45 lines
1.3 KiB
Rust
45 lines
1.3 KiB
Rust
type point = { x: int, y: int };
|
|
|
|
fn a() {
|
|
let mut p = {x: 3, y: 4};
|
|
let _q = &p; //! NOTE loan of mutable local variable granted here
|
|
|
|
// This assignment is illegal because the field x is not
|
|
// inherently mutable; since `p` was made immutable, `p.x` is now
|
|
// immutable. Otherwise the type of &_q.x (&int) would be wrong.
|
|
p.x = 5; //! ERROR assigning to mutable field prohibited due to outstanding loan
|
|
}
|
|
|
|
fn b() {
|
|
let mut p = {x: 3, mut y: 4};
|
|
let _q = &p;
|
|
|
|
// This assignment is legal because `y` is inherently mutable (and
|
|
// hence &_q.y is &mut int).
|
|
p.y = 5;
|
|
}
|
|
|
|
fn c() {
|
|
// this is sort of the opposite. We take a loan to the interior of `p`
|
|
// and then try to overwrite `p` as a whole.
|
|
|
|
let mut p = {x: 3, mut y: 4};
|
|
let _q = &p.y; //! NOTE loan of mutable local variable granted here
|
|
p = {x: 5, mut y: 7};//! ERROR assigning to mutable local variable prohibited due to outstanding loan
|
|
copy p;
|
|
}
|
|
|
|
fn d() {
|
|
// just for completeness's sake, the easy case, where we take the
|
|
// address of a subcomponent and then modify that subcomponent:
|
|
|
|
let mut p = {x: 3, mut y: 4};
|
|
let _q = &p.y; //! NOTE loan of mutable field granted here
|
|
p.y = 5; //! ERROR assigning to mutable field prohibited due to outstanding loan
|
|
copy p;
|
|
}
|
|
|
|
fn main() {
|
|
}
|
|
|