Auto merge of #61105 - Centril:rollup-t9lemjf, r=Centril

Rollup of 6 pull requests

Successful merges:

 - #59545 (Use arenas to avoid Lrc in queries #2)
 - #61054 (Suggest dereferencing on assignment to mutable borrow)
 - #61056 (tweak discriminant on non-nullary enum diagnostic)
 - #61082 (fix dangling reference in Vec::append)
 - #61086 (Box::into_unique: do the reborrow-to-raw *after* destroying the Box)
 - #61098 (Fix overflowing literal lint in loops)

Failed merges:

r? @ghost

Author: bors

Cargo.lock

@@ -2877,6 +2877,7 @@ dependencies = [
  "rustc_errors 0.0.0",
  "rustc_target 0.0.0",
  "serialize 0.0.0",
+ "smallvec 0.6.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "stable_deref_trait 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "syntax 0.0.0",
  "syntax_ext 0.0.0",

src/liballoc/boxed.rs

@@ -253,15 +253,16 @@ impl<T: ?Sized> Box<T> {
     #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
     #[inline]
     #[doc(hidden)]
-    pub fn into_unique(mut b: Box<T>) -> Unique<T> {
+    pub fn into_unique(b: Box<T>) -> Unique<T> {
+        let mut unique = b.0;
+        mem::forget(b);
         // Box is kind-of a library type, but recognized as a "unique pointer" by
         // Stacked Borrows.  This function here corresponds to "reborrowing to
         // a raw pointer", but there is no actual reborrow here -- so
         // without some care, the pointer we are returning here still carries
-        // the `Uniq` tag.  We round-trip through a mutable reference to avoid that.
-        let unique = unsafe { b.0.as_mut() as *mut T };
-        mem::forget(b);
-        unsafe { Unique::new_unchecked(unique) }
+        // the tag of `b`, with `Unique` permission.
+        // We round-trip through a mutable reference to avoid that.
+        unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
     }
 
     /// Consumes and leaks the `Box`, returning a mutable reference,