More conservative impl of Replacer for closures

This is an alternative flavor of PR #1048:

Require `Replacer` closures to take a `&'a Captures<'b>` (for all `'a,
'b: 'a`) as argument and have a return type that must not depend on `'b`
(but only on `'a`).

src/regex/string.rs

@@ -2376,16 +2376,21 @@ mod replacer_closure {
     use super::*;
     /// If a closure implements this for all `'a`, then it also implements
     /// [`Replacer`].
+    // TODO: Use two lifetimes `'a` and `'b`: one for the reference and one for
+    // the lifetime argument `'b` of `Captures<'b>`. This requires a syntax
+    // like `for<'a, 'b: 'a> ReplacerClosure<'a, 'b> when using this trait,
+    // which currently doesn't exist.
+    // See also: https://github.com/rust-lang/rfcs/pull/3261
     pub trait ReplacerClosure<'a>
     where
-        Self: FnMut(&'a Captures<'a>) -> <Self as ReplacerClosure<'a>>::Output,
+        Self: FnMut(&'a Captures<'_>) -> <Self as ReplacerClosure<'a>>::Output,
     {
         /// Return type of the closure (may depend on lifetime `'a`).
         type Output: AsRef<str>;
     }
     impl<'a, F: ?Sized, O> ReplacerClosure<'a> for F
     where
-        F: FnMut(&'a Captures<'a>) -> O,
+        F: FnMut(&'a Captures<'_>) -> O,
         O: AsRef<str>,
     {
         type Output = O;
@@ -2430,7 +2435,8 @@ use replacer_closure::*;
 ///
 /// Closures that take an argument of type  `&'a Captures<'b>` for any `'a` and
 /// `'b: 'a` and which return a type `T: AsRef<str>` (that may depend on `'a`
-/// or `'b`) implement the `Replacer` trait through a [blanket implementation].
+/// but not on `'b`) implement the `Replacer` trait through a [blanket
+/// implementation].
 ///
 /// [blanket implementation]: Self#impl-Replacer-for-F
 ///
@@ -2580,7 +2586,7 @@ impl<'a> Replacer for &'a Cow<'a, str> {
 /// ```ignore
 /// impl<F, T> Replacer for F
 /// where
-///     F: for<'a> FnMut(&'a Captures<'a>) -> T,
+///     F: for<'a, 'b> FnMut(&'a Captures<'b>) -> T,
 ///     T: AsRef<str>, // `T` may also depend on `'a`, which cannot be expressed easily
 /// {
 ///     /* … */

tests/misc.rs

@@ -159,6 +159,12 @@ mod replacer_closure_lifetimes {
             .replace_all("x", coerce(|caps| Cow::Borrowed(&caps[0])));
         assert_eq!(s, "x");
     }
+    // The following test is commented out yet because currently `Replacer` is
+    // not implemented for closures whose return type depends on the lifetime
+    // parameter `'b` of the `Captures<'b>` type.
+    // TODO: Fix this when/if the hidden `ReplacerClosure` helper trait gets
+    // two lifetime parameters.
+    /*
     #[test]
     fn parameter_lifetime() {
         fn coerce<F: for<'b> FnMut(&Captures<'b>) -> Cow<'b, str>>(f: F) -> F {
@@ -170,20 +176,5 @@ mod replacer_closure_lifetimes {
         );
         assert_eq!(s, "x");
     }
-    // Additionally demand that its sufficient if the closure accepts a single
-    // lifetime `'u` which is used both for the reference to and the lifetime
-    // argument of the `Captures` argument. Note that `Captures<'u>` is
-    // covariant over `'u`.
-    #[test]
-    fn unified_lifetime() {
-        fn coerce<F: for<'u> FnMut(&'u Captures<'u>) -> Cow<'u, str>>(
-            f: F,
-        ) -> F {
-            f
-        }
-        let s = Regex::new("x")
-            .unwrap()
-            .replace_all("x", coerce(|caps| Cow::Borrowed(&caps[0])));
-        assert_eq!(s, "x");
-    }
+    */
 }