Adjust error message grammar to be less awkward

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -518,7 +518,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     .with_span_help(
                         self.get_closure_bound_clause_span(*def_id),
                         "`Fn` and `FnMut` closures require captured values to be able to be \
-                         consumed multiple times, but an `FnOnce` consume them only once",
+                         consumed multiple times, but `FnOnce` closures may consume them only once",
                     )
             }
             _ => {

tests/ui/borrowck/borrowck-in-static.stderr

@@ -10,7 +10,7 @@ LL |     Box::new(|| x)
    |              |
    |              captured by this `Fn` closure
    |
-   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
 help: consider cloning the value if the performance cost is acceptable
    |
 LL |     Box::new(|| x.clone())

tests/ui/borrowck/borrowck-move-by-capture.stderr

@@ -12,7 +12,7 @@ LL |         let _h = to_fn_once(move || -> isize { *bar });
    |                             |
    |                             `bar` is moved here
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/borrowck-move-by-capture.rs:3:37
    |
 LL | fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }

tests/ui/borrowck/issue-103624.stderr

@@ -13,7 +13,7 @@ LL |
 LL |             self.b;
    |             ^^^^^^ `self.b` is moved here
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/issue-103624.rs:7:36
    |
 LL | async fn spawn_blocking<T>(f: impl (Fn() -> T) + Send + Sync + 'static) -> T {

tests/ui/borrowck/issue-87456-point-to-closure.stderr

@@ -10,7 +10,7 @@ LL |
 LL |         let _foo: String = val;
    |                            ^^^ move occurs because `val` has type `String`, which does not implement the `Copy` trait
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/issue-87456-point-to-closure.rs:3:24
    |
 LL | fn take_mut(_val: impl FnMut()) {}

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

@@ -10,7 +10,7 @@ LL |         y.into_iter();
    |         |
    |         move occurs because `y` has type `Vec<String>`, which does not implement the `Copy` trait
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/unboxed-closures-move-upvar-from-non-once-ref-closure.rs:5:28
    |
 LL | fn call<F>(f: F) where F : Fn() {

tests/ui/issues/issue-4335.stderr

@@ -10,7 +10,7 @@ LL |     id(Box::new(|| *v))
    |                 |
    |                 captured by this `FnMut` closure
    |
-   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
 help: if `T` implemented `Clone`, you could clone the value
   --> $DIR/issue-4335.rs:5:10
    |

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

@@ -10,7 +10,7 @@ LL |     let _f = to_fn(|| test(i));
    |                    |
    |                    captured by this `Fn` closure
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/moves-based-on-type-move-out-of-closure-env-issue-1965.rs:3:33
    |
 LL | fn to_fn<A:std::marker::Tuple,F:Fn<A>>(f: F) -> F { f }

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

@@ -10,7 +10,7 @@ LL |        expect_fn(|| drop(x.0));
    |                  |
    |                  captured by this `Fn` closure
    |
-help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
   --> $DIR/issue-52663-span-decl-captured-variable.rs:1:33
    |
 LL | fn expect_fn<F>(f: F) where F : Fn() {

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

@@ -44,7 +44,7 @@ LL |
 LL |         foo(f);
    |             ^ move occurs because `f` has type `{closure@$DIR/borrowck-call-is-borrow-issue-12224.rs:52:17: 52:58}`, which does not implement the `Copy` trait
    |
-   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but an `FnOnce` consume them only once
+   = help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
 help: consider cloning the value if the performance cost is acceptable
    |
 LL |         foo(f.clone());