Rollup merge of #87460 - FabianWolff:issue-87456, r=Aaron1011

Point to closure when emitting 'cannot move out' for captured variable

Attempts to fix #87456. The error message now points to the capturing closure, but I was not able to explain _why_ the closure implements `Fn` or `FnMut` (`TypeckResults::closure_kind_origins` did not contain anything for the closure in question).

cc `@Aaron1011`

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -336,15 +336,15 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 if def_id.as_local() == Some(self.mir_def_id()) && upvar_field.is_some() =>
             {
                 let closure_kind_ty = closure_substs.as_closure().kind_ty();
-                let closure_kind = closure_kind_ty.to_opt_closure_kind();
-                let capture_description = match closure_kind {
-                    Some(ty::ClosureKind::Fn) => "captured variable in an `Fn` closure",
-                    Some(ty::ClosureKind::FnMut) => "captured variable in an `FnMut` closure",
+                let closure_kind = match closure_kind_ty.to_opt_closure_kind() {
+                    Some(kind @ (ty::ClosureKind::Fn | ty::ClosureKind::FnMut)) => kind,
                     Some(ty::ClosureKind::FnOnce) => {
                         bug!("closure kind does not match first argument type")
                     }
                     None => bug!("closure kind not inferred by borrowck"),
                 };
+                let capture_description =
+                    format!("captured variable in an `{}` closure", closure_kind);
 
                 let upvar = &self.upvars[upvar_field.unwrap().index()];
                 let upvar_hir_id = upvar.place.get_root_variable();
@@ -368,6 +368,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let mut diag = self.cannot_move_out_of(span, &place_description);
 
                 diag.span_label(upvar_span, "captured outer variable");
+                diag.span_label(
+                    self.body.span,
+                    format!("captured by this `{}` closure", closure_kind),
+                );
 
                 diag
             }

src/test/ui/borrowck/borrowck-in-static.stderr

@@ -4,7 +4,10 @@ error[E0507]: cannot move out of `x`, a captured variable in an `Fn` closure
 LL |     let x = Box::new(0);
    |         - captured outer variable
 LL |     Box::new(|| x)
-   |                 ^ move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
+   |              ---^
+   |              |  |
+   |              |  move occurs because `x` has type `Box<i32>`, which does not implement the `Copy` trait
+   |              captured by this `Fn` closure
 
 error: aborting due to previous error
 

src/test/ui/borrowck/borrowck-move-by-capture.stderr

@@ -1,15 +1,18 @@
 error[E0507]: cannot move out of `bar`, a captured variable in an `FnMut` closure
   --> $DIR/borrowck-move-by-capture.rs:9:29
    |
-LL |     let bar: Box<_> = box 3;
-   |         --- captured outer variable
-LL |     let _g = to_fn_mut(|| {
-LL |         let _h = to_fn_once(move || -> isize { *bar });
-   |                             ^^^^^^^^^^^^^^^^   ----
-   |                             |                  |
-   |                             |                  move occurs because `bar` has type `Box<isize>`, which does not implement the `Copy` trait
-   |                             |                  move occurs due to use in closure
-   |                             move out of `bar` occurs here
+LL |       let bar: Box<_> = box 3;
+   |           --- captured outer variable
+LL |       let _g = to_fn_mut(|| {
+   |  ________________________-
+LL | |         let _h = to_fn_once(move || -> isize { *bar });
+   | |                             ^^^^^^^^^^^^^^^^   ----
+   | |                             |                  |
+   | |                             |                  move occurs because `bar` has type `Box<isize>`, which does not implement the `Copy` trait
+   | |                             |                  move occurs due to use in closure
+   | |                             move out of `bar` occurs here
+LL | |     });
+   | |_____- captured by this `FnMut` closure
 
 error: aborting due to previous error
 

src/test/ui/borrowck/issue-87456-point-to-closure.rs

@@ -0,0 +1,14 @@
+// Regression test for #87456.
+
+fn take_mut(_val: impl FnMut()) {}
+
+fn main() {
+    let val = String::new();
+    //~^ NOTE: captured outer variable
+    take_mut(|| {
+    //~^ NOTE: captured by this `FnMut` closure
+        let _foo: String = val;
+        //~^ ERROR: cannot move out of `val`, a captured variable in an `FnMut` closure [E0507]
+        //~| NOTE: move occurs because
+    })
+}

src/test/ui/borrowck/issue-87456-point-to-closure.stderr

@@ -0,0 +1,22 @@
+error[E0507]: cannot move out of `val`, a captured variable in an `FnMut` closure
+  --> $DIR/issue-87456-point-to-closure.rs:10:28
+   |
+LL |       let val = String::new();
+   |           --- captured outer variable
+LL |
+LL |       take_mut(|| {
+   |  ______________-
+LL | |
+LL | |         let _foo: String = val;
+   | |                            ^^^
+   | |                            |
+   | |                            move occurs because `val` has type `String`, which does not implement the `Copy` trait
+   | |                            help: consider borrowing here: `&val`
+LL | |
+LL | |
+LL | |     })
+   | |_____- captured by this `FnMut` closure
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.

src/test/ui/borrowck/unboxed-closures-move-upvar-from-non-once-ref-closure.stderr

@@ -1,11 +1,15 @@
 error[E0507]: cannot move out of `y`, a captured variable in an `Fn` closure
   --> $DIR/unboxed-closures-move-upvar-from-non-once-ref-closure.rs:11:9
    |
-LL |     let y = vec![format!("World")];
-   |         - captured outer variable
-LL |     call(|| {
-LL |         y.into_iter();
-   |         ^ move occurs because `y` has type `Vec<String>`, which does not implement the `Copy` trait
+LL |       let y = vec![format!("World")];
+   |           - captured outer variable
+LL |       call(|| {
+   |  __________-
+LL | |         y.into_iter();
+   | |         ^ move occurs because `y` has type `Vec<String>`, which does not implement the `Copy` trait
+LL | |
+LL | |     });
+   | |_____- captured by this `Fn` closure
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-4335.stderr

@@ -4,7 +4,10 @@ error[E0507]: cannot move out of `*v`, as `v` is a captured variable in an `FnMu
 LL | fn f<'r, T>(v: &'r T) -> Box<dyn FnMut() -> T + 'r> {
    |             - captured outer variable
 LL |     id(Box::new(|| *v))
-   |                    ^^ move occurs because `*v` has type `T`, which does not implement the `Copy` trait
+   |                 ---^^
+   |                 |  |
+   |                 |  move occurs because `*v` has type `T`, which does not implement the `Copy` trait
+   |                 captured by this `FnMut` closure
 
 error: aborting due to previous error
 

src/test/ui/moves/moves-based-on-type-move-out-of-closure-env-issue-1965.stderr

@@ -4,7 +4,10 @@ error[E0507]: cannot move out of `i`, a captured variable in an `Fn` closure
 LL |     let i = box 3;
    |         - captured outer variable
 LL |     let _f = to_fn(|| test(i));
-   |                            ^ move occurs because `i` has type `Box<usize>`, which does not implement the `Copy` trait
+   |                    --------^-
+   |                    |       |
+   |                    |       move occurs because `i` has type `Box<usize>`, which does not implement the `Copy` trait
+   |                    captured by this `Fn` closure
 
 error: aborting due to previous error
 

src/test/ui/nll/issue-52663-span-decl-captured-variable.stderr

@@ -4,7 +4,10 @@ error[E0507]: cannot move out of `x.0`, as `x` is a captured variable in an `Fn`
 LL |        let x = (vec![22], vec![44]);
    |            - captured outer variable
 LL |        expect_fn(|| drop(x.0));
-   |                          ^^^ move occurs because `x.0` has type `Vec<i32>`, which does not implement the `Copy` trait
+   |                  --------^^^-
+   |                  |       |
+   |                  |       move occurs because `x.0` has type `Vec<i32>`, which does not implement the `Copy` trait
+   |                  captured by this `Fn` closure
 
 error: aborting due to previous error
 

src/test/ui/span/borrowck-call-is-borrow-issue-12224.stderr

@@ -29,11 +29,17 @@ LL |     f.f.call_mut(())
 error[E0507]: cannot move out of `f`, a captured variable in an `FnMut` closure
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:57:13
    |
-LL |     let mut f = move |g: Box<dyn FnMut(isize)>, b: isize| {
-   |         ----- captured outer variable
+LL |       let mut f = move |g: Box<dyn FnMut(isize)>, b: isize| {
+   |           ----- captured outer variable
 ...
-LL |         foo(f);
-   |             ^ move occurs because `f` has type `[closure@$DIR/borrowck-call-is-borrow-issue-12224.rs:52:17: 54:6]`, which does not implement the `Copy` trait
+LL |       f(Box::new(|a| {
+   |  ________________-
+LL | |
+LL | |         foo(f);
+   | |             ^ move occurs because `f` has type `[closure@$DIR/borrowck-call-is-borrow-issue-12224.rs:52:17: 54:6]`, which does not implement the `Copy` trait
+LL | |
+LL | |     }), 3);
+   | |_____- captured by this `FnMut` closure
 
 error[E0505]: cannot move out of `f` because it is borrowed
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:55:16