Macro expansion

N.B. rustc_ast, rustc_expand, and rustc_builtin_macros are all undergoing refactoring, so some of the links in this chapter may be broken.

Rust has a very powerful macro system. In the previous chapter, we saw how the parser sets aside macros to be expanded (using temporary placeholders). This chapter is about the process of expanding those macros iteratively until we have a complete Abstract Syntax Tree (AST) for our crate with no unexpanded macros (or a compile error).

First, we discuss the algorithm that expands and integrates macro output into ASTs. Next, we take a look at how hygiene data is collected. Finally, we look at the specifics of expanding different types of macros.

Many of the algorithms and data structures described below are in rustc_expand, with fundamental data structures in rustc_expand::base.

Also of note, cfg and cfg_attr are treated specially from other macros, and are handled in rustc_expand::config.

Expansion and AST Integration

Firstly, expansion happens at the crate level. Given a raw source code for a crate, the compiler will produce a massive AST with all macros expanded, all modules inlined, etc. The primary entry point for this process is the MacroExpander::fully_expand_fragment method. With few exceptions, we use this method on the whole crate (see "Eager Expansion" below for more detailed discussion of edge case expansion issues).

At a high level, fully_expand_fragment works in iterations. We keep a queue of unresolved macro invocations (i.e. macros we haven't found the definition of yet). We repeatedly try to pick a macro from the queue, resolve it, expand it, and integrate it back. If we can't make progress in an iteration, this represents a compile error. Here is the algorithm:

  1. Initialize a queue of unresolved macros.
  2. Repeat until queue is empty (or we make no progress, which is an error):
    1. Resolve imports in our partially built crate as much as possible.
    2. Collect as many macro Invocations as possible from our partially built crate (fn-like, attributes, derives) and add them to the queue.
    3. Dequeue the first element and attempt to resolve it.
    4. If it's resolved:
      1. Run the macro's expander function that consumes a TokenStream or AST and produces a TokenStream or AstFragment (depending on the macro kind). (A TokenStream is a collection of TokenTrees, each of which are a token (punctuation, identifier, or literal) or a delimited group (anything inside ()/[]/{})).
        • At this point, we know everything about the macro itself and can call set_expn_data to fill in its properties in the global data; that is the hygiene data associated with ExpnId (see Hygiene below).
      2. Integrate that piece of AST into the currently-existing though partially-built AST. This is essentially where the "token-like mass" becomes a proper set-in-stone AST with side-tables. It happens as follows:
        • If the macro produces tokens (e.g. a proc macro), we parse into an AST, which may produce parse errors.
        • During expansion, we create SyntaxContexts (hierarchy 2) (see Hygiene below).
        • These three passes happen one after another on every AST fragment freshly expanded from a macro:
      3. After expanding a single macro and integrating its output, continue to the next iteration of fully_expand_fragment.
    5. If it's not resolved:
      1. Put the macro back in the queue.
      2. Continue to next iteration...

Error Recovery

If we make no progress in an iteration we have reached a compilation error (e.g. an undefined macro). We attempt to recover from failures (i.e. unresolved macros or imports) with the intent of generating diagnostics. Failure recovery happens by expanding unresolved macros into ExprKind::Err and allows compilation to continue past the first error so that rustc can report more errors than just the original failure.

Name Resolution

Notice that name resolution is involved here: we need to resolve imports and macro names in the above algorithm. This is done in rustc_resolve::macros, which resolves macro paths, validates those resolutions, and reports various errors (e.g. "not found", "found, but it's unstable", "expected x, found y"). However, we don't try to resolve other names yet. This happens later, as we will see in the chapter: Name Resolution.

Eager Expansion

Eager expansion means we expand the arguments of a macro invocation before the macro invocation itself. This is implemented only for a few special built-in macros that expect literals; expanding arguments first for some of these macro results in a smoother user experience. As an example, consider the following:

macro bar($i: ident) { $i }
macro foo($i: ident) { $i }

foo!(bar!(baz));

A lazy-expansion would expand foo! first. An eager-expansion would expand bar! first.

Eager-expansion is not a generally available feature of Rust. Implementing eager-expansion more generally would be challenging, so we implement it for a few special built-in macros for the sake of user-experience. The built-in macros are implemented in rustc_builtin_macros, along with some other early code generation facilities like injection of standard library imports or generation of test harness. There are some additional helpers for building AST fragments in rustc_expand::build. Eager-expansion generally performs a subset of the things that lazy (normal) expansion does. It is done by invoking fully_expand_fragment on only part of a crate (as opposed to the whole crate, like we normally do).

Other Data Structures

Here are some other notable data structures involved in expansion and integration:

  • ResolverExpand - a trait used to break crate dependencies. This allows the resolver services to be used in rustc_ast, despite rustc_resolve and pretty much everything else depending on rustc_ast.
  • ExtCtxt/ExpansionData - holds various intermediate expansion infrastructure data.
  • Annotatable - a piece of AST that can be an attribute target, almost the same thing as AstFragment except for types and patterns that can be produced by macros but cannot be annotated with attributes.
  • MacResult - a "polymorphic" AST fragment, something that can turn into a different AstFragment depending on its AstFragmentKind (i.e. an item, expression, pattern, etc).

Hygiene and Hierarchies

If you have ever used the C/C++ preprocessor macros, you know that there are some annoying and hard-to-debug gotchas! For example, consider the following C code:

#define DEFINE_FOO struct Bar {int x;}; struct Foo {Bar bar;};

// Then, somewhere else
struct Bar {
    ...
};

DEFINE_FOO

Most people avoid writing C like this – and for good reason: it doesn't compile. The struct Bar defined by the macro clashes names with the struct Bar defined in the code. Consider also the following example:

#define DO_FOO(x) {\
    int y = 0;\
    foo(x, y);\
    }

// Then elsewhere
int y = 22;
DO_FOO(y);

Do you see the problem? We wanted to generate a call foo(22, 0), but instead we got foo(0, 0) because the macro defined its own y!

These are both examples of macro hygiene issues. Hygiene relates to how to handle names defined within a macro. In particular, a hygienic macro system prevents errors due to names introduced within a macro. Rust macros are hygienic in that they do not allow one to write the sorts of bugs above.

At a high level, hygiene within the Rust compiler is accomplished by keeping track of the context where a name is introduced and used. We can then disambiguate names based on that context. Future iterations of the macro system will allow greater control to the macro author to use that context. For example, a macro author may want to introduce a new name to the context where the macro was called. Alternately, the macro author may be defining a variable for use only within the macro (i.e. it should not be visible outside the macro).

The context is attached to AST nodes. All AST nodes generated by macros have context attached. Additionally, there may be other nodes that have context attached, such as some desugared syntax (non-macro-expanded nodes are considered to just have the "root" context, as described below). Throughout the compiler, we use rustc_span::Spans to refer to code locations. This struct also has hygiene information attached to it, as we will see later.

Because macros invocations and definitions can be nested, the syntax context of a node must be a hierarchy. For example, if we expand a macro and there is another macro invocation or definition in the generated output, then the syntax context should reflect the nesting.

However, it turns out that there are actually a few types of context we may want to track for different purposes. Thus, there are not just one but three expansion hierarchies that together comprise the hygiene information for a crate.

All of these hierarchies need some sort of "macro ID" to identify individual elements in the chain of expansions. This ID is ExpnId. All macros receive an integer ID, assigned continuously starting from 0 as we discover new macro calls. All hierarchies start at ExpnId::root, which is its own parent.

The rustc_span::hygiene crate contains all of the hygiene-related algorithms (with the exception of some hacks in Resolver::resolve_crate_root) and structures related to hygiene and expansion that are kept in global data.

The actual hierarchies are stored in HygieneData. This is a global piece of data containing hygiene and expansion info that can be accessed from any Ident without any context.

The Expansion Order Hierarchy

The first hierarchy tracks the order of expansions, i.e., when a macro invocation is in the output of another macro.

Here, the children in the hierarchy will be the "innermost" tokens. The ExpnData struct itself contains a subset of properties from both macro definition and macro call available through global data. ExpnData::parent tracks the child-to-parent link in this hierarchy.

For example:

macro_rules! foo { () => { println!(); } }

fn main() { foo!(); }

In this code, the AST nodes that are finally generated would have hierarchy root -> id(foo) -> id(println).

The Macro Definition Hierarchy

The second hierarchy tracks the order of macro definitions, i.e., when we are expanding one macro another macro definition is revealed in its output. This one is a bit tricky and more complex than the other two hierarchies.

SyntaxContext represents a whole chain in this hierarchy via an ID. SyntaxContextData contains data associated with the given SyntaxContext; mostly it is a cache for results of filtering that chain in different ways. SyntaxContextData::parent is the child-to-parent link here, and SyntaxContextData::outer_expns are individual elements in the chain. The "chaining-operator" is SyntaxContext::apply_mark in compiler code.

A Span, mentioned above, is actually just a compact representation of a code location and SyntaxContext. Likewise, an Ident is just an interned Symbol + Span (i.e. an interned string + hygiene data).

For built-in macros, we use the context: SyntaxContext::empty().apply_mark(expn_id), and such macros are considered to be defined at the hierarchy root. We do the same for proc macros because we haven't implemented cross-crate hygiene yet.

If the token had context X before being produced by a macro then after being produced by the macro it has context X -> macro_id. Here are some examples:

Example 0:

macro m() { ident }

m!();

Here ident which initially has context SyntaxContext::root has context ROOT -> id(m) after it's produced by m.

Example 1:

macro m() { macro n() { ident } }

m!();
n!();

In this example the ident has context ROOT initially, then ROOT -> id(m) after the first expansion, then ROOT -> id(m) -> id(n).

Example 2:

Note that these chains are not entirely determined by their last element, in other words ExpnId is not isomorphic to SyntaxContext.

macro m($i: ident) { macro n() { ($i, bar) } }

m!(foo);

After all expansions, foo has context ROOT -> id(n) and bar has context ROOT -> id(m) -> id(n).

Currently this hierarchy for tracking macro definitions is subject to the so-called "context transplantation hack". Modern (i.e. experimental) macros have stronger hygiene than the legacy "Macros By Example" (MBE) system which can result in weird interactions between the two. The hack is intended to make things "just work" for now.

The Call-site Hierarchy

The third and final hierarchy tracks the location of macro invocations.

In this hierarchy ExpnData::call_site is the child -> parent link.

Here is an example:

macro bar($i: ident) { $i }
macro foo($i: ident) { $i }

foo!(bar!(baz));

For the baz AST node in the final output, the expansion-order hierarchy is ROOT -> id(foo) -> id(bar) -> baz, while the call-site hierarchy is ROOT -> baz.

Macro Backtraces

Macro backtraces are implemented in rustc_span using the hygiene machinery in rustc_span::hygiene.

Producing Macro Output

Above, we saw how the output of a macro is integrated into the AST for a crate, and we also saw how the hygiene data for a crate is generated. But how do we actually produce the output of a macro? It depends on the type of macro.

There are two types of macros in Rust:

  1. macro_rules! macros (a.k.a. "Macros By Example" (MBE)), and,
  2. procedural macros (proc macros); including custom derives.

During the parsing phase, the normal Rust parser will set aside the contents of macros and their invocations. Later, macros are expanded using these portions of the code.

Some important data structures/interfaces here:

Macros By Example

MBEs have their own parser distinct from the Rust parser. When macros are expanded, we may invoke the MBE parser to parse and expand a macro. The MBE parser, in turn, may call the Rust parser when it needs to bind a metavariable (e.g. $my_expr) while parsing the contents of a macro invocation. The code for macro expansion is in compiler/rustc_expand/src/mbe/.

Example

macro_rules! printer {
    (print $mvar:ident) => {
        println!("{}", $mvar);
    };
    (print twice $mvar:ident) => {
        println!("{}", $mvar);
        println!("{}", $mvar);
    };
}

Here $mvar is called a metavariable. Unlike normal variables, rather than binding to a value at runtime, a metavariable binds at compile time to a tree of tokens. A token is a single "unit" of the grammar, such as an identifier (e.g. foo) or punctuation (e.g. =>). There are also other special tokens, such as EOF, which its self indicates that there are no more tokens. There are token trees resulting from the paired parentheses-like characters ((...), [...], and {...}) – they include the open and close and all the tokens in between (Rust requires that parentheses-like characters be balanced). Having macro expansion operate on token streams rather than the raw bytes of a source-file abstracts away a lot of complexity. The macro expander (and much of the rest of the compiler) doesn't consider the exact line and column of some syntactic construct in the code; it considers which constructs are used in the code. Using tokens allows us to care about what without worrying about where. For more information about tokens, see the Parsing chapter of this book.

printer!(print foo); // `foo` is a variable

The process of expanding the macro invocation into the syntax tree println!("{}", foo) and then expanding the syntax tree into a call to Display::fmt is one common example of macro expansion.

The MBE parser

There are two parts to MBE expansion done by the macro parser:

  1. parsing the definition, and,
  2. parsing the invocations.

We think of the MBE parser as a nondeterministic finite automaton (NFA) based regex parser since it uses an algorithm similar in spirit to the Earley parsing algorithm. The macro parser is defined in compiler/rustc_expand/src/mbe/macro_parser.rs.

The interface of the macro parser is as follows (this is slightly simplified):

fn parse_tt(
    &mut self,
    parser: &mut Cow<'_, Parser<'_>>,
    matcher: &[MatcherLoc]
) -> ParseResult

We use these items in macro parser:

  • a parser variable is a reference to the state of a normal Rust parser, including the token stream and parsing session. The token stream is what we are about to ask the MBE parser to parse. We will consume the raw stream of tokens and output a binding of metavariables to corresponding token trees. The parsing session can be used to report parser errors.
  • a matcher variable is a sequence of MatcherLocs that we want to match the token stream against. They're converted from token trees before matching.

In the analogy of a regex parser, the token stream is the input and we are matching it against the pattern defined by matcher. Using our examples, the token stream could be the stream of tokens containing the inside of the example invocation print foo, while matcher might be the sequence of token (trees) print $mvar:ident.

The output of the parser is a ParseResult, which indicates which of three cases has occurred:

  • Success: the token stream matches the given matcher and we have produced a binding from metavariables to the corresponding token trees.
  • Failure: the token stream does not match matcher and results in an error message such as "No rule expected token ...".
  • Error: some fatal error has occurred in the parser. For example, this happens if there is more than one pattern match, since that indicates the macro is ambiguous.

The full interface is defined here.

The macro parser does pretty much exactly the same as a normal regex parser with one exception: in order to parse different types of metavariables, such as ident, block, expr, etc., the macro parser must call back to the normal Rust parser. Both the definition and invocation of macros are parsed using the parser in a process which is non-intuitively self-referential.

The code to parse macro definitions is in compiler/rustc_expand/src/mbe/macro_rules.rs. It defines the pattern for matching a macro definition as $( $lhs:tt => $rhs:tt );+. In other words, a macro_rules definition should have in its body at least one occurrence of a token tree followed by => followed by another token tree. When the compiler comes to a macro_rules definition, it uses this pattern to match the two token trees per the rules of the definition of the macro, thereby utilizing the macro parser itself. In our example definition, the metavariable $lhs would match the patterns of both arms: (print $mvar:ident) and (print twice $mvar:ident). And $rhs would match the bodies of both arms: { println!("{}", $mvar); } and { println!("{}", $mvar); println!("{}", $mvar); }. The parser keeps this knowledge around for when it needs to expand a macro invocation.

When the compiler comes to a macro invocation, it parses that invocation using a NFA-based macro parser described above. However, the matcher variable used is the first token tree ($lhs) extracted from the arms of the macro definition. Using our example, we would try to match the token stream print foo from the invocation against the matchers print $mvar:ident and print twice $mvar:ident that we previously extracted from the definition. The algorithm is exactly the same, but when the macro parser comes to a place in the current matcher where it needs to match a non-terminal (e.g. $mvar:ident), it calls back to the normal Rust parser to get the contents of that non-terminal. In this case, the Rust parser would look for an ident token, which it finds (foo) and returns to the macro parser. Then, the macro parser proceeds in parsing as normal. Also, note that exactly one of the matchers from the various arms should match the invocation; if there is more than one match, the parse is ambiguous, while if there are no matches at all, there is a syntax error.

For more information about the macro parser's implementation, see the comments in compiler/rustc_expand/src/mbe/macro_parser.rs.

Procedural Macros

Procedural macros are also expanded during parsing. However, rather than having a parser in the compiler, proc macros are implemented as custom, third-party crates. The compiler will compile the proc macro crate and specially annotated functions in them (i.e. the proc macro itself), passing them a stream of tokens. A proc macro can then transform the token stream and output a new token stream, which is synthesized into the AST.

The token stream type used by proc macros is stable, so rustc does not use it internally. The compiler's (unstable) token stream is defined in rustc_ast::tokenstream::TokenStream. This is converted into the stable proc_macro::TokenStream and back in rustc_expand::proc_macro and rustc_expand::proc_macro_server. Since the Rust ABI is currently unstable, we use the C ABI for this conversion.

Custom Derive

Custom derives are a special type of proc macro.

Macros By Example and Macros 2.0

There is an legacy and mostly undocumented effort to improve the MBE system by giving it more hygiene-related features, better scoping and visibility rules, etc. Internally this uses the same machinery as today's MBEs with some additional syntactic sugar and are allowed to be in namespaces.