rustc: use more correct span data in for loop desugaring

Before:

      help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
         |
      LL |     for x in DroppingSlice(&*v).iter(); {
         |                                       +

After:

      help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
         |
      LL |     };
         |      +

This seems like a reasonable fix: since the desugared "expr_drop_temps_mut"
contains the entire desugared loop construct, its span should contain the
entire loop construct as well.

compiler/rustc_ast_lowering/src/expr.rs

@@ -1401,8 +1401,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
             )
         };
 
+        // #82462: to correctly diagnose borrow errors, the block that contains
+        // the iter expr needs to have a span that covers the loop body.
+        let desugared_full_span =
+            self.mark_span_with_reason(DesugaringKind::ForLoop(ForLoopLoc::Head), e.span, None);
+
         let match_expr = self.arena.alloc(self.expr_match(
-            desugared_span,
+            desugared_full_span,
             into_iter_expr,
             arena_vec![self; iter_arm],
             hir::MatchSource::ForLoopDesugar,
@@ -1416,7 +1421,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // surrounding scope of the `match` since the `match` is not a terminating scope.
         //
         // Also, add the attributes to the outer returned expr node.
-        self.expr_drop_temps_mut(desugared_span, match_expr, attrs.into())
+        self.expr_drop_temps_mut(desugared_full_span, match_expr, attrs.into())
     }
 
     /// Desugar `ExprKind::Try` from: `<expr>?` into:

src/test/mir-opt/remove_storage_markers.main.RemoveStorageMarkers.diff

@@ -80,7 +80,7 @@
 -         StorageDead(_7);                 // scope 3 at $DIR/remove_storage_markers.rs:8:18: 8:19
 -         StorageDead(_6);                 // scope 2 at $DIR/remove_storage_markers.rs:10:5: 10:6
 -         StorageDead(_4);                 // scope 1 at $DIR/remove_storage_markers.rs:10:5: 10:6
--         StorageDead(_2);                 // scope 1 at $DIR/remove_storage_markers.rs:8:18: 8:19
+-         StorageDead(_2);                 // scope 1 at $DIR/remove_storage_markers.rs:10:5: 10:6
 -         StorageDead(_1);                 // scope 0 at $DIR/remove_storage_markers.rs:11:1: 11:2
           return;                          // scope 0 at $DIR/remove_storage_markers.rs:11:2: 11:2
       }

src/test/ui/borrowck/issue-82462.nll.stderr

@@ -0,0 +1,22 @@
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/issue-82462.rs:18:9
+   |
+LL |     for x in DroppingSlice(&*v).iter() {
+   |              ------------------
+   |              |               |
+   |              |               immutable borrow occurs here
+   |              a temporary with access to the immutable borrow is created here ...
+LL |         v.push(*x);
+   |         ^ mutable borrow occurs here
+LL |         break;
+LL |     }
+   |     - ... and the immutable borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DroppingSlice`
+   |
+help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
+   |
+LL |     };
+   |      +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/issue-82462.rs

@@ -0,0 +1,21 @@
+struct DroppingSlice<'a>(&'a [i32]);
+
+impl Drop for DroppingSlice<'_> {
+    fn drop(&mut self) {
+        println!("hi from slice");
+    }
+}
+
+impl DroppingSlice<'_> {
+    fn iter(&self) -> std::slice::Iter<'_, i32> {
+        self.0.iter()
+    }
+}
+
+fn main() {
+    let mut v = vec![1, 2, 3, 4];
+    for x in DroppingSlice(&*v).iter() {
+        v.push(*x); //~ERROR
+        break;
+    }
+}

src/test/ui/borrowck/issue-82462.stderr

@@ -0,0 +1,22 @@
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/issue-82462.rs:18:9
+   |
+LL |     for x in DroppingSlice(&*v).iter() {
+   |              ------------------
+   |              |               |
+   |              |               immutable borrow occurs here
+   |              a temporary with access to the immutable borrow is created here ...
+LL |         v.push(*x);
+   |         ^^^^^^^^^^ mutable borrow occurs here
+LL |         break;
+LL |     }
+   |     - ... and the immutable borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DroppingSlice`
+   |
+help: consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped
+   |
+LL |     };
+   |      +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.