Splicing

Backticks mean “evaluate this now, and put the result here.”

`expression`

There are two uses:

• Building names. r`i` with i = 3 is the name r3. This is the most common use, covered in Spliced names.
• Freezing a value into code that runs later. A declaration generated by a macro is evaluated where it ends up, after the macro’s parameters are gone. A splice evaluates part of it while they still exist. See Generating declarations.

macro declare_scaled(n: int) {
    pub const SCALED`n` = `n * 100`
}

declare_scaled 7

macro show(value: int) {
    @emit value
}

show SCALED7

700

Here the name SCALED`n` becomes SCALED7, and the value `n * 100` becomes 700, both while declare_scaled runs.

Where it makes no difference

In ordinary expressions, which are evaluated anyway, a splice changes nothing: `1 + 2` is the same as 1 + 2.

macro show(value: int) {
    @emit value
}

show `1 + 2`

3

A literal backtick

To write a backtick that isn’t a splice, for example in a custom syntax pattern, escape it: \`.