Rollup merge of #152587 - lqd:tiny-things, r=jackh726

A couple of tiny polonius things

Here's a couple of tiny things I had ready to go @jackh726
- a tiny cleanup to avoid round-tripping through `Location`s to check for liveness, since we're already working with points
- while I was there, I fixed the doc which wasn't showing up properly (maybe a rustdoc or bootstrap bug when we build locally or during dist, either way, both don't show up correctly linked most of the time) for `PointIndex`es.
- and in the second commit slightly expand test coverage with [an example](https://internals.rust-lang.org/t/get-mut-map-back-from-entry-api/24003) that would have needed stdlib changes (cc author @Darksonn for visibility here)

More soon.

r? @jackh726

compiler/rustc_borrowck/src/polonius/mod.rs

@@ -182,8 +182,7 @@ fn on_node_traversed(&mut self, loan: BorrowIndex, node: LocalizedNode) {
         //
         // FIXME: analyze potential unsoundness, possibly in concert with a borrowck
         // implementation in a-mir-formality, fuzzing, or manually crafting counter-examples.
-        let location = self.liveness.location_from_point(node.point);
-        if self.liveness.is_live_at(node.region, location) {
+        if self.liveness.is_live_at_point(node.region, node.point) {
             self.live_loans.insert(node.point, loan);
         }
     }

compiler/rustc_borrowck/src/region_infer/values.rs

@@ -131,9 +131,17 @@ pub(crate) fn add_all_points(&mut self, region: RegionVid) {
         }
     }
 
-    /// Returns whether `region` is marked live at the given `location`.
+    /// Returns whether `region` is marked live at the given
+    /// [`location`][rustc_middle::mir::Location].
     pub(crate) fn is_live_at(&self, region: RegionVid, location: Location) -> bool {
         let point = self.location_map.point_from_location(location);
+        self.is_live_at_point(region, point)
+    }
+
+    /// Returns whether `region` is marked live at the given
+    /// [`point`][rustc_mir_dataflow::points::PointIndex].
+    #[inline]
+    pub(crate) fn is_live_at_point(&self, region: RegionVid, point: PointIndex) -> bool {
         if let Some(points) = &self.points {
             points.row(region).is_some_and(|r| r.contains(point))
         } else {

tests/ui/nll/polonius/nll-problem-case-3-issue-21906.nll.stderr

@@ -35,8 +35,21 @@ LL | |         }
 LL | |     }
    | |_____- returning this value requires that `*map` is borrowed for `'r`
 
+error[E0499]: cannot borrow `*map` as mutable more than once at a time
+  --> $DIR/nll-problem-case-3-issue-21906.rs:45:41
+   |
+LL | fn get_priority_mut_entry<'a, K, V>(
+   |                           -- lifetime `'a` defined here
+...
+LL |     match map.entry(key1) {
+   |           --- first mutable borrow occurs here
+LL |         Entry::Occupied(occupied) => Some(occupied.into_mut()),
+   |                                      ------------------------- returning this value requires that `*map` is borrowed for `'a`
+LL |         Entry::Vacant(_vacant) => match map.entry(key2) {
+   |                                         ^^^ second mutable borrow occurs here
+
 error[E0499]: cannot borrow `*self` as mutable more than once at a time
-  --> $DIR/nll-problem-case-3-issue-21906.rs:44:21
+  --> $DIR/nll-problem-case-3-issue-21906.rs:64:21
    |
 LL |     fn two(&mut self) -> &i32 {
    |            - let's call the lifetime of this reference `'1`
@@ -48,7 +61,7 @@ LL |                 return k;
    |                        - returning this value requires that `*self` is borrowed for `'1`
 
 error[E0502]: cannot borrow `x.data` as immutable because it is also borrowed as mutable
-  --> $DIR/nll-problem-case-3-issue-21906.rs:62:22
+  --> $DIR/nll-problem-case-3-issue-21906.rs:82:22
    |
 LL | fn foo(x: &mut Foo) -> Option<&mut i32> {
    |           - let's call the lifetime of this reference `'1`
@@ -61,7 +74,7 @@ LL |     println!("{:?}", x.data);
    |                      ^^^^^^ immutable borrow occurs here
 
 error[E0499]: cannot borrow `*vec` as mutable more than once at a time
-  --> $DIR/nll-problem-case-3-issue-21906.rs:77:9
+  --> $DIR/nll-problem-case-3-issue-21906.rs:97:9
    |
 LL | fn f(vec: &mut Vec<u8>) -> &u8 {
    |           - let's call the lifetime of this reference `'1`
@@ -75,7 +88,7 @@ LL |         vec.push(10);
    |         ^^^ second mutable borrow occurs here
 
 error[E0502]: cannot borrow `*vec` as immutable because it is also borrowed as mutable
-  --> $DIR/nll-problem-case-3-issue-21906.rs:78:9
+  --> $DIR/nll-problem-case-3-issue-21906.rs:98:9
    |
 LL | fn f(vec: &mut Vec<u8>) -> &u8 {
    |           - let's call the lifetime of this reference `'1`
@@ -88,7 +101,7 @@ LL |         n
 LL |         vec.last().unwrap()
    |         ^^^ immutable borrow occurs here
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 
 Some errors have detailed explanations: E0499, E0502.
 For more information about an error, try `rustc --explain E0499`.

tests/ui/nll/polonius/nll-problem-case-3-issue-21906.rs

@@ -29,6 +29,26 @@ fn from_the_rfc<'r, K: Hash + Eq + Copy, V: Default>(
     }
 }
 
+// A variant that's similar to the RFC example above, but using the entry API, and requested in
+// https://internals.rust-lang.org/t/get-mut-map-back-from-entry-api/24003
+fn get_priority_mut_entry<'a, K, V>(
+    map: &'a mut HashMap<K, V>,
+    key1: K,
+    key2: K,
+) -> Option<&'a mut V>
+where
+    K: Eq + Hash,
+{
+    use std::collections::hash_map::Entry;
+    match map.entry(key1) {
+        Entry::Occupied(occupied) => Some(occupied.into_mut()),
+        Entry::Vacant(_vacant) => match map.entry(key2) {
+            Entry::Occupied(occupied2) => Some(occupied2.into_mut()),
+            Entry::Vacant(_) => None,
+        },
+    }
+}
+
 // MCVE 1 from issue #21906
 struct A {
     a: i32,