rustdoc: Add unstable --merge-doctests=yes/no/auto flag

src/librustdoc/doctest/make.rs

@@ -156,21 +156,22 @@ impl<'a> BuildDocTestBuilder<'a> {
 
         // Up until now, we've been dealing with settings for the whole crate.
         // Now, infer settings for this particular test.
+        //
+        // Avoid tests with incompatible attributes.
+        let opt_out = lang_str.is_some_and(|lang_str| {
+            lang_str.compile_fail || lang_str.test_harness || lang_str.standalone_crate
+        });
         let can_be_merged = if can_merge_doctests == MergeDoctests::Auto {
-            let mut can_merge = false;
-            // Avoid tests with incompatible attributes.
-            can_merge |= lang_str.is_some_and(|lang_str| {
-                !lang_str.compile_fail && !lang_str.test_harness && !lang_str.standalone_crate
-            });
-            // If it contains `#[feature]` or `#[no_std]`, we don't want it to be merged either.
-            can_merge &= !has_global_allocator
-                && crate_attrs.is_empty()
+            // We try to look at the contents of the test to detect whether it should be merged.
+            // This is not a complete list of possible failures, but it catches many cases.
+            let will_probably_fail = has_global_allocator
+                || !crate_attrs.is_empty()
                 // If this is a merged doctest and a defined macro uses `$crate`, then the path will
                 // not work, so better not put it into merged doctests.
-                && !(has_macro_def && everything_else.contains("$crate"));
-            can_merge
+                || (has_macro_def && everything_else.contains("$crate"));
+            !opt_out && !will_probably_fail
         } else {
-            can_merge_doctests != MergeDoctests::Never
+            can_merge_doctests != MergeDoctests::Never && !opt_out
         };
         DocTestBuilder {
             supports_color,

tests/rustdoc-ui/doctest/force-merge-default-not-override.rs

@@ -0,0 +1,25 @@
+//@ check-pass
+//@ edition: 2024
+//@ compile-flags: --test --test-args=--test-threads=1 --merge-doctests=yes -Z unstable-options
+//@ normalize-stdout: "tests/rustdoc-ui/doctest" -> "$$DIR"
+//@ normalize-stdout: "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ normalize-stdout: "ran in \d+\.\d+s" -> "ran in $$TIME"
+//@ normalize-stdout: "compilation took \d+\.\d+s" -> "compilation took $$TIME"
+//@ normalize-stdout: ".rs:\d+:\d+" -> ".rs:$$LINE:$$COL"
+
+// FIXME: compiletest doesn't support `// RAW` for doctests because the progress messages aren't
+// emitted as JSON. Instead the .stderr file tests that this doesn't contains a
+// "merged compilation took ..." message.
+
+/// ```standalone_crate
+/// let x = 12;
+/// ```
+///
+/// These two doctests should be not be merged, even though this passes `--merge-doctests=yes`.
+///
+/// ```standalone_crate
+/// fn main() {
+///     println!("owo");
+/// }
+/// ```
+pub struct Foo;

tests/rustdoc-ui/doctest/force-merge-default-not-override.stdout

@@ -0,0 +1,7 @@
+
+running 2 tests
+test $DIR/force-merge-default-not-override.rs - Foo (line 14) ... ok
+test $DIR/force-merge-default-not-override.rs - Foo (line 20) ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+