CombinedLintPass 背景 前一篇文章 介绍了关于 Lint 和 LintPass 的一些概念和实现。基于这些结构，提供了一个简易的 Lint 检查的实现方式。本文主要介绍 CombinedLintPass 这一结构的实现，并基于 CombinedLintPass 进一步优化 Lint 的实现。
CombinedLintpass Rustc 在 LintPass 的中实现了 Lint 工具检查的具体逻辑。并且使用 Visitor 模式遍历 AST 的同时调用 lintpass 中的 check_*方法。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 impl ast_visit ::Visitor for Linter { fn visit_crate (a: ast:crate ){ for lintpass in lintpasses{ lintpass.check_crate (a) } walk_crate (); } fn visit_stmt (a: ast:stmt){ for lintpass in lintpasses{ lintpass.check_stmt (a) } walk_stmt (); } ... }
但是，Rustc 自身和 clippy 提供的 Lint 定义多达550+多个。考虑到性能因素，定义大量的 LintPass，分别注册和调用显然是不合适的。Rustc 提供了一种更优的解决方法：既然可以将多个 Lint 组织为一个 LintPass，同样也可以将多个 LintPass 组合成一个 CombinedLintPass。
Compiler lint passes are combined into one pass
BuiltinCombinedEarlyLintPass CombinedLintPass 同样分为 early 和 late 两类。 以 builtin 的 early lint 为例，Rustc 在 rustc_lint::src::lib.rs 中为这些 lintpass 定义了一个 BuiltinCombinedEarlyLintPass 结构。
1 early_lint_passes!(declare_combined_early_pass, [BuiltinCombinedEarlyLintPass]);
虽然这个定义看起来只有一行，但其中通过若干个宏的展开，汇总了14个 LintPass，并且每个 LintPass 提供了50多个 check_* 方法。接下来一一说明这些宏。
BuiltinCombinedEarlyLintPass 的宏定义 early_lint_passes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 macro_rules! early_lint_passes { ($macro :path, $args:tt) => { $macro!( $args, [ UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns::default (), NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr::new (), WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, ] ); }; }
首先是 early_lint_passes 宏，这个宏的主要作用是定义了所有的 early lintpass。这里的 lintpass 是成对出现的，:左边为 lintpass 的 Identifier，:右边为 lintpass 的constructor。所以会出现 EllipsisInclusiveRangePatterns::default() 和 DeprecatedAttr::new()这种形式。early_lint_passes 会将定义的 early lintpass 和 第二个参数一起传递给下一个宏。BuiltinCombinedEarlyLintPass的定义被展开为：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 declare_combined_early_pass!([BuiltinCombinedEarlyLintPass], [ UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns::default (), NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr::new (), WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, ])
declare_combined_early_pass 1 2 3 4 5 macro_rules! declare_combined_early_pass { ([$name:ident], $passes:tt) => ( early_lint_methods!(declare_combined_early_lint_pass, [pub $name, $passes]); ) }
declare_combined_early_pass 宏接收 early_lint_passes宏传来的 name(BuiltinCombinedEarlyLintPass) 和 passes，并继续传递给 early_lint_methods 宏。BuiltinCombinedEarlyLintPass的定义继续展开为：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 early_lint_methods!(declare_combined_early_lint_pass, [pub BuiltinCombinedEarlyLintPass, [ UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns::default (), NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr::new (), WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, ] ]);
early_lint_methods 1 2 3 4 5 6 7 8 9 10 11 macro_rules! early_lint_methods { ($macro :path, $args:tt) => ( $macro!($args, [ fn check_param (a: &ast::Param); fn check_ident (a: &ast::Ident); fn check_crate (a: &ast::Crate); fn check_crate_post (a: &ast::Crate); ... ]); ) }
earlylint_methods 宏在前一篇文章中也介绍过，它定义了 EarlyLintPass 中需要实现的 `check *函数，并且将这些函数以及接收的参数$args传递给下一个宏。因为BuiltinCombinedEarlyLintPass也是 early lint 的一种，所以同样需要实现这些函数。
通过这个宏，BuiltinCombinedEarlyLintPass`的定义继续展开为：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 declare_combined_early_lint_pass!( [pub BuiltinCombinedEarlyLintPass, [ UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns::default (), NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr::new (), WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, ] ], [ fn check_param (a: &ast::Param); fn check_ident (a: &ast::Ident); fn check_crate (a: &ast::Crate); fn check_crate_post (a: &ast::Crate); ... ] )
declare_combined_early_lint_pass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 macro_rules! declare_combined_early_lint_pass { ([$v:vis $name:ident, [$($passes:ident: $constructor:expr,)*]], $methods:tt) => ( #[allow(non_snake_case)] $v struct $name { $($passes: $passes,)* } impl $name { $v fn new () -> Self { Self { $($passes: $constructor,)* } } $v fn get_lints () -> LintArray { let mut lints = Vec::new (); $(lints.extend_from_slice (&$passes::get_lints ());)* lints } } impl EarlyLintPass for $name { expand_combined_early_lint_pass_methods!([$($passes),*], $methods); } #[allow(rustc::lint_pass_impl_without_macro)] impl LintPass for $name { fn name (&self ) -> &'static str { panic! () } } ) }
declare_combined_early_lint_pass宏是生成 BuiltinCombinedEarlyLintPass 的主体。这个宏中做了以下工作：
生成一个名为 BuiltinCombinedEarlyLintPass 的 struct，其中的属性为宏 early_lint_passes 提供的 lintpass 的 identifier。
实现 fn new() fn name() 和 fn get_lints() 方法。其中 new() 调用了 early_lint_passes 提供的 lintpass 的 constructor。
调用宏 expand_combined_early_lint_pass_methods，实现自身的 check_* 方法。
通过这个宏，BuiltinCombinedEarlyLintPass的定义变为：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 pub struct BuiltinCombinedEarlyLintPass { UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns, NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr, WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, } impl BuiltinCombinedEarlyLintPass { pub fn new () -> Self { Self { UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, UnusedImportBraces: UnusedImportBraces, UnsafeCode: UnsafeCode, AnonymousParameters: AnonymousParameters, EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns::default (), NonCamelCaseTypes: NonCamelCaseTypes, DeprecatedAttr: DeprecatedAttr::new (), WhileTrue: WhileTrue, NonAsciiIdents: NonAsciiIdents, HiddenUnicodeCodepoints: HiddenUnicodeCodepoints, IncompleteFeatures: IncompleteFeatures, RedundantSemicolons: RedundantSemicolons, UnusedDocComment: UnusedDocComment, } } pub fn get_lints () -> LintArray { let mut lints = Vec::new (); lints.extend_from_slice (&UnusedParens::get_lints ()); lints.extend_from_slice (&UnusedBraces::get_lints ()); lints.extend_from_slice (&UnusedImportBraces::get_lints ()); lints.extend_from_slice (&UnsafeCode::get_lints ()); lints.extend_from_slice (&AnonymousParameters::get_lints ()); lints.extend_from_slice (&EllipsisInclusiveRangePatterns::get_lints ()); lints.extend_from_slice (&NonCamelCaseTypes::get_lints ()); lints.extend_from_slice (&DeprecatedAttr::get_lints ()); lints.extend_from_slice (&WhileTrue::get_lints ()); lints.extend_from_slice (&NonAsciiIdents::get_lints ()); lints.extend_from_slice (&HiddenUnicodeCodepoints::get_lints ()); lints.extend_from_slice (&IncompleteFeatures::get_lints ()); lints.extend_from_slice (&RedundantSemicolons::get_lints ()); lints.extend_from_slice (&UnusedDocComment::get_lints ()); lints } } impl EarlyLintPass for BuiltinCombinedEarlyLintPass { expand_combined_early_lint_pass_methods!([$($passes),*], $methods); } #[allow(rustc::lint_pass_impl_without_macro)] impl LintPass for BuiltinCombinedEarlyLintPass { fn name (&self ) -> &'static str { panic! () } }
expand_combined_early_lint_pass_methods 1 2 3 4 5 6 7 macro_rules! expand_combined_early_lint_pass_methods { ($passes:tt, [$($(#[$attr:meta] )* fn $name:ident ($($param:ident: $arg:ty),*);)*]) => ( $(fn $name (&mut self , context: &EarlyContext<'_ >, $($param: $arg),*) { expand_combined_early_lint_pass_method!($passes, self , $name, (context, $($param),*)); })* ) }
expand_combined_early_lint_pass_methods宏在 BuiltinCombinedEarlyLintPass 中展开所有 early_lint_methods 中定义的方法。BuiltinCombinedEarlyLintPass的定义变为（省略其他定义）：
1 2 3 4 5 6 7 8 9 10 11 12 13 impl EarlyLintPass for BuiltinCombinedEarlyLintPass { fn check_param (&mut self , context: &EarlyContext<'_ >, a: &ast::Param) { expand_combined_early_lint_pass_method!($passes, self , $name, (context, $($param),*)); } fn check_ident (&mut self , context: &EarlyContext<'_ >, a: &ast::Ident) { expand_combined_early_lint_pass_method!($passes, self , $name, (context, $($param),*)); } fn check_crate (&mut self , context: &EarlyContext<'_ >, a: &ast::Crate) { expand_combined_early_lint_pass_method!($passes, self , $name, (context, $($param),*)); } ... }
expand_combined_early_lint_pass_method 1 2 3 4 5 macro_rules! expand_combined_early_lint_pass_method { ([$($passes:ident),*], $self : ident, $name: ident, $params:tt) => ({ $($self .$passes.$name $params;)* }) }
expandcombined_early_lint_pass_method：在展开的`check 函数中调用每一个LintPass的check_。
通过这个宏，BuiltinCombinedEarlyLintPass`的定义变为（省略其他定义）：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 impl EarlyLintPass for BuiltinCombinedEarlyLintPass { fn check_param (&mut self , context: &EarlyContext<'_ >, a: &ast::Param) { self .UnusedParens.check_param (context, a); self .UnusedBraces.check_param (context, a); self .UnusedImportBraces.check_param (context, a); ... } fn check_ident (&mut self , context: &EarlyContext<'_ >, a: &ast::Ident) { self .UnusedParens.check_ident (context, a); self .UnusedBraces.check_ident (context, a); self .UnusedImportBraces.check_ident (context, a); ... } fn check_crate (&mut self , context: &EarlyContext<'_ >, a: &ast::Crate) { self .UnusedParens.check_crate (context, a); self .UnusedBraces.check_crate (context, a); self .UnusedImportBraces.check_crate (context, a); ... } ... }
BuiltinCombinedEarlyLintPass 的最终定义 通过以上宏的展开，BuiltinCombinedEarlyLintPass的定义实际为如下形式：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 pub struct BuiltinCombinedEarlyLintPass { UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, ... } impl BuiltinCombinedEarlyLintPass { pub fn new () -> Self { UnusedParens: UnusedParens, UnusedBraces: UnusedBraces, ... } pub fn get_lints () -> LintArray { let mut lints = Vec::new (); lints.extend_from_slice (&UnusedParens::get_lints ()); lints.extend_from_slice (&UnusedBraces::get_lints ()); ... lints } } impl EarlyLintPass for BuiltinCombinedEarlyLintPass { fn check_crates (&mut self , context: &EarlyContext<'_ >, a: &ast::Crate){ self .UnusedParens.check_crates (context, a); self .UnusedBraces.check_crates (context, a); ... } fn check_ident (&mut self , context: &EarlyContext<'_ >, a: Ident){ self .UnusedParens.check_ident (context, a); self .UnusedBraces.check_ident (context, a); ... } .. }
通过这个定义，可以在遍历 AST 时使用 BuiltinCombinedEarlyLintPass 的 check_* 方法实现多个 lintpass 的检查。
Lint 的进一步优化 基于 CombinedLintPass ，可以对上一篇文章中提出的 Linter 的设计做进一步优化。
这里，可以用 CombinedLintPass 的check_* 方法，在 Visitor 遍历 AST 时执行对应的检查。虽然效果与之前一致，但因为宏的关系，所有的 check_* 方法和需要执行的 lintpass 都被收集到了一个结构中，也更容易管理。同样的，因为 CombinedLintPass 实际上调用的是每个 lintpass 各自的 check 方法，虽然调用起来可能下图一样很复杂，但因为 lintpass 中定义的 check 方法大部分是由宏生成的空检查，所以也不会造成性能上的损失。
总结 本文简单介绍了 Rustc 源码中关于 CombinedLintPass 这一结构的定义和实现 ，并以此进一步优化 Linter 的设计。希望能够对理解 Rustc 及 Lint 有所帮助，如有错误，欢迎指正。后续的文章将继续介绍 Rustc 中 Lint 在编译过程中的注册和执行过程，期待继续关注。
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