r[variable]
r[variable.intro] A variable is a component of a stack frame, either a named function parameter, an anonymous temporary, or a named local variable.
r[variable.local] A local variable (or stack-local allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame.
r[variable.local-mut]
Local variables are immutable unless declared otherwise. For example:
let mut x = ....
r[variable.param-mut]
Function parameters are immutable unless declared with mut. The mut keyword
applies only to the following parameter. For example: |mut x, y| and
fn f(mut x: Box<i32>, y: Box<i32>) declare one mutable variable x and one
immutable variable y.
r[variable.init] Local variables are not initialized when allocated. Instead, the entire frame worth of local variables are allocated, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local variables. Local variables can be used only after they have been initialized through all reachable control flow paths.
In this next example, init_after_if is initialized after the if expression
while uninit_after_if is not because it is not initialized in the else case.
# fn random_bool() -> bool { true }
fn initialization_example() {
let init_after_if: ();
let uninit_after_if: ();
if random_bool() {
init_after_if = ();
uninit_after_if = ();
} else {
init_after_if = ();
}
init_after_if; // ok
// uninit_after_if; // err: use of possibly uninitialized `uninit_after_if`
}