improve suggestions output and add ui test

src/librustc_typeck/check/demand.rs

@@ -154,13 +154,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         if no_argument_methods.len() > 0 {
             no_argument_methods.iter()
                                .take(5)
-                               .map(|method| format!("{}", method.name()))
+                               .map(|method| format!(".{}()", method.name()))
                                .collect::<Vec<String>>()
                                .join("\n - ")
         } else {
             methods.iter()
                    .take(5)
-                   .map(|method| format!("{}", method.name()))
+                   .map(|method| format!(".{}()", method.name()))
                    .collect::<Vec<String>>()
                    .join("\n - ")
         }

src/test/ui/span/coerce-suggestions.stderr

@@ -0,0 +1,52 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestions.rs:17:20
+   |
+17 |     let x: usize = String::new();
+   |                    ^^^^^^^^^^^^^ expected usize, found struct `std::string::String`
+   |
+   = note: expected type `usize`
+   = note:    found type `std::string::String`
+   = help: here are some functions which might fulfill your needs:
+ - .capacity()
+ - .len()
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestions.rs:23:19
+   |
+23 |     let x: &str = String::new();
+   |                   ^^^^^^^^^^^^^ expected &str, found struct `std::string::String`
+   |
+   = note: expected type `&str`
+   = note:    found type `std::string::String`
+   = help: try with `&String::new()`
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestions.rs:30:10
+   |
+30 |     test(&y);
+   |          ^^ types differ in mutability
+   |
+   = note: expected type `&mut std::string::String`
+   = note:    found type `&std::string::String`
+   = help: try with `&mut y`
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestions.rs:36:11
+   |
+36 |     test2(&y);
+   |           ^^ types differ in mutability
+   |
+   = note: expected type `&mut i32`
+   = note:    found type `&std::string::String`
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-suggestions.rs:42:9
+   |
+42 |     f = box f;
+   |         ^^^^^ cyclic type of infinite size
+   |
+   = note: expected type `_`
+   = note:    found type `Box<_>`
+
+error: aborting due to 5 previous errors
+