^Syntax
IfExpression :
   if Expression_{except struct expression} BlockExpression
   (else ( BlockExpression | IfExpression | IfLetExpression ) )^?

An if expression is a conditional branch in program control. The form of an if expression is a condition expression, followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block. The condition expressions must have type bool. If a condition expression evaluates to true, the consequent block is executed and any subsequent else if or else block is skipped. If a condition expression evaluates to false, the consequent block is skipped and any subsequent else if condition is evaluated. If all if and else if conditions evaluate to false then any else block is executed. An if expression evaluates to the same value as the executed block, or () if no block is evaluated. An if expression must have the same type in all situations.

# #![allow(unused_variables)]
#fn main() {
# let x = 3;
if x == 4 {
    println!("x is four");
} else if x == 3 {
    println!("x is three");
} else {
    println!("x is something else");
}

let y = if 12 * 15 > 150 {
    "Bigger"
} else {
    "Smaller"
};
assert_eq!(y, "Bigger");
#}

^Syntax
IfLetExpression :
   if let Pattern = Expression_{except struct expression} BlockExpression
   (else ( BlockExpression | IfExpression | IfLetExpression ) )^?

An if let expression is semantically similar to an if expression but in place of a condition expression it expects the keyword let followed by a refutable pattern, an = and an expression. If the value of the expression on the right hand side of the = matches the pattern, the corresponding block will execute, otherwise flow proceeds to the following else block if it exists. Like if expressions, if let expressions have a value determined by the block that is evaluated.

# #![allow(unused_variables)]
#fn main() {
let dish = ("Ham", "Eggs");

// this body will be skipped because the pattern is refuted
if let ("Bacon", b) = dish {
    println!("Bacon is served with {}", b);
} else {
    // This block is evaluated instead.
    println!("No bacon will be served");
}

// this body will execute
if let ("Ham", b) = dish {
    println!("Ham is served with {}", b);
}
#}

if and if let expressions can be intermixed:

# #![allow(unused_variables)]
#fn main() {
let x = Some(3);
let a = if let Some(1) = x {
    1
} else if x == Some(2) {
    2
} else if let Some(y) = x {
    y
} else {
    -1
};
assert_eq!(a, 3);
#}