E001 -- Borrow Checker Error

E001 comes from the ownership and borrow checker. The compiler runs it during compilation (not during --check), and reports the violation with the rule it broke.

Example

Returning a borrow from a function is not allowed:

fn f() -> &Int {
  let x = 1;
  return &x;
}
fn main() { }

warning[E001]: 3:3: cannot return a borrow from a function

Current behaviour

The compiler reports some borrow violations as warnings rather than hard errors today, so a build can still produce a binary. Treat every E001 as an error in your code: the diagnostic states a rule the language does not permit (for example, references cannot be returned from a function or stored in a struct field). --diagnostics=json reports them under borrow_warnings.

Fix

• Return an owned value, or take output parameters by reference (&mut T) and mutate through them.
• For designs that need a borrow to cross a scope, use owned values, handles or indices; see the memory model guide.

Notes

• E001 is also the code used for borrow diagnostics in --diagnostics=json output (kind is borrow_error or borrow_warning).