字段访问表达式

Syntax
FieldExpression :
Expression . IDENTIFIER

A field expression consists of an expression followed by a single dot and an identifier, when not immediately followed by a parenthesized expression-list (the latter is always a method call expression). A field expression denotes a field of a struct or union. To call a function stored in a struct, parentheses are needed around the field expression.

mystruct.myfield;
foo().x;
(Struct {a: 10, b: 20}).a;
mystruct.method();          // Method expression
(mystruct.function_field)() // Call expression containing a field expression

A field access is a place expression referring to the location of that field. When the subexpression is mutable, the field expression is also mutable.

Also, if the type of the expression to the left of the dot is a pointer, it is automatically dereferenced as many times as necessary to make the field access possible. In cases of ambiguity, we prefer fewer autoderefs to more.

Finally, the fields of a struct or a reference to a struct are treated as separate entities when borrowing. If the struct does not implement Drop and is stored in a local variable, this also applies to moving out of each of its fields. This also does not apply if automatic dereferencing is done though user defined types.

struct A { f1: String, f2: String, f3: String }
let mut x: A;
# x = A {
#     f1: "f1".to_string(),
#     f2: "f2".to_string(),
#     f3: "f3".to_string()
# };
let a: &mut String = &mut x.f1; // x.f1 borrowed mutably
let b: &String = &x.f2;         // x.f2 borrowed immutably
let c: &String = &x.f2;         // Can borrow again
let d: String = x.f3;           // Move out of x.f3