Implement lint for regex::Regex compilation inside a loop

CHANGELOG.md

@@ -5873,6 +5873,7 @@ Released 2018-09-13
 [`ref_in_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_in_deref
 [`ref_option_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_option_ref
 [`ref_patterns`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_patterns
+[`regex_compile_in_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#regex_compile_in_loop
 [`regex_macro`]: https://rust-lang.github.io/rust-clippy/master/index.html#regex_macro
 [`renamed_function_params`]: https://rust-lang.github.io/rust-clippy/master/index.html#renamed_function_params
 [`repeat_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#repeat_once

clippy_lints/src/declared_lints.rs

@@ -637,6 +637,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::ref_patterns::REF_PATTERNS_INFO,
     crate::reference::DEREF_ADDROF_INFO,
     crate::regex::INVALID_REGEX_INFO,
+    crate::regex::REGEX_COMPILE_IN_LOOP_INFO,
     crate::regex::TRIVIAL_REGEX_INFO,
     crate::repeat_vec_with_capacity::REPEAT_VEC_WITH_CAPACITY_INFO,
     crate::reserve_after_initialization::RESERVE_AFTER_INITIALIZATION_INFO,

clippy_lints/src/regex.rs

@@ -6,6 +6,7 @@ use clippy_utils::source::SpanRangeExt;
 use clippy_utils::{def_path_def_ids, path_def_id, paths};
 use rustc_ast::ast::{LitKind, StrStyle};
 use rustc_hir::def_id::DefIdMap;
+use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{BorrowKind, Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::impl_lint_pass;
@@ -55,6 +56,42 @@ declare_clippy_lint! {
     "trivial regular expressions"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for [regex](https://crates.io/crates/regex) compilation inside a loop with a literal.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// Compiling a regex is a much more expensive operation than using one, and a compiled regex can be used multiple times.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # let haystacks = [""];
+    /// for haystack in haystacks {
+    ///     let regex = regex::Regex::new(MY_REGEX);
+    ///     if regex.is_match(heystack) {
+    ///         // Perform operation
+    ///     }
+    /// }
+    /// ```
+    /// should be replaced with
+    /// ```no_run
+    /// # let haystacks = [""];
+    /// let regex = regex::Regex::new(MY_REGEX);
+    /// for haystack in haystacks {
+    ///     if regex.is_match(heystack) {
+    ///         // Perform operation
+    ///     }
+    /// }
+    /// ```
+    ///
+    #[clippy::version = "1.83.0"]
+    pub REGEX_COMPILE_IN_LOOP,
+    perf,
+    "regular expression compilation performed in a loop"
+}
+
 #[derive(Copy, Clone)]
 enum RegexKind {
     Unicode,
@@ -68,7 +105,7 @@ pub struct Regex {
     definitions: DefIdMap<RegexKind>,
 }
 
-impl_lint_pass!(Regex => [INVALID_REGEX, TRIVIAL_REGEX]);
+impl_lint_pass!(Regex => [INVALID_REGEX, TRIVIAL_REGEX, REGEX_COMPILE_IN_LOOP]);
 
 impl<'tcx> LateLintPass<'tcx> for Regex {
     fn check_crate(&mut self, cx: &LateContext<'tcx>) {
@@ -92,17 +129,69 @@ impl<'tcx> LateLintPass<'tcx> for Regex {
     }
 
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if let ExprKind::Call(fun, [arg]) = expr.kind
-            && let Some(def_id) = path_def_id(cx, fun)
-            && let Some(regex_kind) = self.definitions.get(&def_id)
-        {
+        if let Some((regex_kind, _, arg)) = extract_regex_call(&self.definitions, cx, expr) {
             match regex_kind {
                 RegexKind::Unicode => check_regex(cx, arg, true),
                 RegexKind::UnicodeSet => check_set(cx, arg, true),
                 RegexKind::Bytes => check_regex(cx, arg, false),
                 RegexKind::BytesSet => check_set(cx, arg, false),
             }
         }
+
+        if let ExprKind::Loop(block, _, _, span) = expr.kind {
+            let mut visitor = RegexCompVisitor {
+                cx,
+                loop_span: span,
+                definitions: &self.definitions,
+            };
+
+            visitor.visit_block(block);
+        }
+    }
+}
+
+struct RegexCompVisitor<'pass, 'tcx> {
+    definitions: &'pass DefIdMap<RegexKind>,
+    cx: &'pass LateContext<'tcx>,
+    loop_span: Span,
+}
+
+impl<'pass, 'tcx> Visitor<'tcx> for RegexCompVisitor<'pass, 'tcx> {
+    type NestedFilter = intravisit::nested_filter::None;
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        if let Some((_, fun, arg)) = extract_regex_call(self.definitions, self.cx, expr)
+            && (matches!(arg.kind, ExprKind::Lit(_)) || const_str(self.cx, arg).is_some())
+        {
+            span_lint_and_help(
+                self.cx,
+                REGEX_COMPILE_IN_LOOP,
+                fun.span,
+                "compiling a regex in a loop",
+                Some(self.loop_span),
+                "move the regex construction outside this loop",
+            );
+        }
+
+        // Avoid recursing into loops, as the LateLintPass::visit_expr will do this already.
+        if !matches!(expr.kind, ExprKind::Loop(..)) {
+            intravisit::walk_expr(self, expr);
+        }
+    }
+}
+
+fn extract_regex_call<'tcx>(
+    definitions: &DefIdMap<RegexKind>,
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+) -> Option<(RegexKind, &'tcx Expr<'tcx>, &'tcx Expr<'tcx>)> {
+    if let ExprKind::Call(fun, [arg]) = expr.kind
+        && let Some(def_id) = path_def_id(cx, fun)
+        && let Some(regex_kind) = definitions.get(&def_id)
+    {
+        Some((*regex_kind, fun, arg))
+    } else {
+        None
     }
 }
 

tests/ui/regex.rs

@@ -5,7 +5,7 @@
     clippy::needless_borrow,
     clippy::needless_borrows_for_generic_args
 )]
-#![warn(clippy::invalid_regex, clippy::trivial_regex)]
+#![warn(clippy::invalid_regex, clippy::trivial_regex, clippy::regex_compile_in_loop)]
 
 extern crate regex;
 
@@ -118,7 +118,31 @@ fn trivial_regex() {
     let _ = BRegex::new(r"\b{start}word\b{end}");
 }
 
+fn regex_compile_in_loop() {
+    loop {
+        let regex = Regex::new("a.b");
+        //~^ ERROR: compiling a regex in a loop
+        let regex = BRegex::new("a.b");
+        //~^ ERROR: compiling a regex in a loop
+
+        if true {
+            let regex = Regex::new("a.b");
+            //~^ ERROR: compiling a regex in a loop
+        }
+
+        for _ in 0..10 {
+            let nested_regex = Regex::new("a.b");
+            //~^ ERROR: compiling a regex in a loop
+        }
+    }
+
+    for i in 0..10 {
+        let dependant_regex = Regex::new(&format!("{i}"));
+    }
+}
+
 fn main() {
     syntax_error();
     trivial_regex();
+    regex_compile_in_loop();
 }

tests/ui/regex.stderr

@@ -195,5 +195,55 @@ LL |     let binary_trivial_empty = BRegex::new("^$");
    |
    = help: consider using `str::is_empty`
 
-error: aborting due to 24 previous errors
+error: compiling a regex in a loop
+  --> tests/ui/regex.rs:123:21
+   |
+LL |         let regex = Regex::new("a.b");
+   |                     ^^^^^^^^^^
+   |
+help: move the regex construction outside this loop
+  --> tests/ui/regex.rs:122:5
+   |
+LL |     loop {
+   |     ^^^^
+   = note: `-D clippy::regex-compile-in-loop` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::regex_compile_in_loop)]`
+
+error: compiling a regex in a loop
+  --> tests/ui/regex.rs:125:21
+   |
+LL |         let regex = BRegex::new("a.b");
+   |                     ^^^^^^^^^^^
+   |
+help: move the regex construction outside this loop
+  --> tests/ui/regex.rs:122:5
+   |
+LL |     loop {
+   |     ^^^^
+
+error: compiling a regex in a loop
+  --> tests/ui/regex.rs:129:25
+   |
+LL |             let regex = Regex::new("a.b");
+   |                         ^^^^^^^^^^
+   |
+help: move the regex construction outside this loop
+  --> tests/ui/regex.rs:122:5
+   |
+LL |     loop {
+   |     ^^^^
+
+error: compiling a regex in a loop
+  --> tests/ui/regex.rs:134:32
+   |
+LL |             let nested_regex = Regex::new("a.b");
+   |                                ^^^^^^^^^^
+   |
+help: move the regex construction outside this loop
+  --> tests/ui/regex.rs:133:9
+   |
+LL |         for _ in 0..10 {
+   |         ^^^^^^^^^^^^^^
+
+error: aborting due to 28 previous errors