Fix ICE for mismatched args on target without span

Commit 7ed00ca improved our error reporting by including the target
function in our error messages when there is an argument count mismatch.
A simple example from the UI tests is:

```
error[E0593]: function is expected to take a single 2-tuple as argument, but it takes 0 arguments
  --> $DIR/closure-arg-count.rs:32:53
   |
32 |     let _it = vec![1, 2, 3].into_iter().enumerate().map(foo);
   |                                                     ^^^ expected function that takes a single 2-tuple as argument
...
44 | fn foo() {}
   | -------- takes 0 arguments
```

However, this assumed the target span was always available. This does
not hold true if the target function is in `std` or another crate. A
simple example from #48046 is assigning `str::split` to a function type
with a different number of arguments.

Fix by removing all of the labels and suggestions related to the target
span when it's not found.

Fixes #48046

src/librustc/traits/error_reporting.rs

@@ -744,8 +744,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 } else {
                     let (closure_span, found) = found_did
                         .and_then(|did| self.tcx.hir.get_if_local(did))
-                        .map(|node| self.get_fn_like_arguments(node))
-                        .unwrap_or((found_span.unwrap(), found));
+                        .map(|node| {
+                            let (found_span, found) = self.get_fn_like_arguments(node);
+                            (Some(found_span), found)
+                        }).unwrap_or((found_span, found));
 
                     self.report_arg_count_mismatch(span,
                                                    closure_span,
@@ -855,7 +857,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     fn report_arg_count_mismatch(
         &self,
         span: Span,
-        found_span: Span,
+        found_span: Option<Span>,
         expected_args: Vec<ArgKind>,
         found_args: Vec<ArgKind>,
         is_closure: bool,
@@ -893,48 +895,51 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         );
 
         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
-        err.span_label(found_span, format!("takes {}", found_str));
-
-        if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
-            if fields.len() == expected_args.len() {
-                let sugg = fields.iter()
-                    .map(|(name, _)| name.to_owned())
-                    .collect::<Vec<String>>().join(", ");
-                err.span_suggestion(found_span,
-                                    "change the closure to take multiple arguments instead of \
-                                     a single tuple",
-                                    format!("|{}|", sugg));
+
+        if let Some(found_span) = found_span {
+            err.span_label(found_span, format!("takes {}", found_str));
+
+            if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
+                if fields.len() == expected_args.len() {
+                    let sugg = fields.iter()
+                        .map(|(name, _)| name.to_owned())
+                        .collect::<Vec<String>>().join(", ");
+                    err.span_suggestion(found_span,
+                                        "change the closure to take multiple arguments instead of \
+                                         a single tuple",
+                                        format!("|{}|", sugg));
+                }
             }
-        }
-        if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
-            if fields.len() == found_args.len() && is_closure {
-                let sugg = format!(
-                    "|({}){}|",
-                    found_args.iter()
-                        .map(|arg| match arg {
-                            ArgKind::Arg(name, _) => name.to_owned(),
-                            _ => "_".to_owned(),
-                        })
-                        .collect::<Vec<String>>()
-                        .join(", "),
-                    // add type annotations if available
-                    if found_args.iter().any(|arg| match arg {
-                        ArgKind::Arg(_, ty) => ty != "_",
-                        _ => false,
-                    }) {
-                        format!(": ({})",
-                                fields.iter()
-                                    .map(|(_, ty)| ty.to_owned())
-                                    .collect::<Vec<String>>()
-                                    .join(", "))
-                    } else {
-                        "".to_owned()
-                    },
-                );
-                err.span_suggestion(found_span,
-                                    "change the closure to accept a tuple instead of individual \
-                                     arguments",
-                                    sugg);
+            if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
+                if fields.len() == found_args.len() && is_closure {
+                    let sugg = format!(
+                        "|({}){}|",
+                        found_args.iter()
+                            .map(|arg| match arg {
+                                ArgKind::Arg(name, _) => name.to_owned(),
+                                _ => "_".to_owned(),
+                            })
+                            .collect::<Vec<String>>()
+                            .join(", "),
+                        // add type annotations if available
+                        if found_args.iter().any(|arg| match arg {
+                            ArgKind::Arg(_, ty) => ty != "_",
+                            _ => false,
+                        }) {
+                            format!(": ({})",
+                                    fields.iter()
+                                        .map(|(_, ty)| ty.to_owned())
+                                        .collect::<Vec<String>>()
+                                        .join(", "))
+                        } else {
+                            "".to_owned()
+                        },
+                    );
+                    err.span_suggestion(found_span,
+                                        "change the closure to accept a tuple instead of \
+                                         individual arguments",
+                                        sugg);
+                }
             }
         }
 

src/test/ui/mismatched_types/closure-arg-count.rs

@@ -36,6 +36,9 @@ fn main() {
     //~^ ERROR closure is expected to take
     let _it = vec![1, 2, 3].into_iter().enumerate().map(qux);
     //~^ ERROR function is expected to take
+
+    let _it = vec![1, 2, 3].into_iter().map(usize::checked_add);
+    //~^ ERROR function is expected to take
 }
 
 fn foo() {}

src/test/ui/mismatched_types/closure-arg-count.stderr

@@ -90,7 +90,7 @@ error[E0593]: function is expected to take a single 2-tuple as argument, but it
 32 |     let _it = vec![1, 2, 3].into_iter().enumerate().map(foo);
    |                                                     ^^^ expected function that takes a single 2-tuple as argument
 ...
-41 | fn foo() {}
+44 | fn foo() {}
    | -------- takes 0 arguments
 
 error[E0593]: closure is expected to take a single 2-tuple as argument, but it takes 3 distinct arguments
@@ -107,8 +107,14 @@ error[E0593]: function is expected to take a single 2-tuple as argument, but it
 37 |     let _it = vec![1, 2, 3].into_iter().enumerate().map(qux);
    |                                                     ^^^ expected function that takes a single 2-tuple as argument
 ...
-42 | fn qux(x: usize, y: usize) {}
+45 | fn qux(x: usize, y: usize) {}
    | -------------------------- takes 2 distinct arguments
 
-error: aborting due to 11 previous errors
+error[E0593]: function is expected to take 1 argument, but it takes 2 arguments
+  --> $DIR/closure-arg-count.rs:40:41
+   |
+40 |     let _it = vec![1, 2, 3].into_iter().map(usize::checked_add);
+   |                                         ^^^ expected function that takes 1 argument
+
+error: aborting due to 12 previous errors