Add note if method is called on a function object

Fixes issue rust-lang#29124.

If method is called on a function type a note is generated to suggest
that the developer may have forgotten to call it.

e.g.

fn main() {
    let mut guess = String::new();
    std::io::stdin.read_line(&mut guess);
}

will generate the note:

note: called method on function type. did you mean `std::io::stdin().read_line(..)`?

Author: Daniel J Rollins

src/librustc_typeck/check/method/suggest.rs

@@ -37,6 +37,42 @@ use std::cmp::Ordering;
 use super::{MethodError, NoMatchData, CandidateSource, impl_item, trait_item};
 use super::probe::Mode;
 
+fn is_fn_ty<'a, 'tcx>(ty: &Ty<'tcx>, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> bool {
+    let cx = fcx.tcx();
+    println!("{:?}", ty);
+    match ty.sty {
+        // Not all of these (e.g. unsafe fns) implement FnOnce
+        // so we look for these beforehand
+        ty::TyClosure(..) | ty::TyFnDef(..) | ty::TyFnPtr(_) => true,
+        // If it's not a simple function, look for things which implement FnOnce
+        _ => {
+            if let Ok(fn_once_trait_did) =
+                    cx.lang_items.require(FnOnceTraitLangItem) {
+                let infcx = fcx.infcx();
+                infcx.probe(|_| {
+                    let fn_once_substs =
+                        Substs::new_trait(vec![infcx.next_ty_var()],
+                                          Vec::new(),
+                                          ty);
+                    let trait_ref =
+                      ty::TraitRef::new(fn_once_trait_did,
+                                        cx.mk_substs(fn_once_substs));
+                    let poly_trait_ref = trait_ref.to_poly_trait_ref();
+                    let obligation = Obligation::misc(span,
+                                                      fcx.body_id,
+                                                      poly_trait_ref
+                                                         .to_predicate());
+                    let mut selcx = SelectionContext::new(infcx);
+
+                    return selcx.evaluate_obligation(&obligation)
+                })
+            } else {
+                false
+            }
+        }
+    }
+}
+
 pub fn report_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                               span: Span,
                               rcvr_ty: Ty<'tcx>,
@@ -79,65 +115,35 @@ pub fn report_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                         // snippet
                     };
 
-                    macro_rules! span_stored_function {
-                        () => {
-                            err.span_note(span,
-                                          &format!("use `({0}.{1})(...)` if you meant to call \
-                                                    the function stored in the `{1}` field",
-                                                   expr_string, item_name));
-                        }
-                    }
-
-                    macro_rules! span_did_you_mean {
-                        () => {
-                            err.span_note(span, &format!("did you mean to write `{0}.{1}`?",
-                                                         expr_string, item_name));
-                        }
-                    }
-
-                    // Determine if the field can be used as a function in some way
                     let field_ty = field.ty(cx, substs);
 
-                    match field_ty.sty {
-                        // Not all of these (e.g. unsafe fns) implement FnOnce
-                        // so we look for these beforehand
-                        ty::TyClosure(..) | ty::TyFnDef(..) | ty::TyFnPtr(_) => {
-                            span_stored_function!();
-                        }
-                        // If it's not a simple function, look for things which implement FnOnce
-                        _ => {
-                            if let Ok(fn_once_trait_did) =
-                                    cx.lang_items.require(FnOnceTraitLangItem) {
-                                let infcx = fcx.infcx();
-                                infcx.probe(|_| {
-                                    let fn_once_substs =
-                                        Substs::new_trait(vec![infcx.next_ty_var()],
-                                                          Vec::new(),
-                                                          field_ty);
-                                    let trait_ref =
-                                      ty::TraitRef::new(fn_once_trait_did,
-                                                        cx.mk_substs(fn_once_substs));
-                                    let poly_trait_ref = trait_ref.to_poly_trait_ref();
-                                    let obligation = Obligation::misc(span,
-                                                                      fcx.body_id,
-                                                                      poly_trait_ref
-                                                                         .to_predicate());
-                                    let mut selcx = SelectionContext::new(infcx);
-
-                                    if selcx.evaluate_obligation(&obligation) {
-                                        span_stored_function!();
-                                    } else {
-                                        span_did_you_mean!();
-                                    }
-                                });
-                            } else {
-                                span_did_you_mean!();
-                            }
-                        }
+                    if is_fn_ty(&field_ty, &fcx, span) {
+                        err.span_note(span,
+                                      &format!("use `({0}.{1})(...)` if you meant to call \
+                                               the function stored in the `{1}` field",
+                                               expr_string, item_name));
+                    } else {
+                        err.span_note(span, &format!("did you mean to write `{0}.{1}`?",
+                                                     expr_string, item_name));
                     }
                 }
             }
 
+            if is_fn_ty(&rcvr_ty, &fcx, span) {
+                let expr_string = match rcvr_expr {
+                    Some(expr) => match cx.sess.codemap().span_to_snippet(expr.span) {
+                        Ok(expr_string) => expr_string,
+                        _ => "s".into()
+                    },
+                    _ => "s".into()
+                };
+                err.fileline_note(
+                    span,
+                    &format!("method invoked on function type. did you \
+                             mean `{}().{}(...)`?",
+                             expr_string, item_name));
+            }
+
             if !static_sources.is_empty() {
                 err.fileline_note(
                     span,