Rollup merge of #64912 - lzutao:unneeded-main-doc, r=jonas-schievink

Remove unneeded `fn main` blocks from docs

## [No whitespace diff](https://github.com/rust-lang/rust/pull/64912/files?w=1)

8 files changed, 54 insertions, 85 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index c61e3183409..b2789a535fe 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -29,10 +29,8 @@
 //!     Nil,
 //! }
 //!
-//! fn main() {
-//!     let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
-//!     println!("{:?}", list);
-//! }
+//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
+//! println!("{:?}", list);
 //! ```
 //!
 //! This will print `Cons(1, Cons(2, Nil))`.
@@ -375,14 +373,12 @@ impl<T: ?Sized> Box<T> {
     /// ```
     /// #![feature(box_into_raw_non_null)]
     ///
-    /// fn main() {
-    ///     let x = Box::new(5);
-    ///     let ptr = Box::into_raw_non_null(x);
+    /// let x = Box::new(5);
+    /// let ptr = Box::into_raw_non_null(x);
     ///
-    ///     // Clean up the memory by converting the NonNull pointer back
-    ///     // into a Box and letting the Box be dropped.
-    ///     let x = unsafe { Box::from_raw(ptr.as_ptr()) };
-    /// }
+    /// // Clean up the memory by converting the NonNull pointer back
+    /// // into a Box and letting the Box be dropped.
+    /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
     /// ```
     #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
     #[inline]
@@ -428,23 +424,19 @@ impl<T: ?Sized> Box<T> {
     /// Simple usage:
     ///
     /// ```
-    /// fn main() {
-    ///     let x = Box::new(41);
-    ///     let static_ref: &'static mut usize = Box::leak(x);
-    ///     *static_ref += 1;
-    ///     assert_eq!(*static_ref, 42);
-    /// }
+    /// let x = Box::new(41);
+    /// let static_ref: &'static mut usize = Box::leak(x);
+    /// *static_ref += 1;
+    /// assert_eq!(*static_ref, 42);
     /// ```
     ///
     /// Unsized data:
     ///
     /// ```
-    /// fn main() {
-    ///     let x = vec![1, 2, 3].into_boxed_slice();
-    ///     let static_ref = Box::leak(x);
-    ///     static_ref[0] = 4;
-    ///     assert_eq!(*static_ref, [4, 2, 3]);
-    /// }
+    /// let x = vec![1, 2, 3].into_boxed_slice();
+    /// let static_ref = Box::leak(x);
+    /// static_ref[0] = 4;
+    /// assert_eq!(*static_ref, [4, 2, 3]);
     /// ```
     #[stable(feature = "box_leak", since = "1.26.0")]
     #[inline]
@@ -780,11 +772,9 @@ impl Box<dyn Any> {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let my_string = "Hello World".to_string();
-    ///     print_if_string(Box::new(my_string));
-    ///     print_if_string(Box::new(0i8));
-    /// }
+    /// let my_string = "Hello World".to_string();
+    /// print_if_string(Box::new(my_string));
+    /// print_if_string(Box::new(0i8));
     /// ```
     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
         if self.is::<T>() {
@@ -814,11 +804,9 @@ impl Box<dyn Any + Send> {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let my_string = "Hello World".to_string();
-    ///     print_if_string(Box::new(my_string));
-    ///     print_if_string(Box::new(0i8));
-    /// }
+    /// let my_string = "Hello World".to_string();
+    /// print_if_string(Box::new(my_string));
+    /// print_if_string(Box::new(0i8));
     /// ```
     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
         <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index ddf012d1502..83fd4485f73 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -2226,14 +2226,12 @@ impl<'a, K: Ord, V: Default> Entry<'a, K, V> {
     /// # Examples
     ///
     /// ```
-    /// # fn main() {
     /// use std::collections::BTreeMap;
     ///
     /// let mut map: BTreeMap<&str, Option<usize>> = BTreeMap::new();
     /// map.entry("poneyland").or_default();
     ///
     /// assert_eq!(map["poneyland"], None);
-    /// # }
     /// ```
     pub fn or_default(self) -> &'a mut V {
         match self {
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index f234ac5ebe5..a28c6d22abb 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -861,11 +861,9 @@ impl Rc<dyn Any> {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let my_string = "Hello World".to_string();
-    ///     print_if_string(Rc::new(my_string));
-    ///     print_if_string(Rc::new(0i8));
-    /// }
+    /// let my_string = "Hello World".to_string();
+    /// print_if_string(Rc::new(my_string));
+    /// print_if_string(Rc::new(0i8));
     /// ```
     pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
         if (*self).is::<T>() {
diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 881d499c074..4e4a285c21d 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -412,20 +412,15 @@ impl<T> [T] {
     ///
     /// ```
     /// #![feature(repeat_generic_slice)]
-    ///
-    /// fn main() {
-    ///     assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
-    /// }
+    /// assert_eq!([1, 2].repeat(3), vec![1, 2, 1, 2, 1, 2]);
     /// ```
     ///
     /// A panic upon overflow:
     ///
     /// ```should_panic
     /// #![feature(repeat_generic_slice)]
-    /// fn main() {
-    ///     // this will panic at runtime
-    ///     b"0123456789abcdef".repeat(usize::max_value());
-    /// }
+    /// // this will panic at runtime
+    /// b"0123456789abcdef".repeat(usize::max_value());
     /// ```
     #[unstable(feature = "repeat_generic_slice",
                reason = "it's on str, why not on slice?",
diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index 9a1342c30d5..9231c2d3f1d 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -500,10 +500,8 @@ impl str {
     /// A panic upon overflow:
     ///
     /// ```should_panic
-    /// fn main() {
-    ///     // this will panic at runtime
-    ///     "0123456789abcdef".repeat(usize::max_value());
-    /// }
+    /// // this will panic at runtime
+    /// "0123456789abcdef".repeat(usize::max_value());
     /// ```
     #[stable(feature = "repeat_str", since = "1.16.0")]
     pub fn repeat(&self, n: usize) -> String {
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index abe50fdb7a3..639124e26cc 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -164,10 +164,8 @@ use crate::vec::Vec;
 ///
 /// fn example_func<A: TraitExample>(example_arg: A) {}
 ///
-/// fn main() {
-///     let example_string = String::from("example_string");
-///     example_func(&example_string);
-/// }
+/// let example_string = String::from("example_string");
+/// example_func(&example_string);
 /// ```
 ///
 /// There are two options that would work instead. The first would be to
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 45f98162e4c..5977e69b7fa 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -1244,11 +1244,9 @@ impl Arc<dyn Any + Send + Sync> {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let my_string = "Hello World".to_string();
-    ///     print_if_string(Arc::new(my_string));
-    ///     print_if_string(Arc::new(0i8));
-    /// }
+    /// let my_string = "Hello World".to_string();
+    /// print_if_string(Arc::new(my_string));
+    /// print_if_string(Arc::new(0i8));
     /// ```
     pub fn downcast<T>(self) -> Result<Arc<T>, Self>
     where
diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 7c6ded08bba..6350b189c5f 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -389,28 +389,26 @@ impl<T> Vec<T> {
     /// use std::ptr;
     /// use std::mem;
     ///
-    /// fn main() {
-    ///     let mut v = vec![1, 2, 3];
-    ///
-    ///     // Pull out the various important pieces of information about `v`
-    ///     let p = v.as_mut_ptr();
-    ///     let len = v.len();
-    ///     let cap = v.capacity();
+    /// let mut v = vec![1, 2, 3];
     ///
-    ///     unsafe {
-    ///         // Cast `v` into the void: no destructor run, so we are in
-    ///         // complete control of the allocation to which `p` points.
-    ///         mem::forget(v);
+    /// // Pull out the various important pieces of information about `v`
+    /// let p = v.as_mut_ptr();
+    /// let len = v.len();
+    /// let cap = v.capacity();
     ///
-    ///         // Overwrite memory with 4, 5, 6
-    ///         for i in 0..len as isize {
-    ///             ptr::write(p.offset(i), 4 + i);
-    ///         }
+    /// unsafe {
+    ///     // Cast `v` into the void: no destructor run, so we are in
+    ///     // complete control of the allocation to which `p` points.
+    ///     mem::forget(v);
     ///
-    ///         // Put everything back together into a Vec
-    ///         let rebuilt = Vec::from_raw_parts(p, len, cap);
-    ///         assert_eq!(rebuilt, [4, 5, 6]);
+    ///     // Overwrite memory with 4, 5, 6
+    ///     for i in 0..len as isize {
+    ///         ptr::write(p.offset(i), 4 + i);
     ///     }
+    ///
+    ///     // Put everything back together into a Vec
+    ///     let rebuilt = Vec::from_raw_parts(p, len, cap);
+    ///     assert_eq!(rebuilt, [4, 5, 6]);
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1391,12 +1389,10 @@ impl<T> Vec<T> {
     /// ```
     /// #![feature(vec_leak)]
     ///
-    /// fn main() {
-    ///     let x = vec![1, 2, 3];
-    ///     let static_ref: &'static mut [usize] = Vec::leak(x);
-    ///     static_ref[0] += 1;
-    ///     assert_eq!(static_ref, &[2, 2, 3]);
-    /// }
+    /// let x = vec![1, 2, 3];
+    /// let static_ref: &'static mut [usize] = Vec::leak(x);
+    /// static_ref[0] += 1;
+    /// assert_eq!(static_ref, &[2, 2, 3]);
     /// ```
     #[unstable(feature = "vec_leak", issue = "62195")]
     #[inline]