; change_val(&mut some_val); println!("{}", some_val); } fn change_val(mut val: &mut i32) { val = &10; } ``` ```rust error[E0308]: mismatched types --> src/main.rs:9:11 | 9 | val = &10; | ^^^ types differ in mutability | = note: expected mutable reference `&mut _` found reference `&_` ``` `val` needs to be a (mutable) reference, so the 10 is made a reference by using a &-Symbol in front of it. However this is not how you assign through a reference in this case but the compiler only suggest you to add a `mut`: ```rust fn change_val(mut val: &mut i32) { val = &mut 10; } ``` ```rust error[E0716]: temporary value dropped while borrowed --> src/main.rs:9:16 | 8 | fn change_val(mut val: &mut i32) { | - let's call the lifetime of this reference `'1` 9 | val = &mut 10; | -----------^^- temporary value is freed at the end of this statement | | | | | creates a temporary value which is freed while still in use | assignment requires that borrow lasts for `'1` ``` This creates another, not working, piece of code. Now `val` doesn't live long enough. Parallel to the errors, the compiler should hint you to assign through the reference, like with this implementation: ```rust fn main() { let mut some_val = 5; change_val(&mut some_val); println!("{}", some_val); } fn change_val(mut val: &mut i32) { val = 10; } ``` ```rust error[E0308]: mismatched types --> src/main.rs:9:11 | 8 | fn change_val(mut val: &mut i32) { | -------- expected due to this parameter type 9 | val = 10; | ^^ expected `&mut i32`, found integer | help: consider dereferencing here to assign to the mutably borrowed value | 9 | *val = 10; | + ``` The current status can be confusing and frustrating for rust-beginners. Especially if they come from languages like Java. A hint like this could help beginners. ---
**Author:** ![ekuber](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/ekuber/32/1646_2.png) [@ekuber](https://internals.rust-lang.org/u/ekuber)\ **Post date:** [January 22, 2025, 7:55pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209/2 "2025-01-22T19:55:18Z")
Note that after [https://github.com/rust-lang/rust/pull/134977](https://github.com/rust-lang/rust/pull/134977), if you match the mutability you get ```rust warning: value assigned to `val` is never read --> src/main.rs:9:5 | 9 | val = &mut 10; | ^^^ | = note: `#[warn(unused_assignments)]` on by default help: you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding | 8 ~ fn change_val(val: &mut mut i32) { 9 ~ *val = 10; | warning: unused variable: `val` --> src/main.rs:8:19 | 8 | fn change_val(mut val: &mut i32) { | ^^^ help: if this is intentional, prefix it with an underscore: `_val` | = note: `#[warn(unused_variables)]` on by default error[E0716]: temporary value dropped while borrowed --> src/main.rs:9:16 | 8 | fn change_val(mut val: &mut i32) { | - let's call the lifetime of this reference `'1` 9 | val = &mut 10; | -----------^^- temporary value is freed at the end of this statement | | | | | creates a temporary value which is freed while still in use | assignment requires that borrow lasts for `'1` ``` Note the suggestion in the lint. * * * Regardless, I agree it'd be nice to detect the case you mentioned. Can you file a ticket with the same content as your thread comment? ---
**Author:** ![ValonPrime](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/valonprime/32/12924_2.png) [@ValonPrime](https://internals.rust-lang.org/u/ValonPrime)\ **Post date:** [January 24, 2025, 8:27pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209/3 "2025-01-24T20:27:32Z")
Thanks for your Input. How exactly do I file a Ticket? Im being lead back here when I try to create a feature request on the github-repo. Do I just use the blank template? ---
**Author:** ![pitaj](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/pitaj/32/11262_2.png) [@pitaj](https://internals.rust-lang.org/u/pitaj)\ **Post date:** [January 24, 2025, 9:00pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209/4 "2025-01-24T21:00:34Z")
"New Issue" then "Diagnostic Issue" will bring up [this form](https://github.com/rust-lang/rust/issues/new?template=diagnostics.yaml) ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [July 18, 2026, 9:01pm UTC](https://internals.rust-lang.org/t/compiler-hint-to-assign-to-a-mutably-borrowed-value/22209/5 "2026-07-18T21:01:34Z")
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