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Summary

Add new syntax to declarative macros to give their authors easy access to additional metadata about macro metavariables, such as the index, length, or count of macro repetitions.

Motivation

Macros with repetitions often expand to code that needs to know or could benefit from knowing how many repetitions there are, or which repetition is currently being expanded. Consider the example macro used in the guide to introduce the concept of macro repetitions: building a vector, recreating the vec! macro from the standard library:

macro_rules! vec {
    ( $( $x:expr ),* ) => {
        {
            let mut temp_vec = Vec::new();
            $(
                temp_vec.push($x);
            )*
            temp_vec
        }
    };
}

This would be more efficient if it could use Vec::with_capacity to preallocate the vector with the correct length. However, there is no standard facility in declarative macros to achieve this, as there is no way to obtain the number of repetitions of $x.

Guide-level explanation

The example vec macro defininition in the guide could be made more efficient if it could use Vec::with_capacity to pre-allocate a vector with the correct capacity. To do this, we need to know the number of repetitions.

Metadata about metavariables, like the number of repetitions, can be accessed using metavariable expressions. The metavariable expression for the count of the number of repetitions of a metavariable x is ${count(x)}, so we can improve the vec macro as follows:

#[macro_export]
macro_rules! vec {
    ( $( $x:expr ),* ) => {
        {
            let mut temp_vec = Vec::with_capacity(${count(x)});
            $(
                temp_vec.push($x);
            )*
            temp_vec
        }
    };
}

The following metavariable expressions are available:

Expression Meaning
${count(ident)} The number of times $ident repeats in total.
${count(ident, depth)} The number of times $ident repeats at up to depth nested repetition depths.
${index()} The current index of the inner-most repetition.
${index(depth)} The current index of the nested repetition at depth steps out.
${length()} The length of the inner-most repetition.
${length(depth)} The length of the nested repetition at depth steps out.
${ignore(ident)} Binds $ident for repetition, but expands to nothing.
$$ Expands to a single $, for removing ambiguity in recursive macro definitions.

Reference-level explanation

Metavariable expressions in declarative macros provide expansions for information about metavariables that are otherwise not easily obtainable.

This is a backwards-compatible change as both $$ and ${ .. } are not currently accepted as valid.

The metavariable expressions added in this RFC are concerned with declarative macro metavariable repetitions, and obtaining the information that the compiler knows about the repetitions that are being processed.

Count

The ${count(x)} metavariable expression shown in the vec example in the previous section counts the number of repetitions that will occur if the identifier is used in a repetition at this depth. This means that in a macro expansion like:

    ${count(x)} $( $x )*

the expression ${count(x)} will expand to an unsuffixed integer literal equal to the number of times the $( $x )* repetition will repeat. For example, if the metavariable $x repeats four times then it will expand to the integer literal 4.

If repetitions are nested, then an optional depth parameter can be used to limit the number of nested repetitions that are counted. For example, a macro expansion like:

    ${count(x, 1)} ${count(x, 2)} ${count(x, 3)} $( a $( b $( $x )* )* )*

The three values this expands to are the number of outer-most repetitions (the number of times a would be generated), the sum of the number of middle repetitions (the number of times b would be generated), and the total number of repetitions of $x.

Index and length

Within a repetition, the ${index()} and ${length()} metavariable expressions give the index of the current repetition and the length of the repetition (i.e., the number of times it will repeat). The index value ranges from 0 to length - 1, and the expanded values are unsuffixed integer literals so they are also suitable for tuple indexing.

For nested repetitions, the ${index()} and ${length()} metavariable expressions expand to the inner-most index and length respectively. If the depth parameter is specified, then the metavariable expression expands to the index or length of the surrounding nested repetition, counting outwards from the inner-most repetition. The expressions ${index()} and ${index(0)} are equivalent.

For example in the expression:

    $( a $( b $( c $x ${index()}/${length()} ${index(1)}/${length(1)} ${index(2)}/${length(2)} )* )* )*

the first pair of values are the index and length of the inner-most repetition, the second pair are the index and length of the middle repetition, and the third pair are the index and length of the outer-most repetition.

Ignore

Sometimes it is desired to repeat an expansion the same number of times as a metavariable repeats but without actually expanding the metavariable. It may be possible to work around this by expanding the metavariable in an expression like { $x ; 1 }, where the expanded value of $x is ignored, but this is only possible if what $x expands to is valid in this kind of expression.

The ${ignore(ident)} metavariable acts as if ident was used for the purposes of repetition, but expands to nothing. This means a macro expansion like:

    $( ${ignore(x)} a )*

will expand to a sequence of a tokens repeated the number of times that x repeats.

Dollar dollar

Since metavariable expressions always apply during the expansion of the macro, they cannot be used in recursive macro definitions. To allow recursive macro definitions to use metavariable expressions, the $$ expression expands to a single $ token.

This is also necessary for unambiguously defining repetitions in nested macros. For example, this resolves issue 35853, as the example in that issue can be expressed as:

macro_rules! foo {
    () => {
        macro_rules! bar {
            ( $$( $$any:tt )* ) => { $$( $$any )* };
        }
    };
}

fn main() { foo!(); }

Larger example

For a larger example of these metavariable expressions in use, consider the following macro that operates over three nested repetitions:

macro_rules! example {
    ( $( [ $( ( $( $x:ident )* ) )* ] )* ) => {
        counts = (${count(x, 1)}, ${count(x, 2)}, ${count(x)})
        nested:
        $(
            indexes = (${index()}/${length()})
            counts = (${count(x, 1)}, ${count(x)})
            nested:
            $(
                indexes = (${index(1)}/${length(1)}, ${index()}/${length()})
                counts = (${count(x)})
                nested:
                $(
                    indexes = (${index(2)}/${length(2)}, ${index(1)}/${length(1)}, ${index()}/${length()})
                    ${ignore(x)}
                )*
            )*
        )*
    };
}

Given this input:

    example! {
        [ ( A B C D ) ( E F G H ) ( I J ) ]
        [ ( K L M ) ]
    }

The macro would expand to:

    counts = (2, 4, 13)
    nested:
        indexes = (0/2)
        counts = (3, 10)
        nested:
            indexes = (0/2, 0/3)
            counts = (4)
            nested:
                indexes = (0/2, 0/3, 0/4)
                indexes = (0/2, 0/3, 1/4)
                indexes = (0/2, 0/3, 2/4)
                indexes = (0/2, 0/3, 3/4)
            indexes = (0/2, 1/3)
            counts = (4)
            nested:
                indexes = (0/2, 1/3, 0/4)
                indexes = (0/2, 1/3, 1/4)
                indexes = (0/2, 1/3, 2/4)
                indexes = (0/2, 1/3, 3/4)
            indexes = (0/2, 2/3)
            counts = (2)
            nested:
                indexes = (0/2, 2/3, 0/2)
                indexes = (0/2, 2/3, 1/2)
        indexes = (1/2)
        counts = (1, 3)
        nested:
            indexes = (1/2, 0/1)
            counts = (3)
            nested:
                indexes = (1/2, 0/1, 0/3)
                indexes = (1/2, 0/1, 1/3)
                indexes = (1/2, 0/1, 2/3)

Drawbacks

This adds additional syntax to the language, that program authors must learn and understand. We may not want to add more syntax.

The author believes it is worth the overhead of new syntax, as even though there exist workarounds for obtaining the information if it's really needed, these workarounds are sometimes difficult to discover and naive implementations can significantly harm compiler performance.

Furthermore, the additional syntax is limited to declarative macros, and its use should be limited to specific circustances where it is more understandable than the alternatives.

Rationale and alternatives

This RFC proposes a modest but powerful extension to macro syntax that makes it possible to obtain information that the compiler already knows, but requires inefficient and complex techniques to obtain in the macro.

The original proposal was for a shorter syntax to provide the count of repetitions: $#ident. During discussions of this syntax, it became clear that it was not obvious as to which number this referred to: the count of repetitions at this level, or the length of the current repetition. It also does not provide a way to discover counts or lengths for other repetition depths. There was also interest in being able to discover the index of the current repetition, and the # character had been used in similar proposals for that. There was some reservation expressed for the use of the # token because of the cognitive burden of another sigil, and its common use in the quote! macro.

The meaning of the depth parameter in index and count originally counted inwards from the outer-most nesting. This was changed to count outwards from the inner-most nesting so that expressions can be copied to a different nesting depth without needing to change them.

This RFC proposes using ${ ... } as the delimiter for metavariable expressions. Available alternatives are:

  • $[ ... ], e.g.: $[count(value)]
  • $:, e.g. $:count(value)
  • $@, e.g. $@count(value)
  • $!, e.g. $!count(value)
  • Another sigil, although # should be avoided to avoid clashes with the quote! macro.

Why not a proc macro or built-in macro?

To avoid extending the language with new syntax, we could consider writing something that looks like a macro invocation, such as count!(value), which would be implemented as a procedural macro or built-in to the compiler.

While this is compelling from a language simplicity perspective, it creates some problems due to the way macro expansions are processed. During macro transcription, other macro invocations are not evaluated, so in the macro:

macro_rules! example {
    ($($x:ident),*) => count!(x)
}

During transcription, example!(a, b, c) would expand to count!(x). At this point, the knowledge of the metavariable x and its repetition is lost, and no procedural macro or built-in macro would be able to work out the count.

To workaround this we would need to re-expand the repetition (count!($($x),*), forcing the count! macro to re-parse and count the repetitions. This is additional unnecessary work that this RFC seeks to address.

Another way to think of metavariable expressions is as "macro transcriber directives". You can think of the macro transcriber as performing the following operations:

  • $var => the value of var
  • $( ... ) ... => a repetition

This RFC adds two more:

  • ${ directive(args) } => a special transcriber directive
  • $$ => $

We could special-case certain macro invocations like count! during transcription, but that feels like a worse solution. It would make it harder to understand what the macro transcriber is going to do with arbitrary code without remembering all of the special macros that don't work like other macros.

Prior art

Declarative macros with repetition are commonly used in Rust for things that are implemented using variadic functions in other languages. Usually these other languages provide mechanisms for finding the number of variadic arguments, and it is a notable limitation that Rust does not.

Scripting languages, like Bash, which use $var for variables, often use similar ${...} syntax for values based on variables: for example ${#var} is used for the length of $var. This means ${...} expressions should not seem too weird to developers familiar with these scripting languages.

A proposal for counting sequence repetitions was made in RFC 88. That RFC proposed several options for additional syntax, however the issue was postponed to after the 1.0 release. This RFC addresses the needs of RFC 88, and also goes further, as it proposes a more general syntax useful for more than just counting repetitions, such as obtaing the index of the current repetition. Since the generated values are integer literals, it also addresses the ability to index tuples in repetitions (using tup.${index()}), which was noted as an omission in RFC 88. It's also not possible to implement efficiently as a procedural macro, as the procedural macro would not have access to the repetition counts without generating a sequence and then counting it again.

Unresolved questions

No unresolved questions at present.

While more expressions are possible, expressions beyond those defined in this RFC are out-of-scope.

Future possibilities

The metavariable expression syntax (${...}) is purposefully generic, and may be extended in future RFCs to anything that may be useful for the macro expander to produce.

The syntax $[...] is still invalid, and so remains available for any other extensions which may come in the future and don't fit in with metavariable expression syntax. Additionally, any symbol after $ is also invalid, so other sequences, such as $@, are available.