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Fix text in TRPL macros guide to match the example

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Also, update description of macro invocation syntax:
after https://github.com/rust-lang/rust/pull/20563 there is
a number of additional limitations on macro syntax.
This commit is contained in:
Alexander Korolkov 2015-01-12 00:51:25 +03:00
parent 0aec4db1c0
commit a3d87fa448
1 changed files with 12 additions and 3 deletions
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15
src/doc/trpl/macros.md
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@ -42,7 +42,7 @@ the pattern in the above code:
# let input_1 = T::SpecialA(0);
# let input_2 = T::SpecialA(0);
macro_rules! early_return {
($inp:expr, $sp:path) => ( // invoke it like `(input_5 SpecialE)`
($inp:expr, $sp:path) => ( // invoke it like `(input_5, SpecialE)`
match $inp {
$sp(x) => { return x; }
_ => {}
@ -59,7 +59,7 @@ early_return!(input_2, T::SpecialB);
~~~~
Macros are defined in pattern-matching style: in the above example, the text
`($inp:expr $sp:ident)` that appears on the left-hand side of the `=>` is the
`($inp:expr, $sp:path)` that appears on the left-hand side of the `=>` is the
*macro invocation syntax*, a pattern denoting how to write a call to the
macro. The text on the right-hand side of the `=>`, beginning with `match
$inp`, is the *macro transcription syntax*: what the macro expands to.
@ -74,6 +74,8 @@ conforms to the following rules:
2. `$` has special meaning (described below).
3. The `()`s, `[]`s, and `{}`s it contains must balance. For example, `([)` is
forbidden.
4. Some arguments can be followed only by a limited set of separators, to
avoid ambiguity (described below).
Otherwise, the invocation syntax is free-form.
@ -86,7 +88,8 @@ To take a fragment of Rust code as an argument, write `$` followed by a name
`foo`.)
* `expr` (an expression. Examples: `2 + 2`; `if true then { 1 } else { 2 }`;
`f(42)`.)
* `ty` (a type. Examples: `int`, `Vec<(char, String)>`, `&T`.)
* `ty` (a type. Examples: `i32`, `Vec<(char, String)>`, `&T`.)
* `path` (a path to struct or enum variant. Example: `T::SpecialA`)
* `pat` (a pattern, usually appearing in a `match` or on the left-hand side of
a declaration. Examples: `Some(t)`; `(17, 'a')`; `_`.)
* `block` (a sequence of actions. Example: `{ log(error, "hi"); return 12; }`)
@ -97,6 +100,12 @@ rules of tokenization apply,
So `($x:ident -> (($e:expr)))`, though excessively fancy, would designate a macro
that could be invoked like: `my_macro!(i->(( 2+2 )))`.
To avoid ambiguity, macro invocation syntax must conform to the following rules:
* `expr` must be followed by `=>`, `,` or `;`.
* `ty` and `path` must be followed by `=>`, `,`, `:`, `=`, `>` or `as`.
* `pat` must be followed by `=>`, `,` or `=`.
* `ident` and `block` can be followed by any token.
## Invocation location
A macro invocation may take the place of (and therefore expand to) an