Greatly speed up doctests by compiling compatible doctests in one file

library/core/src/panic/location.rs

@@ -44,7 +44,7 @@ impl<'a> Location<'a> {
     ///
     /// # Examples
     ///
-    /// ```
+    /// ```standalone
     /// use std::panic::Location;
     ///
     /// /// Returns the [`Location`] at which it is called.

library/test/src/types.rs

@@ -250,3 +250,37 @@ pub struct TestDescAndFn {
     pub desc: TestDesc,
     pub testfn: TestFn,
 }
+
+impl TestDescAndFn {
+    pub const fn new_doctest(
+        test_name: &'static str,
+        ignore: bool,
+        source_file: &'static str,
+        start_line: usize,
+        no_run: bool,
+        should_panic: bool,
+        testfn: TestFn,
+    ) -> Self {
+        Self {
+            desc: TestDesc {
+                name: StaticTestName(test_name),
+                ignore,
+                ignore_message: None,
+                source_file,
+                start_line,
+                start_col: 0,
+                end_line: 0,
+                end_col: 0,
+                compile_fail: false,
+                no_run,
+                should_panic: if should_panic {
+                    options::ShouldPanic::Yes
+                } else {
+                    options::ShouldPanic::No
+                },
+                test_type: TestType::DocTest,
+            },
+            testfn,
+        }
+    }
+}

src/doc/rustdoc/src/write-documentation/documentation-tests.md

@@ -376,6 +376,57 @@ that the code sample should be compiled using the respective edition of Rust.
 # fn foo() {}
 ```
 
+Starting in the 2024 edition[^edition-note], compatible doctests are merged as one before being
+run. We combine doctests for performance reasons: the slowest part of doctests is to compile them.
+Merging all of them into one file and compiling this new file, then running the doctests is much
+faster. Whether doctests are merged or not, they are run in their own process.
+
+An example of time spent when running doctests:
+
+[sysinfo crate](https://crates.io/crates/sysinfo):
+
+```text
+wall-time duration: 4.59s
+total compile time: 27.067s
+total runtime: 3.969s
+```
+
+Rust core library:
+
+```text
+wall-time duration: 102s
+total compile time: 775.204s
+total runtime: 15.487s
+```
+
+[^edition-note]: This is based on the edition of the whole crate, not the edition of the individual
+test case that may be specified in its code attribute.
+
+In some cases, doctests cannot be merged. For example, if you have:
+
+```rust
+//! ```
+//! let location = std::panic::Location::caller();
+//! assert_eq!(location.line(), 4);
+//! ```
+```
+
+The problem with this code is that, if you change any other doctests, it'll likely break when
+runing `rustdoc --test`, making it tricky to maintain.
+
+This is where the `standalone` attribute comes in: it tells `rustdoc` that a doctest
+should not be merged with the others. So the previous code should use it:
+
+```rust
+//! ```standalone
+//! let location = std::panic::Location::caller();
+//! assert_eq!(location.line(), 4);
+//! ```
+```
+
+In this case, it means that the line information will not change if you add/remove other
+doctests.
+
 ### Custom CSS classes for code blocks
 
 ```rust