If Expressions

An if expression conditionally evaluates one of two branches based on a boolean condition.

if_expr     = "if" expression "{" block "}" [ else_clause ] ;
else_clause = "else" ( "{" block "}" | if_expr ) ;

The condition expression MUST have type bool.

If an else branch is present, both branches MUST have the same type. (A diverging branch has type !, which coerces to the other branch's type — 3.4:3; core calculus docs/formal/01-core-calculus.md §5.7, rule (Sub-Never).)

The type of an if expression with an else branch is the common type of its branches (core calculus docs/formal/01-core-calculus.md §5.5, rule (If)).

If no else branch is present, the then branch MUST have type ().

fn main() -> i32 {
    let x = if true { 42 } else { 0 };
    x
}

The condition is evaluated first. If it evaluates to true, the then-branch is evaluated and the if expression evaluates to the then-branch's value; otherwise the else-branch is evaluated and the if expression evaluates to the else-branch's value (core calculus docs/formal/01-core-calculus.md §6.6, rules (D-If-T)/(D-If-F)).

An if expression with no else branch evaluates to (): when the condition is false no branch is evaluated and the expression's value is (), and when it is true the then-branch's value is () by rule 4.6:5.

fn main() -> i32 {
    let n = 5;
    if n > 3 { 100 } else { 0 }
}

If expressions can be chained using else if:

fn main() -> i32 {
    let x = 5;
    if x < 3 { 1 }
    else if x < 7 { 2 }
    else { 3 }
}

The branches of an if expression also reconcile their ownership effects: each branch is checked under the same incoming ownership state, and the states are joined after the branch (3.8:50, 3.8:73; core calculus docs/formal/01-core-calculus.md §5.5). That interaction is specified in section 3.8, not here.