Update attribute documentation.

Author: ehuss

src/attributes.md

@@ -1,44 +1,39 @@
 # Attributes
 
 > **<sup>Syntax</sup>**\
-> _Attribute_ :\
-> &nbsp;&nbsp; _InnerAttribute_ | _OuterAttribute_
->
 > _InnerAttribute_ :\
-> &nbsp;&nbsp; `#![` MetaItem `]`
+> &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`
 >
 > _OuterAttribute_ :\
-> &nbsp;&nbsp; `#[` MetaItem `]`
+> &nbsp;&nbsp; `#` `[` _Attr_ `]`
 >
-> _MetaItem_ :\
-> &nbsp;&nbsp; &nbsp;&nbsp; [_SimplePath_]\
-> &nbsp;&nbsp; | [_SimplePath_] `=` [_LiteralExpression_]<sub>_without suffix_</sub>\
-> &nbsp;&nbsp; | [_SimplePath_] `(` _MetaSeq_<sup>?</sup> `)`
+> _Attr_ :\
+> &nbsp;&nbsp; [_SimplePath_] _AttrInput_<sup>?</sup>
 >
-> _MetaSeq_ :\
-> &nbsp;&nbsp; _MetaItemInner_ ( `,` MetaItemInner )<sup>\*</sup> `,`<sup>?</sup>
->
-> _MetaItemInner_ :\
-> &nbsp;&nbsp; &nbsp;&nbsp; _MetaItem_\
-> &nbsp;&nbsp; | [_LiteralExpression_]<sub>_without suffix_</sub>
+> _AttrInput_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; [_DelimTokenTree_]\
+> &nbsp;&nbsp; | `=` [_LiteralExpression_]<sub>_without suffix_</sub>
 
 An _attribute_ is a general, free-form metadatum that is interpreted according
 to name, convention, and language and compiler version. Attributes are modeled
 on Attributes in [ECMA-335], with the syntax coming from [ECMA-334] \(C#).
 
-Attributes may appear as any of:
+_Inner attributes_, written with a bang (`!`) after the hash (`#`), apply to the
+item that the attribute is declared within. _Outer attributes_, written without
+the bang after the hash, apply to the thing that follows the attribute.
 
-* A single identifier, the _attribute name_
-* An identifier followed by the equals sign '=' and a literal, providing a
-  key/value pair
-* An identifier followed by a parenthesized list of sub-attribute arguments
-  which include literals
+The attribute consists of a path to the attribute, followed by an optional
+delimited token tree whose interpretation is defined by the attribute.
+Attributes other than macro attributes also allow the input to be an equals
+sign (`=`) followed by a literal expression. See the [meta item
+syntax](#meta-item-attribute-syntax) below for more details.
 
-Literal values must not include integer or float type suffixes.
+Attributes can be classified into the following kinds:
 
-_Inner attributes_, written with a bang ("!") after the hash ("#"), apply to the
-item that the attribute is declared within. _Outer attributes_, written without
-the bang after the hash, apply to the thing that follows the attribute.
+* Built-in attributes
+* [Macro attributes][attribute macro]
+* [Derive mode helper attributes]
+* [Tool attributes](#tool-attributes)
 
 Attributes may be applied to many things in the language:
 
@@ -87,11 +82,33 @@ fn some_unused_variables() {
 }
 ```
 
-There are three kinds of attributes:
+## Meta Item Attribute Syntax
 
-* Built-in attributes
-* Macro attributes
-* Derive mode helper attributes
+A "meta item" is the syntax used for the _Attr_ rule by built-in attributes
+and the [`meta` macro fragment specifier]. It has the following grammar:
+
+> **<sup>Syntax</sup>**\
+> _MetaItem_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; [_SimplePath_]\
+> &nbsp;&nbsp; | [_SimplePath_] `=` [_LiteralExpression_]<sub>_without suffix_</sub>\
+> &nbsp;&nbsp; | [_SimplePath_] `(` _MetaSeq_<sup>?</sup> `)`
+>
+> _MetaSeq_ :\
+> &nbsp;&nbsp; _MetaItemInner_ ( `,` MetaItemInner )<sup>\*</sup> `,`<sup>?</sup>
+>
+> _MetaItemInner_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; _MetaItem_\
+> &nbsp;&nbsp; | [_LiteralExpression_]<sub>_without suffix_</sub>
+
+Literal expressions in meta items must not include integer or float type
+suffixes.
+
+Some examples of meta items are:
+- `no_std`
+- `doc = "example"`
+- `cfg(any())`
+- `deprecated(since = "1.2.0", note = "text")`
+- `repr(align(32))`
 
 ## Active and inert attributes
 
@@ -139,28 +156,18 @@ names have meaning.
 
 On an `extern` block, the following attributes are interpreted:
 
-- `link_args` - specify arguments to the linker, rather than just the library
-  name and type. This is feature gated and the exact behavior is
-  implementation-defined (due to variety of linker invocation syntax).
 - `link` - indicate that a native library should be linked to for the
   declarations in this block to be linked correctly. `link` supports an optional
   `kind` key with three possible values: `dylib`, `static`, and `framework`. See
   [external blocks](items/external-blocks.html) for more about external blocks.
   Two examples: `#[link(name = "readline")]` and
   `#[link(name = "CoreFoundation", kind = "framework")]`.
-- `linked_from` - indicates what native library this block of FFI items is
-  coming from. This attribute is of the form `#[linked_from = "foo"]` where
-  `foo` is the name of a library in either `#[link]` or a `-l` flag. This
-  attribute is currently required to export symbols from a Rust dynamic library
-  on Windows, and it is feature gated behind the `linked_from` feature.
 
 On declarations inside an `extern` block, the following attributes are
 interpreted:
 
 - `link_name` - the name of the symbol that this function or static should be
   imported as.
-- `linkage` - on a static, this specifies the [linkage
-  type](http://llvm.org/docs/LangRef.html#linkage-types).
 
 See [type layout](type-layout.html) for documentation on the `repr` attribute
 which can be used to control type layout.
@@ -175,8 +182,6 @@ which can be used to control type layout.
   macros named.  The `extern crate` must appear at the crate root, not inside
   `mod`, which ensures proper function of the `$crate` macro variable.
 
-- `macro_reexport` on an `extern crate` — re-export the named macros.
-
 - `macro_export` - export a `macro_rules` macro for cross-crate usage.
 
 - `no_link` on an `extern crate` — even if we load this crate for macros, don't
@@ -371,8 +376,7 @@ They only get checked when the associated tool is active, so if you try to use a
 
 Otherwise, they work just like regular lint attributes:
 
-
-```rust,ignore
+```rust
 // set the entire `pedantic` clippy lint group to warn
 #![warn(clippy::pedantic)]
 // silence warnings from the `filter_map` clippy lint
@@ -552,6 +556,32 @@ impl<T: PartialEq> PartialEq for Foo<T> {
 
 You can implement `derive` for your own traits through [procedural macros].
 
+## Tool attributes
+
+The compiler may allow attributes for external tools where each tool resides
+in its own namespace. The first segment of the attribute path is the name of
+the tool, with one or more additional segments whose interpretation is up to
+the tool.
+
+When a tool is not in use, the tool's attributes are accepted without a
+warning. When the tool is in use, the tool is responsible for processing and
+interpretation of its attributes.
+
+Tool attributes are not available if the [`no_implicit_prelude`] attribute is
+used.
+
+```rust
+// Tells the rustfmt tool to not format the following element.
+#[rustfmt::skip]
+struct S {
+}
+
+// Controls the "cyclomatic complexity" threshold for the cLippy tool.
+#[clippy::cyclomatic_complexity = "100"]
+pub fn f() {}
+```
+
+[_DelimTokenTree_]: macros.html
 [_LiteralExpression_]: expressions/literal-expr.html
 [_SimplePath_]: paths.html#simple-paths
 [`no_implicit_prelude`]: items/modules.html#prelude-items
@@ -585,6 +615,7 @@ You can implement `derive` for your own traits through [procedural macros].
 [external blocks]: items/external-blocks.html
 [items]: items.html
 [attribute macro]: procedural-macros.html#attribute-macros
+[derive mode helper attributes]: procedural-macros.html#derive-mode-helper-attributes
 [function-like macro]: procedural-macros.html#function-like-procedural-macros
 [conditional compilation]: conditional-compilation.html
 [derive mode macro]: procedural-macros.html#derive-mode-macros
@@ -594,3 +625,4 @@ You can implement `derive` for your own traits through [procedural macros].
 [where clause]: items/where-clauses.html
 [trait or lifetime bounds]: trait-bounds.html
 [Expression Attributes]: expressions.html#expression-attributes
+[`meta` macro fragment specifier]: macros-by-example.html

src/conditional-compilation.md

@@ -104,7 +104,7 @@ Example values:
 * `"android"`
 * `"freebsd"`
 * `"dragonfly"`
-* `"bitrig"` 
+* `"bitrig"`
 * `"openbsd"`
 * `"netbsd"`
 
@@ -151,7 +151,7 @@ Key-value option set once with the target's pointer width in bits. For example,
 for targets with 32-bit pointers, this is set to `"32"`. Likewise, it is set
 to `"64"` for targets with 64-bit pointers.
 
-<!-- Are there targets that have a different bit number? --> 
+<!-- Are there targets that have a different bit number? -->
 
 ### `target_has_atomic`
 
@@ -249,16 +249,16 @@ generic parameters.
 
 > **<sup>Syntax</sup>**\
 > _CfgAttrAttribute_ :\
-> &nbsp;&nbsp; `cfg_attr` `(` _ConfigurationPredicate_ `,` [_MetaItem_] `,`<sup>?</sup> `)`
+> &nbsp;&nbsp; `cfg_attr` `(` _ConfigurationPredicate_ `,` _CfgAttrs_<sup>?</sup> `)`
+>
+> _CfgAttrs_ :\
+> &nbsp;&nbsp; [_Attr_]&nbsp;(`,` [_Attr_])<sup>\*</sup> `,`<sup>?</sup>
 
 The `cfg_attr` [attribute] conditionally includes [attributes] based on a
 configuration predicate.
 
-It is written as `cfg_attr` followed by `(`, a configuration predicate, a
-[metaitem], an optional `,`, and finally a `)`.
-
-When the configuration predicate is true, this attribute expands out to be an
-attribute of the attribute metaitem. For example, the following module will
+When the configuration predicate is true, this attribute expands out to the
+attributes listed after the predicate. For example, the following module will
 either be found at `linux.rs` or `windows.rs` based on the target.
 
 ```rust,ignore
@@ -267,6 +267,19 @@ either be found at `linux.rs` or `windows.rs` based on the target.
 mod os;
 ```
 
+Zero, one, or more attributes may be listed. Multiple attributes will each be
+expanded into separate attributes. For example:
+
+```rust,ignore
+#[cfg_attr(feature = "magic", sparkles, crackles)]
+fn bewitched() {}
+
+// When the `magic` feature flag is enabled, the above will expand to:
+#[sparkles]
+#[crackles]
+fn bewitched() {}
+```
+
 > **Note**: The `cfg_attr` can expand to another `cfg_attr`. For example,
 > `#[cfg_attr(linux, cfg_attr(feature = "multithreaded", some_other_attribute))`
 > is valid. This example would be equivalent to
@@ -298,7 +311,7 @@ println!("I'm running on a {} machine!", machine_kind);
 [IDENTIFIER]: identifiers.html
 [RAW_STRING_LITERAL]: tokens.html#raw-string-literals
 [STRING_LITERAL]: tokens.html#string-literals
-[_MetaItem_]: attributes.html
+[_Attr_]: attributes.html
 [`--cfg`]: ../rustc/command-line-arguments.html#a--cfg-configure-the-compilation-environment
 [`--test`]: ../rustc/command-line-arguments.html#a--test-build-a-test-harness
 [`cfg`]: #the-cfg-attribute
@@ -310,4 +323,3 @@ println!("I'm running on a {} machine!", machine_kind);
 [crate type]: linkage.html
 [expressions]: expressions.html
 [items]: items.html
-[metaitem]: attributes.html

src/macros-by-example.md

@@ -80,7 +80,7 @@ syntax named by _designator_. Valid designators are:
 [_Expression_]: expressions.html
 [_Item_]: items.html
 [_LiteralExpression_]: expressions/literal-expr.html
-[_MetaItem_]: attributes.html
+[_MetaItem_]: attributes.html#meta-item-attribute-syntax
 [_Pattern_]: patterns.html
 [_Statement_]: statements.html
 [_TokenTree_]: macros.html#macro-invocation

src/procedural-macros.md

@@ -77,7 +77,7 @@ These macros are defined by a [public] [function] with the `proc_macro`
 [`TokenStream`] is what is inside the delimiters of the macro invocation and the
 output [`TokenStream`] replaces the entire macro invocation. It may contain an
 arbitrary number of [items]. These macros cannot expand to syntax that defines
-new `macro_rule` style macros.
+new `macro_rules` style macros.
 
 For example, the following macro definition ignores its input and outputs a
 function `answer` into its scope.
@@ -195,14 +195,14 @@ struct Struct {
 *Attribute macros* define new [attributes] which can be attached to [items].
 
 Attribute macros are defined by a [public] [function] with the
-`proc_macro_attribute` [attribute] that a signature of
-`(TokenStream, TokenStream) -> TokenStream`. The first [`TokenStream`] is the
-attribute's metaitems, not including the delimiters. If the attribute is written
-without a metaitem, the attribute [`TokenStream`] is empty. The second
-[`TokenStream`] is of the rest of the [item] including other [attributes] on the
-[item]. The returned [`TokenStream`] replaces the [item] with an arbitrary
-number of [items]. These macros cannot expand to syntax that defines new
-`macro_rule` style macros.
+`proc_macro_attribute` [attribute] that a signature of `(TokenStream,
+TokenStream) -> TokenStream`. The first [`TokenStream`] is the delimited token
+tree following the attribute's name, not including the outer delimiters. If
+the attribute is written as a bare attribute name, the attribute
+[`TokenStream`] is empty. The second [`TokenStream`] is the rest of the [item]
+including other [attributes] on the [item]. The returned [`TokenStream`]
+replaces the [item] with an arbitrary number of [items]. These macros cannot
+expand to syntax that defines new `macro_rules` style macros.
 
 For example, this attribute macro takes the input stream and returns it as is,
 effectively being the no-op of attributes.
@@ -247,16 +247,16 @@ fn invoke1() {}
 // out: attr: ""
 // out: item: "fn invoke1() { }"
 
-// Example: Attribute has a metaitem
+// Example: Attribute with input
 #[show_streams(bar)]
 fn invoke2() {}
 // out: attr: "bar"
 // out: item: "fn invoke2() {}"
 
-// Example: Multiple words in metaitem
-#[show_streams(multiple words)]
+// Example: Multiple tokens in the input
+#[show_streams(multiple => tokens)]
 fn invoke3() {}
-// out: attr: "multiple words"
+// out: attr: "multiple => tokens"
 // out: item: "fn invoke3() {}"
 
 // Example:

src/type-layout.md

@@ -116,7 +116,7 @@ The possible representations for a type are the default representation, `C`, the
 primitive representations, and `packed`. Multiple representations can be applied
 to a single type.
 
-The representation of a type can be changed by applying the [`repr` attribute]
+The representation of a type can be changed by applying the `repr` attribute
 to it. The following example shows a struct with a `C` representation.
 
 ```
@@ -332,4 +332,4 @@ used with any other representation.
 [zero-variant enumerations]: items/enumerations.html#zero-variant-enums
 [undefined behavior]: behavior-considered-undefined.html
 [27060]: https://github.com/rust-lang/rust/issues/27060
-[`PhantomData<T>`]: special-types-and-traits.html#phantomdatat
+[`PhantomData<T>`]: special-types-and-traits.html#phantomdatat