Suggest similarly named assoc items in trait impls

Previously, the compiler didn't suggest similarly named associated items
unlike we do in many situations. This patch adds such diagnostics for
associated functions, types and constants.

src/test/ui/error-codes/E0407.stderr

@@ -2,7 +2,10 @@ error[E0407]: method `b` is not a member of trait `Foo`
   --> $DIR/E0407.rs:9:5
    |
 LL |     fn b() {}
-   |     ^^^^^^^^^ not a member of trait `Foo`
+   |     ^^^-^^^^^
+   |     |  |
+   |     |  help: there is an associated function with a similar name: `a`
+   |     not a member of trait `Foo`
 
 error: aborting due to previous error
 

src/test/ui/hygiene/assoc_item_ctxt.stderr

@@ -2,7 +2,10 @@ error[E0407]: method `method` is not a member of trait `Tr`
   --> $DIR/assoc_item_ctxt.rs:35:13
    |
 LL |             fn method() {}
-   |             ^^^^^^^^^^^^^^ not a member of trait `Tr`
+   |             ^^^------^^^^^
+   |             |  |
+   |             |  help: there is an associated function with a similar name: `method`
+   |             not a member of trait `Tr`
 ...
 LL |     mac_trait_impl!();
    |     ------------------ in this macro invocation

src/test/ui/suggestions/suggest-trait-items.rs

@@ -0,0 +1,48 @@
+trait Foo {
+    type Type;
+
+    fn foo();
+    fn bar();
+    fn qux();
+}
+
+struct A;
+
+impl Foo for A {
+//~^ ERROR not all trait items implemented
+    type Typ = ();
+    //~^ ERROR type `Typ` is not a member of trait
+    //~| HELP there is an associated type with a similar name
+
+    fn fooo() {}
+    //~^ ERROR method `fooo` is not a member of trait
+    //~| HELP there is an associated function with a similar name
+
+    fn barr() {}
+    //~^ ERROR method `barr` is not a member of trait
+    //~| HELP there is an associated function with a similar name
+
+    fn quux() {}
+    //~^ ERROR method `quux` is not a member of trait
+    //~| HELP there is an associated function with a similar name
+}
+//~^ HELP implement the missing item
+//~| HELP implement the missing item
+//~| HELP implement the missing item
+//~| HELP implement the missing item
+
+trait Bar {
+    const Const: i32;
+}
+
+struct B;
+
+impl Bar for B {
+//~^ ERROR not all trait items implemented
+    const Cnst: i32 = 0;
+    //~^ ERROR const `Cnst` is not a member of trait
+    //~| HELP there is an associated constant with a similar name
+}
+//~^ HELP implement the missing item
+
+fn main() {}

src/test/ui/suggestions/suggest-trait-items.stderr

@@ -0,0 +1,74 @@
+error[E0437]: type `Typ` is not a member of trait `Foo`
+  --> $DIR/suggest-trait-items.rs:13:5
+   |
+LL |     type Typ = ();
+   |     ^^^^^---^^^^^^
+   |     |    |
+   |     |    help: there is an associated type with a similar name: `Type`
+   |     not a member of trait `Foo`
+
+error[E0407]: method `fooo` is not a member of trait `Foo`
+  --> $DIR/suggest-trait-items.rs:17:5
+   |
+LL |     fn fooo() {}
+   |     ^^^----^^^^^
+   |     |  |
+   |     |  help: there is an associated function with a similar name: `foo`
+   |     not a member of trait `Foo`
+
+error[E0407]: method `barr` is not a member of trait `Foo`
+  --> $DIR/suggest-trait-items.rs:21:5
+   |
+LL |     fn barr() {}
+   |     ^^^----^^^^^
+   |     |  |
+   |     |  help: there is an associated function with a similar name: `bar`
+   |     not a member of trait `Foo`
+
+error[E0407]: method `quux` is not a member of trait `Foo`
+  --> $DIR/suggest-trait-items.rs:25:5
+   |
+LL |     fn quux() {}
+   |     ^^^----^^^^^
+   |     |  |
+   |     |  help: there is an associated function with a similar name: `qux`
+   |     not a member of trait `Foo`
+
+error[E0438]: const `Cnst` is not a member of trait `Bar`
+  --> $DIR/suggest-trait-items.rs:42:5
+   |
+LL |     const Cnst: i32 = 0;
+   |     ^^^^^^----^^^^^^^^^^
+   |     |     |
+   |     |     help: there is an associated constant with a similar name: `Const`
+   |     not a member of trait `Bar`
+
+error[E0046]: not all trait items implemented, missing: `Type`, `foo`, `bar`, `qux`
+  --> $DIR/suggest-trait-items.rs:11:1
+   |
+LL |     type Type;
+   |     ---------- `Type` from trait
+LL | 
+LL |     fn foo();
+   |     --------- `foo` from trait
+LL |     fn bar();
+   |     --------- `bar` from trait
+LL |     fn qux();
+   |     --------- `qux` from trait
+...
+LL | impl Foo for A {
+   | ^^^^^^^^^^^^^^ missing `Type`, `foo`, `bar`, `qux` in implementation
+
+error[E0046]: not all trait items implemented, missing: `Const`
+  --> $DIR/suggest-trait-items.rs:40:1
+   |
+LL |     const Const: i32;
+   |     ----------------- `Const` from trait
+...
+LL | impl Bar for B {
+   | ^^^^^^^^^^^^^^ missing `Const` in implementation
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0046, E0407, E0437, E0438.
+For more information about an error, try `rustc --explain E0046`.