fix span for invalid number of parameters in trait method

src/librustc_typeck/check/compare_method.rs

@@ -648,12 +648,19 @@ fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         let trait_span = if let Some(trait_id) = trait_m_node_id {
             match tcx.hir.expect_trait_item(trait_id).node {
                 TraitItemKind::Method(ref trait_m_sig, _) => {
-                    if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
+                    let pos = if trait_number_args > 0 {
                         trait_number_args - 1
                     } else {
                         0
-                    }) {
-                        Some(arg.span)
+                    };
+                    if let Some(arg) = trait_m_sig.decl.inputs.get(pos) {
+                        Some(if pos == 0 {
+                            arg.span
+                        } else {
+                            Span::new(trait_m_sig.decl.inputs[0].span.lo(),
+                                      arg.span.hi(),
+                                      arg.span.ctxt())
+                        })
                     } else {
                         trait_item_span
                     }
@@ -666,12 +673,19 @@ fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
             ImplItemKind::Method(ref impl_m_sig, _) => {
-                if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
+                let pos = if impl_number_args > 0 {
                     impl_number_args - 1
                 } else {
                     0
-                }) {
-                    arg.span
+                };
+                if let Some(arg) = impl_m_sig.decl.inputs.get(pos) {
+                    if pos == 0 {
+                        arg.span
+                    } else {
+                        Span::new(impl_m_sig.decl.inputs[0].span.lo(),
+                                  arg.span.hi(),
+                                  arg.span.ctxt())
+                    }
                 } else {
                     impl_m_span
                 }

src/test/ui/error-codes/E0050.stderr

@@ -2,7 +2,7 @@ error[E0050]: method `foo` has 1 parameter but the declaration in trait `Foo::fo
   --> $DIR/E0050.rs:20:12
    |
 LL |     fn foo(&self, x: u8) -> bool;
-   |                      -- trait requires 2 parameters
+   |            ------------ trait requires 2 parameters
 ...
 LL |     fn foo(&self) -> bool { true } //~ ERROR E0050
    |            ^^^^^ expected 2 parameters, found 1
@@ -11,19 +11,19 @@ error[E0050]: method `bar` has 1 parameter but the declaration in trait `Foo::ba
   --> $DIR/E0050.rs:21:12
    |
 LL |     fn bar(&self, x: u8, y: u8, z: u8);
-   |                                    -- trait requires 4 parameters
+   |            -------------------------- trait requires 4 parameters
 ...
 LL |     fn bar(&self) { } //~ ERROR E0050
    |            ^^^^^ expected 4 parameters, found 1
 
 error[E0050]: method `less` has 4 parameters but the declaration in trait `Foo::less` has 1
-  --> $DIR/E0050.rs:22:37
+  --> $DIR/E0050.rs:22:13
    |
 LL |     fn less(&self);
    |             ----- trait requires 1 parameter
 ...
 LL |     fn less(&self, x: u8, y: u8, z: u8) { } //~ ERROR E0050
-   |                                     ^^ expected 1 parameter, found 4
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 parameter, found 4
 
 error: aborting due to 3 previous errors
 

src/test/ui/trait-method-number-parameters.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Foo {
+    fn foo(&mut self, x: i32, y: i32) -> i32;
+}
+
+impl Foo for i32 {
+    fn foo(
+        &mut self, //~ ERROR
+        x: i32,
+    ) {
+    }
+}
+
+fn main() {}

src/test/ui/trait-method-number-parameters.stderr

@@ -0,0 +1,13 @@
+error[E0050]: method `foo` has 2 parameters but the declaration in trait `Foo::foo` has 3
+  --> $DIR/trait-method-number-parameters.rs:17:9
+   |
+LL |       fn foo(&mut self, x: i32, y: i32) -> i32;
+   |              ------------------------- trait requires 3 parameters
+...
+LL | /         &mut self, //~ ERROR
+LL | |         x: i32,
+   | |______________^ expected 3 parameters, found 2
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0050`.

src/test/ui/traits/trait-impl-different-num-params.stderr

@@ -2,7 +2,7 @@ error[E0050]: method `bar` has 1 parameter but the declaration in trait `foo::ba
   --> $DIR/trait-impl-different-num-params.rs:15:12
    |
 LL |     fn bar(&self, x: usize) -> Self;
-   |                      ----- trait requires 2 parameters
+   |            --------------- trait requires 2 parameters
 ...
 LL |     fn bar(&self) -> isize {
    |            ^^^^^ expected 2 parameters, found 1