Auto merge of rust-lang#126245 - GuillaumeGomez:doctest-improvement-v2, r=t-rustdoc

Greatly speed up doctests by compiling compatible doctests in one file

Fixes rust-lang#75341.

Take 2 at rust-lang#123974. It should now be much simpler to review since it's based on rust-lang#125798.

I split the changes as much as possible to make it as easy as possible to review (even though it's a huge lot of code changes...).

The following tests are not included into the combined doctests:
 * `compile_fail`
 * If there are crate attributes (`deny`/`allow`/`warn` are ok)
 * have invalid AST
 * `test_harness`
 * no capture
 * `--show-output` (because the output is nicer without the extra code surrounding it)

Everything else is merged into one file. If this one file fails to compile, we go back to the current strategy: compile each doctest separately.

Because of the `edition` codeblock attribute, I couldn't make them all into one file directly so I grouped them by edition, but it should be pretty anecdotic.

In case the users want a specific doctest to be opted-out from this doctest merge, I added the `standalone` codeblock attribute:

```rust
/// ```rust,standalone
/// // This doctest will be run in its own process!
/// ```
```

Now the interesting part, I ran it on a few crates and here are the results (with `cargo test --doc` to only include doctests):

| crate | nb doctests | before this PR | with this PR |
| - | - | - | - |
| sysinfo | 227 | 4.6s | 1.11s |
| geos | 157 | 3.95s | 0.45s |
| core | 4604 | 54.08s | 13.5s (merged: 0.9s, standalone: 12.6s) |
| std | 1147 | 12s | 3.56s (merged: 2.1s, standalone: 1.46s) |

r? `@camelid`

try-job: x86_64-msvc
try-job: aarch64-apple

Author: bors

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