rust: doctest: fix incorrect pattern in replacement

The `-> Result<(), impl core::fmt::Debug>` string is generated by rustdoc
and by adding "::" into the string it no longer finds anything, making
the line useless.

Remove the "::" in the pattern. Omit it in the replacement too, for
consistency with upstream rustdoc.

Fixes: de7cd3e ("rust: use absolute paths in macros referencing core and kernel")
Signed-off-by: Gary Guo <gary@garyguo.net>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260616132559.2245814-1-gary@kernel.org
[ Added link in code comment to `rustdoc`'s 1.87 PR that fully qualified
  it for context. Improved comments for consistency. Reworded to drop
  changelog and to fix typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

scripts/rustdoc_test_builder.rs

@@ -28,7 +28,7 @@ fn main() {
     //
     // ```
     // fn main() { #[allow(non_snake_case)] fn _doctest_main_rust_kernel_file_rs_28_0() {
-    // fn main() { #[allow(non_snake_case)] fn _doctest_main_rust_kernel_file_rs_37_0() -> Result<(), impl ::core::fmt::Debug> {
+    // fn main() { #[allow(non_snake_case)] fn _doctest_main_rust_kernel_file_rs_37_0() -> Result<(), impl core::fmt::Debug> {
     // ```
     //
     // It should be unlikely that doctest code matches such lines (when code is formatted properly).
@@ -47,12 +47,16 @@ fn main() {
         })
         .expect("No test function found in `rustdoc`'s output.");
 
-    // Qualify `Result` to avoid the collision with our own `Result` coming from the prelude.
+    // Replicate `rustdoc` 1.87+ behaviour [1] by fully qualifying `Result` to avoid the collision
+    // with our own `Result` coming from the prelude.
+    //
+    // [1]: https://github.com/rust-lang/rust/pull/137807
+    //
+    // TODO: Remove this when MSRV is bumped above 1.87.
     let body = body.replace(
-        &format!("{rustdoc_function_name}() -> Result<(), impl ::core::fmt::Debug> {{"),
-        &format!(
-            "{rustdoc_function_name}() -> ::core::result::Result<(), impl ::core::fmt::Debug> {{"
-        ),
+        &format!("{rustdoc_function_name}() -> Result<(), impl core::fmt::Debug> {{"),
+        // This intentionally does not use absolute paths to match `rustdoc` 1.87 behaviour.
+        &format!("{rustdoc_function_name}() -> core::result::Result<(), impl core::fmt::Debug> {{"),
     );
 
     // For tests that get generated with `Result`, like above, `rustdoc` generates an `unwrap()` on