Improve message for closure returning a closure.

Now when a `FnMut` closure is returning a closure that contains a
reference to a captured variable, we provide an error that makes it more
clear what is happening.

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -310,9 +310,19 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             "captured variable cannot escape `FnMut` closure body",
         );
 
+        // We should check if the return type of this closure is in fact a closure - in that
+        // case, we can special case the error further.
+        let return_type_is_closure = self.universal_regions.unnormalized_output_ty.is_closure();
+        let message = if return_type_is_closure {
+            "returns a closure that contains a reference to a captured variable, which then \
+             escapes the closure body"
+        } else {
+            "returns a reference to a captured variable which escapes the closure body"
+        };
+
         diag.span_label(
             span,
-            "creates a reference to a captured variable which escapes the closure body",
+            message,
         );
 
         match self.give_region_a_name(infcx, mir, mir_def_id, outlived_fr, &mut 1).source {

src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr

@@ -32,7 +32,7 @@ LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ creates a reference to a captured variable which escapes the closure body
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape

src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr

@@ -32,7 +32,7 @@ LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ creates a reference to a captured variable which escapes the closure body
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape

src/test/ui/issues/issue-40510-1.nll.stderr

@@ -4,7 +4,7 @@ error: captured variable cannot escape `FnMut` closure body
 LL |     || {
    |      - inferred to be a `FnMut` closure
 LL |         &mut x
-   |         ^^^^^^ creates a reference to a captured variable which escapes the closure body
+   |         ^^^^^^ returns a reference to a captured variable which escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape

src/test/ui/issues/issue-40510-3.nll.stderr

@@ -6,7 +6,7 @@ LL |       || {
 LL | /         || {
 LL | |             x.push(())
 LL | |         }
-   | |_________^ creates a reference to a captured variable which escapes the closure body
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape

src/test/ui/issues/issue-49824.nll.stderr

@@ -6,7 +6,7 @@ LL |       || {
 LL | /         || {
 LL | |             let _y = &mut x;
 LL | |         }
-   | |_________^ creates a reference to a captured variable which escapes the closure body
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
    = note: `FnMut` closures only have access to their captured variables while they are executing...
    = note: ...therefore, they cannot allow references to captured variables to escape

src/test/ui/nll/issue-53040.stderr

@@ -2,7 +2,7 @@ error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-53040.rs:15:8
    |
 LL |     || &mut v;
-   |      - ^^^^^^ creates a reference to a captured variable which escapes the closure body
+   |      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
    |      |
    |      inferred to be a `FnMut` closure
    |

src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.nll.stderr

@@ -2,7 +2,7 @@ error: captured variable cannot escape `FnMut` closure body
   --> $DIR/regions-return-ref-to-upvar-issue-17403.rs:17:24
    |
 LL |         let mut f = || &mut x; //~ ERROR cannot infer
-   |                      - ^^^^^^ creates a reference to a captured variable which escapes the closure body
+   |                      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
    |                      |
    |                      inferred to be a `FnMut` closure
    |