rust: doctest: fix incorrect pattern in replacement

index f7540bcf595a..5457679c12f9 100644
@@ -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 +47,14 @@ 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 by fully qualify `Result` (to avoid the collision with our
+    // own `Result` coming from the prelude).
+    //
+    // 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

index 5457679c12f9..79cef75f5af3 100644
@@ -70,6 +70,11 @@ fn main() {
     // Figure out a smaller test name based on the generated function name.
     let name = rustdoc_function_name.split_once("_rust_kernel_").unwrap().1;
 
+    // The rustdoc function name can include the absolute path when building with `O=` which is
+    // undesireable and create overlong symbol names. Remap it to use relative path.
+    let trimmed_function_name = format!("_doctest_main_rust_kernel_{name}");
+    let body = body.replace(&rustdoc_function_name, &trimmed_function_name);
+
     let path = format!("rust/test/doctests/kernel/{name}");
 
     std::fs::write(path, body.as_bytes()).unwrap();