More generic impl of Replacer for closures

src/regex/string.rs

@@ -2371,6 +2371,28 @@ impl<'c, 'h> ExactSizeIterator for SubCaptureMatches<'c, 'h> {}
 
 impl<'c, 'h> core::iter::FusedIterator for SubCaptureMatches<'c, 'h> {}
 
+/// Contains helper trait for blanket implementation for [`Replacer`].
+mod replacer_closure {
+    use super::*;
+    /// If a closure implements this for all `&'a Captures<'b>`, then it also
+    /// implements [`Replacer`].
+    pub trait ReplacerClosure<Arg>
+    where
+        Self: FnMut(Arg) -> <Self as ReplacerClosure<Arg>>::Output,
+    {
+        /// Return type of the closure (may depend on lifetime `'a` or `'b`).
+        type Output: AsRef<str>;
+    }
+    impl<'a, 'b, F, O> ReplacerClosure<&'a Captures<'b>> for F
+    where
+        F: ?Sized + FnMut(&'a Captures<'b>) -> O,
+        O: AsRef<str>,
+    {
+        type Output = O;
+    }
+}
+use replacer_closure::*;
+
 /// A trait for types that can be used to replace matches in a haystack.
 ///
 /// In general, users of this crate shouldn't need to implement this trait,
@@ -2403,6 +2425,49 @@ impl<'c, 'h> core::iter::FusedIterator for SubCaptureMatches<'c, 'h> {}
 /// let result = re.replace("Springsteen, Bruce", NameSwapper);
 /// assert_eq!(result, "Bruce Springsteen");
 /// ```
+///
+/// # Implementation by closures
+///
+/// Closures that take an argument of type  `&'a Captures<'b>` (for all `'a`
+/// and `'b`) and which return a type `T: AsRef<str>` (that may depend on `'a`
+/// or `'b`) implement the `Replacer` trait through a [blanket implementation].
+///
+/// [blanket implementation]: Self#impl-Replacer-for-F
+///
+/// A simple example looks like this:
+///
+/// ```
+/// use regex::{Captures, Regex};
+///
+/// let re = Regex::new(r"[0-9]+").unwrap();
+/// let result = re.replace_all("1234,12345", |caps: &Captures<'_>| {
+///     format!("[number with {} digits]", caps[0].len())
+/// });
+/// assert_eq!(result, "[number with 4 digits],[number with 5 digits]");
+/// ```
+///
+/// The return type of the closure may depend on the lifetime of the reference
+/// that is passed as an argument to the closure. Using a function, this can be
+/// expressed:
+///
+/// ```
+/// use regex::{Captures, Regex};
+/// use std::borrow::Cow;
+///
+/// let re = Regex::new(r"[0-9]+").unwrap();
+/// fn func<'a, 'b>(caps: &'a Captures<'b>) -> Cow<'a, str> {
+///     if caps[0].len() % 2 == 1 {
+///         Cow::Owned(format!("0{}", &caps[0]))
+///     } else {
+///         Cow::Borrowed(&caps[0])
+///     }
+/// }
+/// let result = re.replace_all("1234,12345", func);
+/// assert_eq!(result, "1234,012345");
+/// ```
+///
+/// *Note:* Using a closure instead of a function in the last example can be
+/// more tricky and requires a coercing helper function as of yet.
 pub trait Replacer {
     /// Appends possibly empty data to `dst` to replace the current match.
     ///
@@ -2501,11 +2566,21 @@ impl<'a> Replacer for &'a Cow<'a, str> {
     }
 }
 
-impl<F, T> Replacer for F
-where
-    F: FnMut(&Captures<'_>) -> T,
-    T: AsRef<str>,
-{
+/// Blanket implementation of `Replacer` for closures.
+///
+/// This implementation is basically the following, except that the return type
+/// `T` may optionally depend on the lifetimes `'a` and `'b`.
+///
+/// ```ignore
+/// impl<F, T> Replacer for F
+/// where
+///     F: for<'a, 'b> FnMut(&'a Captures<'b>) -> T,
+///     T: AsRef<str>, // `T` may depend on `'a` or `'b`, which can't be expressed easily
+/// {
+///     /* … */
+/// }
+/// ```
+impl<F: for<'a, 'b> ReplacerClosure<&'a Captures<'b>>> Replacer for F {
     fn replace_append(&mut self, caps: &Captures<'_>, dst: &mut String) {
         dst.push_str((*self)(caps).as_ref());
     }

tests/misc.rs

@@ -141,3 +141,33 @@ fn dfa_handles_pathological_case() {
     };
     assert!(re.is_match(&text));
 }
+
+// Test if implementation of `Replacer` for closures covers any reasonable
+// lifetime combination in regard to the argument and return type.
+mod replacer_closure_lifetimes {
+    use regex::{Captures, Regex};
+    use std::borrow::Cow;
+    #[test]
+    fn reference_lifetime() {
+        fn coerce<F: for<'a> FnMut(&'a Captures<'_>) -> Cow<'a, str>>(
+            f: F,
+        ) -> F {
+            f
+        }
+        let s = Regex::new("x")
+            .unwrap()
+            .replace_all("x", coerce(|caps| Cow::Borrowed(&caps[0])));
+        assert_eq!(s, "x");
+    }
+    #[test]
+    fn parameter_lifetime() {
+        fn coerce<F: for<'b> FnMut(&Captures<'b>) -> Cow<'b, str>>(f: F) -> F {
+            f
+        }
+        let s = Regex::new("x").unwrap().replace_all(
+            "x",
+            coerce(|caps| Cow::Borrowed(caps.get(0).unwrap().as_str())),
+        );
+        assert_eq!(s, "x");
+    }
+}