Note when a a move/borrow error is caused by a deref coercion

Fixes #73268

When a deref coercion occurs, we may end up with a move error if the
base value has been partially moved out of. However, we do not indicate
anywhere that a deref coercion is occuring, resulting in an error
message with a confusing span.

This PR adds an explicit note to move errors when a deref coercion is
involved. We mention the name of the type that the deref-coercion
resolved to, as well as the `Deref::Target` associated type being used.

5 files changed, 87 insertions, 5 deletions

diff --git a/src/test/ui/lint/lint-unconditional-recursion.stderr b/src/test/ui/lint/lint-unconditional-recursion.stderr
index 1770d71e2e2..fb884e31299 100644
--- a/src/test/ui/lint/lint-unconditional-recursion.stderr
+++ b/src/test/ui/lint/lint-unconditional-recursion.stderr
@@ -149,7 +149,7 @@ error: function cannot return without recursing
 LL |     fn deref(&self) -> &Baz {
    |     ^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
 LL |         self.as_ref()
-   |         ---- recursive call site
+   |         ------------- recursive call site
    |
    = help: a `loop` may express intention better if this is on purpose
 
diff --git a/src/test/ui/moves/move-deref-coercion.rs b/src/test/ui/moves/move-deref-coercion.rs
new file mode 100644
index 00000000000..41154388f56
--- /dev/null
+++ b/src/test/ui/moves/move-deref-coercion.rs
@@ -0,0 +1,33 @@
+use std::ops::Deref;
+
+struct NotCopy {
+    inner: bool
+}
+
+impl NotCopy {
+    fn inner_method(&self) {}
+}
+
+struct Foo {
+    first: NotCopy,
+    second: NotCopy
+}
+
+impl Deref for Foo {
+    type Target = NotCopy;
+    fn deref(&self) -> &NotCopy {
+        &self.second
+    }
+}
+
+fn use_field(val: Foo) {
+    let _val = val.first;
+    val.inner; //~ ERROR borrow of
+}
+
+fn use_method(val: Foo) {
+    let _val = val.first;
+    val.inner_method(); //~ ERROR borrow of
+}
+
+fn main() {}
diff --git a/src/test/ui/moves/move-deref-coercion.stderr b/src/test/ui/moves/move-deref-coercion.stderr
new file mode 100644
index 00000000000..e3bdf6d7832
--- /dev/null
+++ b/src/test/ui/moves/move-deref-coercion.stderr
@@ -0,0 +1,35 @@
+error[E0382]: borrow of partially moved value: `val`
+  --> $DIR/move-deref-coercion.rs:25:5
+   |
+LL |     let _val = val.first;
+   |                --------- value partially moved here
+LL |     val.inner;
+   |     ^^^^^^^^^ value borrowed here after partial move
+   |
+   = note: partial move occurs because `val.first` has type `NotCopy`, which does not implement the `Copy` trait
+   = note: borrow occurs due to deref coercion to `NotCopy`
+note: deref defined here
+  --> $DIR/move-deref-coercion.rs:17:5
+   |
+LL |     type Target = NotCopy;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0382]: borrow of partially moved value: `val`
+  --> $DIR/move-deref-coercion.rs:30:5
+   |
+LL |     let _val = val.first;
+   |                --------- value partially moved here
+LL |     val.inner_method();
+   |     ^^^^^^^^^^^^^^^^^^ value borrowed here after partial move
+   |
+   = note: partial move occurs because `val.first` has type `NotCopy`, which does not implement the `Copy` trait
+   = note: borrow occurs due to deref coercion to `NotCopy`
+note: deref defined here
+  --> $DIR/move-deref-coercion.rs:17:5
+   |
+LL |     type Target = NotCopy;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0382`.
diff --git a/src/test/ui/no-capture-arc.stderr b/src/test/ui/no-capture-arc.stderr
index 0081841fec0..37032e73f19 100644
--- a/src/test/ui/no-capture-arc.stderr
+++ b/src/test/ui/no-capture-arc.stderr
@@ -1,5 +1,5 @@
 error[E0382]: borrow of moved value: `arc_v`
-  --> $DIR/no-capture-arc.rs:14:18
+  --> $DIR/no-capture-arc.rs:14:16
    |
 LL |     let arc_v = Arc::new(v);
    |         ----- move occurs because `arc_v` has type `Arc<Vec<i32>>`, which does not implement the `Copy` trait
@@ -10,7 +10,14 @@ LL |         assert_eq!((*arc_v)[3], 4);
    |                      ----- variable moved due to use in closure
 ...
 LL |     assert_eq!((*arc_v)[2], 3);
-   |                  ^^^^^ value borrowed here after move
+   |                ^^^^^^^^ value borrowed here after move
+   |
+   = note: borrow occurs due to deref coercion to `Vec<i32>`
+note: deref defined here
+  --> $SRC_DIR/alloc/src/sync.rs:LL:COL
+   |
+LL |     type Target = T;
+   |     ^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/no-reuse-move-arc.stderr b/src/test/ui/no-reuse-move-arc.stderr
index bb4e0871224..6f37d4c9d86 100644
--- a/src/test/ui/no-reuse-move-arc.stderr
+++ b/src/test/ui/no-reuse-move-arc.stderr
@@ -1,5 +1,5 @@
 error[E0382]: borrow of moved value: `arc_v`
-  --> $DIR/no-reuse-move-arc.rs:12:18
+  --> $DIR/no-reuse-move-arc.rs:12:16
    |
 LL |     let arc_v = Arc::new(v);
    |         ----- move occurs because `arc_v` has type `Arc<Vec<i32>>`, which does not implement the `Copy` trait
@@ -10,7 +10,14 @@ LL |         assert_eq!((*arc_v)[3], 4);
    |                      ----- variable moved due to use in closure
 ...
 LL |     assert_eq!((*arc_v)[2], 3);
-   |                  ^^^^^ value borrowed here after move
+   |                ^^^^^^^^ value borrowed here after move
+   |
+   = note: borrow occurs due to deref coercion to `Vec<i32>`
+note: deref defined here
+  --> $SRC_DIR/alloc/src/sync.rs:LL:COL
+   |
+LL |     type Target = T;
+   |     ^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error