Auto merge of #60377 - Centril:rollup-42fxe9u, r=Centril

Rollup of 9 pull requests

Successful merges:

 - #59946 (Fix equivalent string in escape_default docs)
 - #60256 (Option::flatten)
 - #60305 (hir: remove LoweredNodeId)
 - #60334 (Stabilized vectored IO)
 - #60353 (Add test not to forget resolved ICE)
 - #60356 (Stabilize str::as_mut_ptr)
 - #60358 (Clarify the short explanation of E0207)
 - #60359 (resolve: Consider erroneous imports used to avoid duplicate diagnostics)
 - #60360 (Add test case for labeled break in const assignment)

Failed merges:

r? @ghost

src/libcore/option.rs

@@ -136,7 +136,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
-use crate::{hint, mem, ops::{self, Deref}};
+use crate::{convert, hint, mem, ops::{self, Deref}};
 use crate::pin::Pin;
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
@@ -1413,3 +1413,33 @@ impl<T> ops::Try for Option<T> {
         None
     }
 }
+
+impl<T> Option<Option<T>> {
+    /// Converts from `Option<Option<T>>` to `Option<T>`
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// #![feature(option_flattening)]
+    /// let x: Option<Option<u32>> = Some(Some(6));
+    /// assert_eq!(Some(6), x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = Some(None);
+    /// assert_eq!(None, x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = None;
+    /// assert_eq!(None, x.flatten());
+    /// ```
+    /// Flattening once only removes one level of nesting:
+    /// ```
+    /// #![feature(option_flattening)]
+    /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
+    /// assert_eq!(Some(Some(6)), x.flatten());
+    /// assert_eq!(Some(6), x.flatten().flatten());
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_flattening", issue = "60258")]
+    pub fn flatten(self) -> Option<T> {
+        self.and_then(convert::identity)
+    }
+}

src/libcore/str/mod.rs

@@ -2214,7 +2214,7 @@ impl str {
     /// modified in a way that it remains valid UTF-8.
     ///
     /// [`u8`]: primitive.u8.html
-    #[unstable(feature = "str_as_mut_ptr", issue = "58215")]
+    #[stable(feature = "str_as_mut_ptr", since = "1.36.0")]
     #[inline]
     pub fn as_mut_ptr(&mut self) -> *mut u8 {
         self as *mut str as *mut u8
@@ -4061,7 +4061,7 @@ impl str {
     /// Both are equivalent to:
     ///
     /// ```
-    /// println!("\\u{{2764}}\n!");
+    /// println!("\\u{{2764}}\\n!");
     /// ```
     ///
     /// Using `to_string`: