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)

1 files changed, 20 insertions, 24 deletions

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]