Removing Tail Expressions

After the previous desugarings, any block that returns a value is the target of an assignment. In this step we move the assignment inside the block so as to remove all tail expressions.

#![allow(unused)]
fn main() {
$place = {
    $statements;
    $expr
};

// becomes:
{
    $statements;
    $place = $expr;
}
}

#![allow(unused)]
fn main() {
$place = if $bool {
    $then
} else {
    $else
};

// becomes:
if $bool {
    $place = $then;
} else {
    $place = $else;
}
}

#![allow(unused)]
fn main() {
$place = loop {
    $statements;
    if $bool {
        break $expr;
    }
};

// becomes
loop {
    $statements;
    if $bool {
        $place = $expr;
        break;
    }
}
}

The one block that is special is the whole function. Since the tail of a block is a value context we know the tail expression of the function, if any, is a local variable. We then simply add an explicit return statement.

#![allow(unused)]
fn main() {
fn $name($args..) -> $ty {
    $statements;
    $local
}

// becomes
fn $name($args..) -> $ty {
    $statements;
    return $local;
}
}

After this step, all blocks end in a statement rather than an expression, and all blocks and control-flow expressions have type ().