Appendix: Macro Follow-Set Ambiguity Formal Specification

This page documents the formal specification of the follow rules for Macros By Example. They were originally specified in RFC 550, from which the bulk of this text is copied, and expanded upon in subsequent RFCs.

Definitions & Conventions

  • macro: anything invokable as foo!(...) in source code.
  • MBE: macro-by-example, a macro defined by macro_rules.
  • matcher: the left-hand-side of a rule in a macro_rules invocation, or a subportion thereof.
  • macro parser: the bit of code in the Rust parser that will parse the input using a grammar derived from all of the matchers.
  • fragment: The class of Rust syntax that a given matcher will accept (or “match”).
  • repetition : a fragment that follows a regular repeating pattern
  • NT: non-terminal, the various “meta-variables” or repetition matchers that can appear in a matcher, specified in MBE syntax with a leading $ character.
  • simple NT: a “meta-variable” non-terminal (further discussion below).
  • complex NT: a repetition matching non-terminal, specified via repetition operators (*, +, ?).
  • token: an atomic element of a matcher; i.e. identifiers, operators, open/close delimiters, and simple NT’s.
  • token tree: a tree structure formed from tokens (the leaves), complex NT’s, and finite sequences of token trees.
  • delimiter token: a token that is meant to divide the end of one fragment and the start of the next fragment.
  • separator token: an optional delimiter token in an complex NT that separates each pair of elements in the matched repetition.
  • separated complex NT: a complex NT that has its own separator token.
  • delimited sequence: a sequence of token trees with appropriate open- and close-delimiters at the start and end of the sequence.
  • empty fragment: The class of invisible Rust syntax that separates tokens, i.e. whitespace, or (in some lexical contexts), the empty token sequence.
  • fragment specifier: The identifier in a simple NT that specifies which fragment the NT accepts.
  • language: a context-free language.

Example:

#![allow(unused)]
fn main() {
macro_rules! i_am_an_mbe {
    (start $foo:expr $($i:ident),* end) => ($foo)
}
}

(start $foo:expr $($i:ident),* end) is a matcher. The whole matcher is a delimited sequence (with open- and close-delimiters ( and )), and $foo and $i are simple NT’s with expr and ident as their respective fragment specifiers.

$(i:ident),* is also an NT; it is a complex NT that matches a comma-separated repetition of identifiers. The , is the separator token for the complex NT; it occurs in between each pair of elements (if any) of the matched fragment.

Another example of a complex NT is $(hi $e:expr ;)+, which matches any fragment of the form hi <expr>; hi <expr>; ... where hi <expr>; occurs at least once. Note that this complex NT does not have a dedicated separator token.

(Note that Rust’s parser ensures that delimited sequences always occur with proper nesting of token tree structure and correct matching of open- and close-delimiters.)

We will tend to use the variable “M” to stand for a matcher, variables “t” and “u” for arbitrary individual tokens, and the variables “tt” and “uu” for arbitrary token trees. (The use of “tt” does present potential ambiguity with its additional role as a fragment specifier; but it will be clear from context which interpretation is meant.)

“SEP” will range over separator tokens, “OP” over the repetition operators *, +, and ?, “OPEN”/“CLOSE” over matching token pairs surrounding a delimited sequence (e.g. [ and ]).

Greek letters “α” “β” “γ” “δ” stand for potentially empty token-tree sequences. (However, the Greek letter “ε” (epsilon) has a special role in the presentation and does not stand for a token-tree sequence.)

  • This Greek letter convention is usually just employed when the presence of a sequence is a technical detail; in particular, when we wish to emphasize that we are operating on a sequence of token-trees, we will use the notation “tt …” for the sequence, not a Greek letter.

Note that a matcher is merely a token tree. A “simple NT”, as mentioned above, is an meta-variable NT; thus it is a non-repetition. For example, $foo:ty is a simple NT but $($foo:ty)+ is a complex NT.

Note also that in the context of this formalism, the term “token” generally includes simple NTs.

Finally, it is useful for the reader to keep in mind that according to the definitions of this formalism, no simple NT matches the empty fragment, and likewise no token matches the empty fragment of Rust syntax. (Thus, the only NT that can match the empty fragment is a complex NT.) This is not actually true, because the vis matcher can match an empty fragment. Thus, for the purposes of the formalism, we will treat $v:vis as actually being $($v:vis)?, with a requirement that the matcher match an empty fragment.

The Matcher Invariants

To be valid, a matcher must meet the following three invariants. The definitions of FIRST and FOLLOW are described later.

  1. For any two successive token tree sequences in a matcher M (i.e. M = ... tt uu ...) with uu ... nonempty, we must have FOLLOW(... tt) ∪ {ε} ⊇ FIRST(uu ...).
  2. For any separated complex NT in a matcher, M = ... $(tt ...) SEP OP ..., we must have SEP ∈ FOLLOW(tt ...).
  3. For an unseparated complex NT in a matcher, M = ... $(tt ...) OP ..., if OP = * or +, we must have FOLLOW(tt ...) ⊇ FIRST(tt ...).

The first invariant says that whatever actual token that comes after a matcher, if any, must be somewhere in the predetermined follow set. This ensures that a legal macro definition will continue to assign the same determination as to where ... tt ends and uu ... begins, even as new syntactic forms are added to the language.

The second invariant says that a separated complex NT must use a separator token that is part of the predetermined follow set for the internal contents of the NT. This ensures that a legal macro definition will continue to parse an input fragment into the same delimited sequence of tt ...’s, even as new syntactic forms are added to the language.

The third invariant says that when we have a complex NT that can match two or more copies of the same thing with no separation in between, it must be permissible for them to be placed next to each other as per the first invariant. This invariant also requires they be nonempty, which eliminates a possible ambiguity.

NOTE: The third invariant is currently unenforced due to historical oversight and significant reliance on the behaviour. It is currently undecided what to do about this going forward. Macros that do not respect the behaviour may become invalid in a future edition of Rust. See the tracking issue.

FIRST and FOLLOW, informally

A given matcher M maps to three sets: FIRST(M), LAST(M) and FOLLOW(M).

Each of the three sets is made up of tokens. FIRST(M) and LAST(M) may also contain a distinguished non-token element ε (“epsilon”), which indicates that M can match the empty fragment. (But FOLLOW(M) is always just a set of tokens.)

Informally:

  • FIRST(M): collects the tokens potentially used first when matching a fragment to M.
  • LAST(M): collects the tokens potentially used last when matching a fragment to M.

  • FOLLOW(M): the set of tokens allowed to follow immediately after some fragment matched by M.

    In other words: t ∈ FOLLOW(M) if and only if there exists (potentially empty) token sequences α, β, γ, δ where:

    • M matches β,

    • t matches γ, and

    • The concatenation α β γ δ is a parseable Rust program.

We use the shorthand ANYTOKEN to denote the set of all tokens (including simple NTs). For example, if any token is legal after a matcher M, then FOLLOW(M) = ANYTOKEN.

(To review one’s understanding of the above informal descriptions, the reader at this point may want to jump ahead to the examples of FIRST/LAST before reading their formal definitions.)

FIRST, LAST

Below are formal inductive definitions for FIRST and LAST.

“A ∪ B” denotes set union, “A ∩ B” denotes set intersection, and “A \ B” denotes set difference (i.e. all elements of A that are not present in B).

FIRST

FIRST(M) is defined by case analysis on the sequence M and the structure of its first token-tree (if any):

  • if M is the empty sequence, then FIRST(M) = { ε },

  • if M starts with a token t, then FIRST(M) = { t },

    (Note: this covers the case where M starts with a delimited token-tree sequence, M = OPEN tt ... CLOSE ..., in which case t = OPEN and thus FIRST(M) = { OPEN }.)

    (Note: this critically relies on the property that no simple NT matches the empty fragment.)

  • Otherwise, M is a token-tree sequence starting with a complex NT: M = $( tt ... ) OP α, or M = $( tt ... ) SEP OP α, (where α is the (potentially empty) sequence of token trees for the rest of the matcher).

    • Let SEP_SET(M) = { SEP } if SEP is present and ε ∈ FIRST(tt ...); otherwise SEP_SET(M) = {}.

  • Let ALPHA_SET(M) = FIRST(α) if OP = * or ? and ALPHA_SET(M) = {} if OP = +.

  • FIRST(M) = (FIRST(tt ...) \ {ε}) ∪ SEP_SET(M) ∪ ALPHA_SET(M).

The definition for complex NTs deserves some justification. SEP_SET(M) defines the possibility that the separator could be a valid first token for M, which happens when there is a separator defined and the repeated fragment could be empty. ALPHA_SET(M) defines the possibility that the complex NT could be empty, meaning that M’s valid first tokens are those of the following token-tree sequences α. This occurs when either * or ? is used, in which case there could be zero repetitions. In theory, this could also occur if + was used with a potentially-empty repeating fragment, but this is forbidden by the third invariant.

From there, clearly FIRST(M) can include any token from SEP_SET(M) or ALPHA_SET(M), and if the complex NT match is nonempty, then any token starting FIRST(tt ...) could work too. The last piece to consider is ε. SEP_SET(M) and FIRST(tt ...) \ {ε} cannot contain ε, but ALPHA_SET(M) could. Hence, this definition allows M to accept ε if and only if ε ∈ ALPHA_SET(M) does. This is correct because for M to accept ε in the complex NT case, both the complex NT and α must accept it. If OP = +, meaning that the complex NT cannot be empty, then by definition ε ∉ ALPHA_SET(M). Otherwise, the complex NT can accept zero repetitions, and then ALPHA_SET(M) = FOLLOW(α). So this definition is correct with respect to \varepsilon as well.

LAST

LAST(M), defined by case analysis on M itself (a sequence of token-trees):

  • if M is the empty sequence, then LAST(M) = { ε }
  • if M is a singleton token t, then LAST(M) = { t }

  • if M is the singleton complex NT repeating zero or more times, M = $( tt ... ) *, or M = $( tt ... ) SEP *

    • Let sep_set = { SEP } if SEP present; otherwise sep_set = {}.

    • if ε ∈ LAST(tt ...) then LAST(M) = LAST(tt ...) ∪ sep_set

    • otherwise, the sequence tt ... must be non-empty; LAST(M) = LAST(tt ...) ∪ {ε}.

  • if M is the singleton complex NT repeating one or more times, M = $( tt ... ) +, or M = $( tt ... ) SEP +

    • Let sep_set = { SEP } if SEP present; otherwise sep_set = {}.

    • if ε ∈ LAST(tt ...) then LAST(M) = LAST(tt ...) ∪ sep_set

    • otherwise, the sequence tt ... must be non-empty; LAST(M) = LAST(tt ...)

  • if M is the singleton complex NT repeating zero or one time, M = $( tt ...) ?, then LAST(M) = LAST(tt ...) ∪ {ε}.
  • if M is a delimited token-tree sequence OPEN tt ... CLOSE, then LAST(M) = { CLOSE }.

  • if M is a non-empty sequence of token-trees tt uu ...,

    • If ε ∈ LAST(uu ...), then LAST(M) = LAST(tt) ∪ (LAST(uu ...) \ { ε }).

    • Otherwise, the sequence uu ... must be non-empty; then LAST(M) = LAST(uu ...).

Examples of FIRST and LAST

Below are some examples of FIRST and LAST. (Note in particular how the special ε element is introduced and eliminated based on the interaction between the pieces of the input.)

Our first example is presented in a tree structure to elaborate on how the analysis of the matcher composes. (Some of the simpler subtrees have been elided.)

INPUT:  $(  $d:ident   $e:expr   );*    $( $( h )* );*    $( f ; )+   g
            ~~~~~~~~   ~~~~~~~                ~
                |         |                   |
FIRST:   { $d:ident }  { $e:expr }          { h }


INPUT:  $(  $d:ident   $e:expr   );*    $( $( h )* );*    $( f ; )+
            ~~~~~~~~~~~~~~~~~~             ~~~~~~~           ~~~
                        |                      |               |
FIRST:          { $d:ident }               { h, ε }         { f }

INPUT:  $(  $d:ident   $e:expr   );*    $( $( h )* );*    $( f ; )+   g
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~    ~~~~~~~~~~~~~~    ~~~~~~~~~   ~
                        |                       |              |       |
FIRST:        { $d:ident, ε }            {  h, ε, ;  }      { f }   { g }


INPUT:  $(  $d:ident   $e:expr   );*    $( $( h )* );*    $( f ; )+   g
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                        |
FIRST:                       { $d:ident, h, ;,  f }

Thus:

  • FIRST($($d:ident $e:expr );* $( $(h)* );* $( f ;)+ g) = { $d:ident, h, ;, f }

Note however that:

  • FIRST($($d:ident $e:expr );* $( $(h)* );* $($( f ;)+ g)*) = { $d:ident, h, ;, f, ε }

Here are similar examples but now for LAST.

  • LAST($d:ident $e:expr) = { $e:expr }
  • LAST($( $d:ident $e:expr );*) = { $e:expr, ε }
  • LAST($( $d:ident $e:expr );* $(h)*) = { $e:expr, ε, h }
  • LAST($( $d:ident $e:expr );* $(h)* $( f ;)+) = { ; }
  • LAST($( $d:ident $e:expr );* $(h)* $( f ;)+ g) = { g }

FOLLOW(M)

Finally, the definition for FOLLOW(M) is built up as follows. pat, expr, etc. represent simple nonterminals with the given fragment specifier.

  • FOLLOW(pat) = {=>, ,, =, |, if, in}`.
  • FOLLOW(expr) = FOLLOW(expr_2021) = FOLLOW(stmt) = {=>, ,, ;}`.
  • FOLLOW(ty) = FOLLOW(path) = {{, [, ,, =>, :, =, >, >>, ;, |, as, where, block nonterminals}.
  • FOLLOW(vis) = {,l any keyword or identifier except a non-raw priv; any token that can begin a type; ident, ty, and path nonterminals}.
  • FOLLOW(t) = ANYTOKEN for any other simple token, including block, ident, tt, item, lifetime, literal and meta simple nonterminals, and all terminals.
  • FOLLOW(M), for any other M, is defined as the intersection, as t ranges over (LAST(M) \ {ε}), of FOLLOW(t).

The tokens that can begin a type are, as of this writing, {(, [, !, *, &, &&, ?, lifetimes, >, >>, ::, any non-keyword identifier, super, self, Self, extern, crate, $crate, _, for, impl, fn, unsafe, typeof, dyn}, although this list may not be complete because people won’t always remember to update the appendix when new ones are added.

Examples of FOLLOW for complex M:

  • FOLLOW($( $d:ident $e:expr )*) = FOLLOW($e:expr)
  • FOLLOW($( $d:ident $e:expr )* $(;)*) = FOLLOW($e:expr) ∩ ANYTOKEN = FOLLOW($e:expr)
  • FOLLOW($( $d:ident $e:expr )* $(;)* $( f |)+) = ANYTOKEN

Examples of valid and invalid matchers

With the above specification in hand, we can present arguments for why particular matchers are legal and others are not.

  • ($ty:ty < foo ,) : illegal, because FIRST(< foo ,) = { < } ⊈ FOLLOW(ty)

  • ($ty:ty , foo <) : legal, because FIRST(, foo <) = { , } is ⊆ FOLLOW(ty).

  • ($pa:pat $pb:pat $ty:ty ,) : illegal, because FIRST($pb:pat $ty:ty ,) = { $pb:pat } ⊈ FOLLOW(pat), and also FIRST($ty:ty ,) = { $ty:ty } ⊈ FOLLOW(pat).

  • ( $($a:tt $b:tt)* ; ) : legal, because FIRST($b:tt) = { $b:tt } is ⊆ FOLLOW(tt) = ANYTOKEN, as is FIRST(;) = { ; }.

  • ( $($t:tt),* , $(t:tt),* ) : legal, (though any attempt to actually use this macro will signal a local ambiguity error during expansion).

  • ($ty:ty $(; not sep)* -) : illegal, because FIRST($(; not sep)* -) = { ;, - } is not in FOLLOW(ty).

  • ($($ty:ty)-+) : illegal, because separator - is not in FOLLOW(ty).

  • ($($e:expr)*) : illegal, because expr NTs are not in FOLLOW(expr NT).