Appendix: Macro FollowSet 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 asfoo!(...)
in source code.MBE
: macrobyexample, a macro defined bymacro_rules
.matcher
: the lefthandside of a rule in amacro_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 patternNT
: nonterminal, the various "metavariables" or repetition matchers that can appear in a matcher, specified in MBE syntax with a leading$
character.simple NT
: a "metavariable" nonterminal (further discussion below).complex NT
: a repetition matching nonterminal, 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 closedelimiters 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 contextfree language.
Example:
#![allow(unused_variables)] 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 closedelimiters (
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
commaseparated 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 closedelimiters.)
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 tokentree sequences. (However, the Greek letter "ε" (epsilon) has a special role in the presentation and does not stand for a tokentree 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 tokentrees, 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 metavariable NT; thus it is a nonrepetition. 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.
 For any two successive token tree sequences in a matcher
M
(i.e.M = ... tt uu ...
) withuu ...
nonempty, we must have FOLLOW(... tt
) ∪ {ε} ⊇ FIRST(uu ...
).  For any separated complex NT in a matcher,
M = ... $(tt ...) SEP OP ...
, we must haveSEP
∈ FOLLOW(tt ...
).  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 nontoken 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 tokentree (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 tokentree sequence,
M = OPEN tt ... CLOSE ...
, in which caset = 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 tokentree sequence starting with a complex NT:
M = $( tt ... ) OP α
, orM = $( tt ... ) SEP OP α
, (whereα
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 SEP_SET(M) = { SEP } if SEP is present and ε ∈ FIRST(

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 tokentree
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 potentiallyempty 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 tokentrees):

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 ... ) *
, orM = $( 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 nonempty; LAST(M) = LAST(tt ...
) ∪ {ε}.


if M is the singleton complex NT repeating one or more times,
M = $( tt ... ) +
, orM = $( 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 nonempty; 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 tokentree sequence
OPEN tt ... CLOSE
, then LAST(M) = {CLOSE
}. 
if M is a nonempty sequence of tokentrees
tt uu ...
,
If ε ∈ LAST(
uu ...
), then LAST(M) = LAST(tt
) ∪ (LAST(uu ...
) \ { ε }). 
Otherwise, the sequence
uu ...
must be nonempty; 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(stmt) = {
=>
,,
,;
}`. 
FOLLOW(ty) = FOLLOW(path) = {
{
,[
,,
,=>
,:
,=
,>
,>>
,;
,
,as
,where
, block nonterminals}. 
FOLLOW(vis) = {
,
l any keyword or identifier except a nonrawpriv
; 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 nonkeyword 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).