Mitigate impact of subtle invalid call suggestion logic

There's some subtle interaction between inferred expressions being
passed as an argument to fn calls with fewer than expected arguments. To
avoid the ICE, I'm changing indexing operations with `.get(idx)`, but
the underlying logic still needs to be audited as it was written with
the assumption that `final_arg_types` and `provided_args` have the right
length.

Address 96638.

compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

@@ -429,9 +429,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             errors.drain_filter(|error| {
                 let Error::Invalid(input_idx, Compatibility::Incompatible(error)) = error else { return false };
                 let expected_ty = expected_input_tys[*input_idx];
-                let provided_ty = final_arg_types[*input_idx].map(|ty| ty.0).unwrap();
+                let Some(Some((provided_ty, _))) = final_arg_types.get(*input_idx) else { return false };
                 let cause = &self.misc(provided_args[*input_idx].span);
-                let trace = TypeTrace::types(cause, true, expected_ty, provided_ty);
+                let trace = TypeTrace::types(cause, true, expected_ty, *provided_ty);
                 if let Some(e) = error {
                     if !matches!(trace.cause.as_failure_code(e), FailureCode::Error0308(_)) {
                         self.report_and_explain_type_error(trace, e).emit();
@@ -679,8 +679,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     Error::Invalid(input_idx, compatibility) => {
                         let expected_ty = expected_input_tys[input_idx];
                         if let Compatibility::Incompatible(error) = &compatibility {
-                            let provided_ty = final_arg_types[input_idx].map(|ty| ty.0).unwrap();
-                            let cause = &self.misc(provided_args[input_idx].span);
+                            let provided_ty = final_arg_types
+                                .get(input_idx)
+                                .and_then(|x| x.as_ref())
+                                .map(|ty| ty.0)
+                                .unwrap_or(tcx.ty_error());
+                            let cause = &self.misc(
+                                provided_args.get(input_idx).map(|i| i.span).unwrap_or(call_span),
+                            );
                             let trace = TypeTrace::types(cause, true, expected_ty, provided_ty);
                             if let Some(e) = error {
                                 self.note_type_err(
@@ -695,14 +701,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             }
                         }
 
-                        self.emit_coerce_suggestions(
-                            &mut err,
-                            &provided_args[input_idx],
-                            final_arg_types[input_idx].map(|ty| ty.0).unwrap(),
-                            final_arg_types[input_idx].map(|ty| ty.1).unwrap(),
-                            None,
-                            None,
-                        );
+                        if let Some(expr) = provided_args.get(input_idx) {
+                            self.emit_coerce_suggestions(
+                                &mut err,
+                                &expr,
+                                final_arg_types[input_idx].map(|ty| ty.0).unwrap(),
+                                final_arg_types[input_idx].map(|ty| ty.1).unwrap(),
+                                None,
+                                None,
+                            );
+                        }
                     }
                     Error::Extra(arg_idx) => {
                         let arg_type = if let Some((_, ty)) = final_arg_types[arg_idx] {

src/test/ui/argument-suggestions/issue-96638.rs

@@ -0,0 +1,9 @@
+fn f(_: usize, _: &usize, _: usize) {}
+
+fn arg<T>() -> T { todo!() }
+
+fn main() {
+    let x = arg(); // `x` must be inferred
+    // The reference on `&x` is important to reproduce the ICE
+    f(&x, ""); //~ ERROR this function takes 3 arguments but 2 arguments were supplied
+}

src/test/ui/argument-suggestions/issue-96638.stderr

@@ -0,0 +1,19 @@
+error[E0061]: this function takes 3 arguments but 2 arguments were supplied
+  --> $DIR/issue-96638.rs:8:5
+   |
+LL |     f(&x, "");
+   |     ^ -- an argument of type `usize` is missing
+   |
+note: function defined here
+  --> $DIR/issue-96638.rs:1:4
+   |
+LL | fn f(_: usize, _: &usize, _: usize) {}
+   |    ^ --------  ---------  --------
+help: provide the argument
+   |
+LL |     f({usize}, &x, {usize});
+   |     ~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0061`.