diff --git a/src/macro-expansion.md b/src/macro-expansion.md index e7eaf197..ebab56ad 100644 --- a/src/macro-expansion.md +++ b/src/macro-expansion.md @@ -5,11 +5,11 @@ > 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 `macro`s to be expanded (using temporary [placeholders]). -This chapter is about the process of expanding those `macro`s iteratively until -we have a complete [*Abstract Syntax Tree* (`AST`)][ast] for our crate with no -unexpanded `macro`s (or a compile error). +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)][ast] for our crate with no +unexpanded macros (or a compile error). [ast]: ./ast-validation.md [`rustc_ast`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/index.html @@ -17,14 +17,14 @@ unexpanded `macro`s (or a compile error). [`rustc_builtin_macros`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_builtin_macros/index.html [placeholders]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/placeholders/index.html -First, we discuss the algorithm that expands and integrates `macro` output into -`AST`s. Next, we take a look at how hygiene data is collected. Finally, we look -at the specifics of expanding different types of `macro`s. +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`][base]. -Also of note, `cfg` and `cfg_attr` are treated specially from other `macro`s, and are +Also of note, `cfg` and `cfg_attr` are treated specially from other macros, and are handled in [`rustc_expand::config`][cfg]. [`rustc_expand`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/index.html @@ -34,7 +34,7 @@ handled in [`rustc_expand::config`][cfg]. ## 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 `macro`s expanded, all +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`][fef] method. With few exceptions, we use this method on the whole crate (see ["Eager Expansion"](#eager-expansion) @@ -44,53 +44,53 @@ below for more detailed discussion of edge case expansion issues). [reb]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/build/index.html At a high level, [`fully_expand_fragment`][fef] works in iterations. We keep a -queue of unresolved `macro` invocations (i.e. `macro`s we haven't found the -definition of yet). We repeatedly try to pick a `macro` from the queue, resolve +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][original]: [fef]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/expand/struct.MacroExpander.html#method.fully_expand_fragment [original]: https://github.com/rust-lang/rust/pull/53778#issuecomment-419224049 -1. Initialize a `queue` of unresolved `macro`s. +1. Initialize a `queue` of unresolved macros. 2. Repeat until `queue` is empty (or we make no progress, which is an error): 1. [Resolve](./name-resolution.md) imports in our partially built crate as much as possible. - 2. Collect as many `macro` [`Invocation`s][inv] as possible from our + 2. Collect as many macro [`Invocation`s][inv] 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 [`TokenTree`s][tt], + 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 [`TokenTree`s][tt], 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 + - 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][hybelow] 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 + 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. + - If the macro produces tokens (e.g. a proc macro), we parse into + an AST, which may produce parse errors. - During expansion, we create [`SyntaxContext`]s (hierarchy 2) (see [Hygiene][hybelow] below). - - These three passes happen one after another on every `AST` fragment - freshly expanded from a `macro`: + - These three passes happen one after another on every AST fragment + freshly expanded from a macro: - [`NodeId`]s are assigned by [`InvocationCollector`]. This - also collects new `macro` calls from this new `AST` piece and + also collects new macro calls from this new AST piece and adds them to the queue. - ["Def paths"][defpath] are created and [`DefId`]s are assigned to them by [`DefCollector`]. - Names are put into modules (from the resolver's point of view) by [`BuildReducedGraphVisitor`]. - 3. After expanding a single `macro` and integrating its output, continue + 3. After expanding a single macro and integrating its output, continue to the next iteration of [`fully_expand_fragment`][fef]. 5. If it's not resolved: - 1. Put the `macro` back in the queue. + 1. Put the macro back in the queue. 2. Continue to next iteration... [`AstFragment`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/expand/enum.AstFragment.html @@ -112,9 +112,9 @@ iteration, this represents a compile error. Here is the [algorithm][original]: ### 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 `macro`s or imports) with the intent of generating diagnostics. -Failure recovery happens by expanding unresolved `macro`s into +(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`][err] and allows compilation to continue past the first error so that `rustc` can report more errors than just the original failure. @@ -123,8 +123,8 @@ 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`][mresolve], which resolves `macro` paths, validates +macro names in the above algorithm. This is done in +[`rustc_resolve::macros`][mresolve], 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 @@ -134,10 +134,10 @@ Resolution](./name-resolution.md). ### 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 `macro`s that expect literals; expanding arguments first for some of -these `macro` results in a smoother user experience. As an example, consider +_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: ```rust,ignore @@ -152,11 +152,11 @@ A lazy-expansion would expand `foo!` first. An eager-expansion would expand 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 `macro`s for the sake of user-experience. The built-in -`macro`s are implemented in [`rustc_builtin_macros`], along with some other +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`][reb]. Eager-expansion generally +AST fragments in [`rustc_expand::build`][reb]. Eager-expansion generally performs a subset of the things that lazy (normal) expansion does. It is done by invoking [`fully_expand_fragment`][fef] on only part of a crate (as opposed to the whole crate, like we normally do). @@ -170,10 +170,10 @@ integration: 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 `type`s and patterns that can be produced by - `macro`s but cannot be annotated with attributes. -- [`MacResult`] - a "polymorphic" `AST` fragment, something that can turn into +- [`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). @@ -223,16 +223,16 @@ 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 `macro`s are hygienic +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`). +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). [code_dir]: https://github.com/rust-lang/rust/tree/master/compiler/rustc_expand/src/mbe [code_mp]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/mbe/macro_parser @@ -240,18 +240,18 @@ only within the `macro` (i.e. it should not be visible outside the `macro`). [code_parse_int]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/mbe/macro_parser/struct.TtParser.html#method.parse_tt [parsing]: ./the-parser.html -The context is attached to `AST` nodes. All `AST` nodes generated by `macro`s have +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 +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::Span`s][span] to refer to code locations. This struct also has hygiene information attached to it, as we will see later. [span]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html -Because `macro`s 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 +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 @@ -259,13 +259,13 @@ 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 `macro`s receive -an integer ID, assigned continuously starting from 0 as we discover new `macro` +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`][rootid], which is its own parent. -The [`rustc_span::hygiene`][hy] library contains all of the hygiene-related algorithms +The [`rustc_span::hygiene`][hy] crate contains all of the hygiene-related algorithms (with the exception of some hacks in [`Resolver::resolve_crate_root`][hacks]) and structures related to hygiene and expansion that are kept in global data. @@ -283,12 +283,12 @@ 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`. +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`] struct itself contains a subset of properties from both macro +definition and macro call available through global data. [`ExpnData::parent`][edp] tracks the child-to-parent link in this hierarchy. [`ExpnData`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/hygiene/struct.ExpnData.html @@ -302,13 +302,13 @@ macro_rules! foo { () => { println!(); } } fn main() { foo!(); } ``` -In this code, the `AST` nodes that are finally generated would have hierarchy +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 +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`][sc] represents a whole chain in this hierarchy via an ID. @@ -330,15 +330,15 @@ a code location and [`SyntaxContext`][sc]. Likewise, an [`Ident`] is just an int [scdoe]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/hygiene/struct.SyntaxContextData.html#structfield.outer_expn [am]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/hygiene/struct.SyntaxContext.html#method.apply_mark -For built-in `macro`s, we use the context: -[`SyntaxContext::empty().apply_mark(expn_id)`], and such `macro`s are +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 macro`s because we haven't implemented cross-crate hygiene yet. [`SyntaxContext::empty().apply_mark(expn_id)`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/hygiene/struct.SyntaxContext.html#method.apply_mark -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: +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: @@ -379,9 +379,9 @@ 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 +Currently this hierarchy for tracking macro definitions is subject to the so-called ["context transplantation hack"][hack]. Modern (i.e. experimental) -`macro`s have stronger hygiene than the legacy "Macros By Example" (`MBE`) +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. @@ -390,7 +390,7 @@ intended to make things "just work" for now. ### The Call-site Hierarchy -The third and final hierarchy tracks the location of `macro` invocations. +The third and final hierarchy tracks the location of macro invocations. In this hierarchy [`ExpnData::call_site`][callsite] is the `child -> parent` link. @@ -406,38 +406,38 @@ macro foo($i: ident) { $i } foo!(bar!(baz)); ``` -For the `baz` `AST` node in the final output, the expansion-order hierarchy is +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 +Macro backtraces are implemented in [`rustc_span`] using the hygiene machinery in [`rustc_span::hygiene`][hy]. [`rustc_span`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/index.html ## Producing Macro Output -Above, we saw how the output of a `macro` is integrated into the `AST` for a crate, +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`. +actually produce the output of a macro? It depends on the type of macro. -There are two types of `macro`s in Rust: - 1. `macro_rules!` macros, and, - 2. procedural `macro`s (`proc macro`s); including custom derives. +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 -`macro`s and their invocations. Later, `macro`s are expanded using these +macros and their invocations. Later, macros are expanded using these portions of the code. Some important data structures/interfaces here: -- [`SyntaxExtension`] - a lowered `macro` representation, contains its expander - function, which transforms a [`TokenStream`] or `AST` into another - [`TokenStream`] or `AST` + some additional data like stability, or a list of - unstable features allowed inside the `macro`. +- [`SyntaxExtension`] - a lowered macro representation, contains its expander + function, which transforms a [`TokenStream`] or AST into another + [`TokenStream`] or AST + some additional data like stability, or a list of + unstable features allowed inside the macro. - [`SyntaxExtensionKind`] - expander functions may have several different - signatures (take one token stream, or two, or a piece of `AST`, etc). This is + signatures (take one token stream, or two, or a piece of AST, etc). This is an `enum` that lists them. - [`BangProcMacro`]/[`TTMacroExpander`]/[`AttrProcMacro`]/[`MultiItemModifier`] - `trait`s representing the expander function signatures. @@ -451,11 +451,11 @@ Some important data structures/interfaces here: ## Macros By Example -`MBE`s have their own parser distinct from the Rust parser. When `macro`s 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 +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/`][code_dir]. ### Example @@ -480,9 +480,9 @@ 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 +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 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 @@ -492,23 +492,23 @@ the [Parsing][parsing] chapter of this book. printer!(print foo); // `foo` is a variable ``` -The process of expanding the `macro` invocation into the syntax tree +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_. +`Display::fmt` is one common example of _macro expansion_. ### The MBE parser -There are two parts to `MBE` expansion done by the `macro` 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 +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](https://en.wikipedia.org/wiki/Earley_parser). The `macro` +parsing algorithm](https://en.wikipedia.org/wiki/Earley_parser). The macro parser is defined in [`compiler/rustc_expand/src/mbe/macro_parser.rs`][code_mp]. -The interface of the `macro` parser is as follows (this is slightly simplified): +The interface of the macro parser is as follows (this is slightly simplified): ```rust,ignore fn parse_tt( @@ -518,11 +518,11 @@ fn parse_tt( ) -> ParseResult ``` -We use these items in `macro` parser: +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 + 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 [`MatcherLoc`]s that we want to match @@ -531,73 +531,73 @@ We use these items in `macro` parser: [`MatcherLoc`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/mbe/macro_parser/enum.MatcherLoc.html 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 +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) +invocation `print foo`, while matcher might be the sequence of token (trees) `print $mvar:ident`. The output of the parser is a [`ParseResult`], which indicates which of three cases has occurred: -- **Success**: the token stream matches the given `matcher` and we have produced a +- **Success**: the token stream matches the given matcher and we have produced a binding from metavariables to the corresponding token trees. -- **Failure**: the token stream does not match `matcher` and results in an error +- **Failure**: the token stream does not match matcher and results in an error message such as "No rule expected token ...". - **Error**: some fatal error has occurred _in the parser_. For example, this happens if there is more than one pattern match, since that indicates the - `macro` is ambiguous. + macro is ambiguous. The full interface is defined [here][code_parse_int]. -The `macro` parser does pretty much exactly the same as a normal regex parser +The macro parser does pretty much exactly the same as a normal regex parser with one exception: in order to parse different types of metavariables, such as -`ident`, `block`, `expr`, etc., the `macro` parser must call back to the normal -Rust parser. Both the definition and invocation of `macro`s are parsed using +`ident`, `block`, `expr`, etc., the macro parser must call back to the normal +Rust parser. Both the definition and invocation of macros are parsed using the parser in a process which is non-intuitively self-referential. -The code to parse `macro` _definitions_ is in +The code to parse macro _definitions_ is in [`compiler/rustc_expand/src/mbe/macro_rules.rs`][code_mr]. It defines the -pattern for matching a `macro` definition as `$( $lhs:tt => $rhs:tt );+`. In +pattern for matching a macro definition as `$( $lhs:tt => $rhs:tt );+`. In other words, a `macro_rules` definition should have in its body at least one occurrence of a token tree followed by `=>` followed by another token tree. When the compiler comes to a `macro_rules` definition, it uses this pattern to -match the two token trees per the rules of the definition of the `macro`, _thereby -utilizing the `macro` parser itself_. In our example definition, the +match the two token trees per the rules of the definition of the macro, _thereby +utilizing the macro parser itself_. In our example definition, the metavariable `$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!("{}", $mvar); }`. The parser keeps this knowledge around for when it -needs to expand a `macro` invocation. +needs to expand a macro invocation. -When the compiler comes to a `macro` invocation, it parses that invocation using -a NFA-based `macro` parser described above. However, the `matcher` variable -used is the first token tree (`$lhs`) extracted from the arms of the `macro` +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 `matcher`s `print $mvar:ident` and `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`), +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 `matcher`s from +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 +For more information about the macro parser's implementation, see the comments in [`compiler/rustc_expand/src/mbe/macro_parser.rs`][code_mp]. ## Procedural Macros -Procedural `macro`s are also expanded during parsing. However, rather than -having a parser in the compiler, `proc macro`s 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`. +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 macro`s is _stable_, so `rustc` does not +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`][rustcts]. This is converted into the stable [`proc_macro::TokenStream`][stablets] and back in @@ -615,13 +615,13 @@ 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`. +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 +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 `MBE`s with some +rules, etc. Internally this uses the same machinery as today's MBEs with some additional syntactic sugar and are allowed to be in namespaces.