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)

5 files changed, 93 insertions, 121 deletions

diff --git a/src/libcore/any.rs b/src/libcore/any.rs
index 0afbf4f1346..85b59162620 100644
--- a/src/libcore/any.rs
+++ b/src/libcore/any.rs
@@ -87,10 +87,8 @@ pub trait Any: 'static {
     ///     TypeId::of::<String>() == s.type_id()
     /// }
     ///
-    /// fn main() {
-    ///     assert_eq!(is_string(&0), false);
-    ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
-    /// }
+    /// assert_eq!(is_string(&0), false);
+    /// assert_eq!(is_string(&"cookie monster".to_string()), true);
     /// ```
     #[stable(feature = "get_type_id", since = "1.34.0")]
     fn type_id(&self) -> TypeId;
@@ -145,10 +143,8 @@ impl dyn Any {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     is_string(&0);
-    ///     is_string(&"cookie monster".to_string());
-    /// }
+    /// is_string(&0);
+    /// is_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -179,10 +175,8 @@ impl dyn Any {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     print_if_string(&0);
-    ///     print_if_string(&"cookie monster".to_string());
-    /// }
+    /// print_if_string(&0);
+    /// print_if_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -210,16 +204,14 @@ impl dyn Any {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let mut x = 10u32;
-    ///     let mut s = "starlord".to_string();
+    /// let mut x = 10u32;
+    /// let mut s = "starlord".to_string();
     ///
-    ///     modify_if_u32(&mut x);
-    ///     modify_if_u32(&mut s);
+    /// modify_if_u32(&mut x);
+    /// modify_if_u32(&mut s);
     ///
-    ///     assert_eq!(x, 42);
-    ///     assert_eq!(&s, "starlord");
-    /// }
+    /// assert_eq!(x, 42);
+    /// assert_eq!(&s, "starlord");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -250,10 +242,8 @@ impl dyn Any+Send {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     is_string(&0);
-    ///     is_string(&"cookie monster".to_string());
-    /// }
+    /// is_string(&0);
+    /// is_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -276,10 +266,8 @@ impl dyn Any+Send {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     print_if_string(&0);
-    ///     print_if_string(&"cookie monster".to_string());
-    /// }
+    /// print_if_string(&0);
+    /// print_if_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -300,16 +288,14 @@ impl dyn Any+Send {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let mut x = 10u32;
-    ///     let mut s = "starlord".to_string();
+    /// let mut x = 10u32;
+    /// let mut s = "starlord".to_string();
     ///
-    ///     modify_if_u32(&mut x);
-    ///     modify_if_u32(&mut s);
+    /// modify_if_u32(&mut x);
+    /// modify_if_u32(&mut s);
     ///
-    ///     assert_eq!(x, 42);
-    ///     assert_eq!(&s, "starlord");
-    /// }
+    /// assert_eq!(x, 42);
+    /// assert_eq!(&s, "starlord");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -334,10 +320,8 @@ impl dyn Any+Send+Sync {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     is_string(&0);
-    ///     is_string(&"cookie monster".to_string());
-    /// }
+    /// is_string(&0);
+    /// is_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
     #[inline]
@@ -360,10 +344,8 @@ impl dyn Any+Send+Sync {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     print_if_string(&0);
-    ///     print_if_string(&"cookie monster".to_string());
-    /// }
+    /// print_if_string(&0);
+    /// print_if_string(&"cookie monster".to_string());
     /// ```
     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
     #[inline]
@@ -384,16 +366,14 @@ impl dyn Any+Send+Sync {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     let mut x = 10u32;
-    ///     let mut s = "starlord".to_string();
+    /// let mut x = 10u32;
+    /// let mut s = "starlord".to_string();
     ///
-    ///     modify_if_u32(&mut x);
-    ///     modify_if_u32(&mut s);
+    /// modify_if_u32(&mut x);
+    /// modify_if_u32(&mut s);
     ///
-    ///     assert_eq!(x, 42);
-    ///     assert_eq!(&s, "starlord");
-    /// }
+    /// assert_eq!(x, 42);
+    /// assert_eq!(&s, "starlord");
     /// ```
     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
     #[inline]
@@ -437,10 +417,8 @@ impl TypeId {
     ///     TypeId::of::<String>() == TypeId::of::<T>()
     /// }
     ///
-    /// fn main() {
-    ///     assert_eq!(is_string(&0), false);
-    ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
-    /// }
+    /// assert_eq!(is_string(&0), false);
+    /// assert_eq!(is_string(&"cookie monster".to_string()), true);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature="const_type_id")]
diff --git a/src/libcore/char/convert.rs b/src/libcore/char/convert.rs
index 0a870c67518..c456e14db12 100644
--- a/src/libcore/char/convert.rs
+++ b/src/libcore/char/convert.rs
@@ -111,11 +111,9 @@ impl From<char> for u32 {
     /// ```
     /// use std::mem;
     ///
-    /// fn main() {
-    ///     let c = 'c';
-    ///     let u = u32::from(c);
-    ///     assert!(4 == mem::size_of_val(&u))
-    /// }
+    /// let c = 'c';
+    /// let u = u32::from(c);
+    /// assert!(4 == mem::size_of_val(&u))
     /// ```
     #[inline]
     fn from(c: char) -> Self {
@@ -150,11 +148,9 @@ impl From<u8> for char {
     /// ```
     /// use std::mem;
     ///
-    /// fn main() {
-    ///     let u = 32 as u8;
-    ///     let c = char::from(u);
-    ///     assert!(4 == mem::size_of_val(&c))
-    /// }
+    /// let u = 32 as u8;
+    /// let c = char::from(u);
+    /// assert!(4 == mem::size_of_val(&c))
     /// ```
     #[inline]
     fn from(i: u8) -> Self {
diff --git a/src/libcore/char/decode.rs b/src/libcore/char/decode.rs
index 23059243c61..b71c9c2c40b 100644
--- a/src/libcore/char/decode.rs
+++ b/src/libcore/char/decode.rs
@@ -31,21 +31,23 @@ pub struct DecodeUtf16Error {
 /// ```
 /// use std::char::decode_utf16;
 ///
-/// fn main() {
-///     // 𝄞mus<invalid>ic<invalid>
-///     let v = [0xD834, 0xDD1E, 0x006d, 0x0075,
-///              0x0073, 0xDD1E, 0x0069, 0x0063,
-///              0xD834];
+/// // 𝄞mus<invalid>ic<invalid>
+/// let v = [
+///     0xD834, 0xDD1E, 0x006d, 0x0075, 0x0073, 0xDD1E, 0x0069, 0x0063, 0xD834,
+/// ];
 ///
-///     assert_eq!(decode_utf16(v.iter().cloned())
-///                            .map(|r| r.map_err(|e| e.unpaired_surrogate()))
-///                            .collect::<Vec<_>>(),
-///                vec![Ok('𝄞'),
-///                     Ok('m'), Ok('u'), Ok('s'),
-///                     Err(0xDD1E),
-///                     Ok('i'), Ok('c'),
-///                     Err(0xD834)]);
-/// }
+/// assert_eq!(
+///     decode_utf16(v.iter().cloned())
+///         .map(|r| r.map_err(|e| e.unpaired_surrogate()))
+///         .collect::<Vec<_>>(),
+///     vec![
+///         Ok('𝄞'),
+///         Ok('m'), Ok('u'), Ok('s'),
+///         Err(0xDD1E),
+///         Ok('i'), Ok('c'),
+///         Err(0xD834)
+///     ]
+/// );
 /// ```
 ///
 /// A lossy decoder can be obtained by replacing `Err` results with the replacement character:
@@ -53,17 +55,17 @@ pub struct DecodeUtf16Error {
 /// ```
 /// use std::char::{decode_utf16, REPLACEMENT_CHARACTER};
 ///
-/// fn main() {
-///     // 𝄞mus<invalid>ic<invalid>
-///     let v = [0xD834, 0xDD1E, 0x006d, 0x0075,
-///              0x0073, 0xDD1E, 0x0069, 0x0063,
-///              0xD834];
+/// // 𝄞mus<invalid>ic<invalid>
+/// let v = [
+///     0xD834, 0xDD1E, 0x006d, 0x0075, 0x0073, 0xDD1E, 0x0069, 0x0063, 0xD834,
+/// ];
 ///
-///     assert_eq!(decode_utf16(v.iter().cloned())
-///                    .map(|r| r.unwrap_or(REPLACEMENT_CHARACTER))
-///                    .collect::<String>(),
-///                "𝄞mus�ic�");
-/// }
+/// assert_eq!(
+///     decode_utf16(v.iter().cloned())
+///        .map(|r| r.unwrap_or(REPLACEMENT_CHARACTER))
+///        .collect::<String>(),
+///     "𝄞mus�ic�"
+/// );
 /// ```
 #[stable(feature = "decode_utf16", since = "1.9.0")]
 #[inline]
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index aafa35873bb..5dfdd162306 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -1532,12 +1532,10 @@ impl<'a> Formatter<'a> {
     ///     }
     /// }
     ///
-    /// fn main() {
-    ///     assert_eq!(&format!("{:<}", Foo), "left");
-    ///     assert_eq!(&format!("{:>}", Foo), "right");
-    ///     assert_eq!(&format!("{:^}", Foo), "center");
-    ///     assert_eq!(&format!("{}", Foo), "into the void");
-    /// }
+    /// assert_eq!(&format!("{:<}", Foo), "left");
+    /// assert_eq!(&format!("{:>}", Foo), "right");
+    /// assert_eq!(&format!("{:^}", Foo), "center");
+    /// assert_eq!(&format!("{}", Foo), "into the void");
     /// ```
     #[stable(feature = "fmt_flags_align", since = "1.28.0")]
     pub fn align(&self) -> Option<Alignment> {
diff --git a/src/libcore/ptr/mod.rs b/src/libcore/ptr/mod.rs
index 13ccc9b252a..93391918595 100644
--- a/src/libcore/ptr/mod.rs
+++ b/src/libcore/ptr/mod.rs
@@ -2732,31 +2732,29 @@ impl<T: ?Sized> Eq for *mut T {}
 /// impl Trait for Wrapper {}
 /// impl Trait for i32 {}
 ///
-/// fn main() {
-///     let wrapper = Wrapper { member: 10 };
-///
-///     // Pointers have equal addresses.
-///     assert!(std::ptr::eq(
-///         &wrapper as *const Wrapper as *const u8,
-///         &wrapper.member as *const i32 as *const u8
-///     ));
-///
-///     // Objects have equal addresses, but `Trait` has different implementations.
-///     assert!(!std::ptr::eq(
-///         &wrapper as &dyn Trait,
-///         &wrapper.member as &dyn Trait,
-///     ));
-///     assert!(!std::ptr::eq(
-///         &wrapper as &dyn Trait as *const dyn Trait,
-///         &wrapper.member as &dyn Trait as *const dyn Trait,
-///     ));
-///
-///     // Converting the reference to a `*const u8` compares by address.
-///     assert!(std::ptr::eq(
-///         &wrapper as &dyn Trait as *const dyn Trait as *const u8,
-///         &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
-///     ));
-/// }
+/// let wrapper = Wrapper { member: 10 };
+///
+/// // Pointers have equal addresses.
+/// assert!(std::ptr::eq(
+///     &wrapper as *const Wrapper as *const u8,
+///     &wrapper.member as *const i32 as *const u8
+/// ));
+///
+/// // Objects have equal addresses, but `Trait` has different implementations.
+/// assert!(!std::ptr::eq(
+///     &wrapper as &dyn Trait,
+///     &wrapper.member as &dyn Trait,
+/// ));
+/// assert!(!std::ptr::eq(
+///     &wrapper as &dyn Trait as *const dyn Trait,
+///     &wrapper.member as &dyn Trait as *const dyn Trait,
+/// ));
+///
+/// // Converting the reference to a `*const u8` compares by address.
+/// assert!(std::ptr::eq(
+///     &wrapper as &dyn Trait as *const dyn Trait as *const u8,
+///     &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
+/// ));
 /// ```
 #[stable(feature = "ptr_eq", since = "1.17.0")]
 #[inline]