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.

A struct literal MUST NOT appear as the outermost expression of an if condition; a program that requires one parenthesizes the condition. Consequently, in if v {} the braces denote the if expression's empty then branch, not a struct literal v {}.

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.