For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

Call expressions

CallExpression :
   Expression ( CallParams? )

CallParams :
   Expression ( , Expression )* ,?

A call expression consists of an expression followed by a parenthesized expression-list. It invokes a function, providing zero or more input variables. If the function eventually returns, then the expression completes. For non-function types, the expression f(...) uses the method on one of the std::ops::Fn, std::ops::FnMut or std::ops::FnOnce traits, which differ in whether they take the type by reference, mutable reference, or take ownership respectively. An automatic borrow will be taken if needed. Rust will also automatically dereference f as required. Some examples of call expressions:

# #![allow(unused_variables)]
#fn main() {
# fn add(x: i32, y: i32) -> i32 { 0 }
let three: i32 = add(1i32, 2i32);
let name: &'static str = (|| "Rust")();

Disambiguating Function Calls

Rust treats all function calls as sugar for a more explicit, fully-qualified syntax. Upon compilation, Rust will desugar all function calls into the explicit form. Rust may sometimes require you to qualify function calls with trait, depending on the ambiguity of a call in light of in-scope items.

Note: In the past, the Rust community used the terms "Unambiguous Function Call Syntax", "Universal Function Call Syntax", or "UFCS", in documentation, issues, RFCs, and other community writings. However, the term lacks descriptive power and potentially confuses the issue at hand. We mention it here for searchability's sake.

Several situations often occur which result in ambiguities about the receiver or referent of method or associated function calls. These situations may include:

  • Multiple in-scope traits define methods with the same name for the same types
  • Auto-deref is undesirable; for example, distinguishing between methods on a smart pointer itself and the pointer's referent
  • Methods which take no arguments, like default(), and return properties of a type, like size_of()

To resolve the ambiguity, the programmer may refer to their desired method or function using more specific paths, types, or traits.

For example,

trait Pretty {
    fn print(&self);

trait Ugly {
  fn print(&self);

struct Foo;
impl Pretty for Foo {
    fn print(&self) {}

struct Bar;
impl Pretty for Bar {
    fn print(&self) {}
impl Ugly for Bar{
    fn print(&self) {}

fn main() {
    let f = Foo;
    let b = Bar;

    // we can do this because we only have one item called `print` for `Foo`s
    // more explicit, and, in the case of `Foo`, not necessary
    // if you're not into the whole brevity thing
    <Foo as Pretty>::print(&f);

    // b.print(); // Error: multiple 'print' found
    // Bar::print(&b); // Still an error: multiple `print` found

    // necessary because of in-scope items defining `print`
    <Bar as Pretty>::print(&b);

Refer to RFC 132 for further details and motivations.