E0515 rust type_error ai_generated true

error[E0515]: cannot return value referencing local variable `x`

ID: rust/e0515-cannot-return-value-referencing-local-variable

Also available as: JSON · Markdown · 中文
90%Fix Rate
86%Confidence
1Evidence
2024-01-15First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
rustc 1.75.0 active — — —
rustc 1.78.0 active — — —
rustc 1.80.0 active — — —

Root Cause

Returning a reference to a local variable that will be dropped when the function exits, creating a dangling pointer.

generic

中文

返回了对局部变量的引用，该变量在函数退出时会被释放，导致悬垂指针。

Official Documentation

https://doc.rust-lang.org/error_codes/E0515.html

Workarounds

  1. 95% success Return the owned value instead of a reference. Example: fn create() -> String { let s = String::from("hello"); s } 
    Return the owned value instead of a reference. Example: fn create() -> String { let s = String::from("hello"); s }
  2. 90% success Accept a reference parameter and write into it instead of returning a reference. Example: fn fill(buf: &mut String) { buf.push_str("hello"); } 
    Accept a reference parameter and write into it instead of returning a reference. Example: fn fill(buf: &mut String) { buf.push_str("hello"); }
  3. 85% success Use an arena allocator or a reference-counted pointer like Rc<T> to extend the lifetime. Example: Rc::new(local_var) 
    Use an arena allocator or a reference-counted pointer like Rc<T> to extend the lifetime. Example: Rc::new(local_var)

中文步骤

  1. Return the owned value instead of a reference. Example: fn create() -> String { let s = String::from("hello"); s }
  2. Accept a reference parameter and write into it instead of returning a reference. Example: fn fill(buf: &mut String) { buf.push_str("hello"); }
  3. Use an arena allocator or a reference-counted pointer like Rc<T> to extend the lifetime. Example: Rc::new(local_var)

Dead Ends

Common approaches that don't work:

  1. 90% fail

    Box::leak converts a Box to a 'static reference, but the local variable is still on the stack and will be dropped. The reference becomes dangling.

  2. 95% fail

    Transmuting lifetimes doesn't change the fact that the local variable is dropped. The reference becomes invalid immediately upon return.

  3. 80% fail

    Raw pointers don't have lifetime checks, but the data is still deallocated when the function returns, creating a dangling pointer.