Fix ICE for mismatched args on target without span

Commit 7ed00caacc 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

1 files changed, 49 insertions, 44 deletions

diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index 7b86791026b..f58ac9f00e4 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/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);
+                }
             }
         }