Rollup merge of #127474 - tesuji:foldable-inline-derefs, r=t-rustdoc

doc: Make block of inline Deref methods foldable

After:
![image](https://github.com/rust-lang/rust/assets/15225902/3e8ab320-dbf7-436f-9be0-d0ef82664663)
Before:
![image](https://github.com/rust-lang/rust/assets/15225902/f6f7635d-d4c3-437e-a2d9-147726287b05)

Fix  #127470.

Current status:
- [x] Bug when hovering over title "Methods from ...": The anchor sign $ overlaps with `[-]`: https://github.com/rust-lang/rust/pull/127474#issuecomment-2222930038
    => Fixed by https://github.com/rust-lang/rust/pull/127474#issuecomment-2228886292

tests/rustdoc-gui/deref-block.goml

@@ -0,0 +1,30 @@
+// This test ensures that several clickable items actually have the pointer cursor.
+go-to: "file://" + |DOC_PATH| + "/lib2/struct.Derefer.html"
+
+assert-text: (".big-toggle summary", "Methods from Deref<Target = str>§")
+// We ensure it doesn't go over `§`.
+assert-css: (".big-toggle summary::before", {
+    "left": "-34px",
+    "top": "9px",
+})
+// It should NOT have the same X or Y position as the other toggles.
+compare-elements-position-false: (
+    ".big-toggle summary::before",
+    ".method-toggle summary::before",
+    ["x", "y"],
+)
+
+// We now check that if we're in mobile mode, it gets back to its original X position.
+set-window-size: (600, 600)
+assert-css: (".big-toggle summary::before", {
+    "left": "-11px",
+    "top": "9px",
+})
+// It should have the same X position as the other toggles.
+compare-elements-position: (".big-toggle summary::before", ".method-toggle summary::before", ["x"])
+// But still shouldn't have the same Y position.
+compare-elements-position-false: (
+    ".big-toggle summary::before",
+    ".method-toggle summary::before",
+    ["y"],
+)

tests/rustdoc-gui/src/lib2/lib.rs

@@ -356,3 +356,13 @@ pub mod scroll_traits {
         fn this_is_a_method_with_a_long_name_returning_something() -> String;
     }
 }
+
+pub struct Derefer(String);
+
+impl std::ops::Deref for Derefer {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}