Improve the non_snake_case lint to give better suggestions

Author: ftxqxd

src/librustc/lint/builtin.rs

@@ -887,13 +887,17 @@ impl NonSnakeCase {
         fn to_snake_case(str: &str) -> String {
             let mut words = vec![];
             for s in str.split('_') {
+                let mut last_upper = false;
                 let mut buf = String::new();
                 if s.is_empty() { continue; }
                 for ch in s.chars() {
-                    if !buf.is_empty() && buf.as_slice() != "'" && ch.is_uppercase() {
+                    if !buf.is_empty() && buf.as_slice() != "'"
+                                       && ch.is_uppercase()
+                                       && !last_upper {
                         words.push(buf);
                         buf = String::new();
                     }
+                    last_upper = ch.is_uppercase();
                     buf.push_char(ch.to_lowercase());
                 }
                 words.push(buf);

src/test/compile-fail/lint-non-snake-case-functions.rs

@@ -23,11 +23,14 @@ impl Foo {
 
     pub fn xyZ(&mut self) {}
     //~^ ERROR method `xyZ` should have a snake case name such as `xy_z`
+
+    fn render_HTML() {}
+    //~^ ERROR method `render_HTML` should have a snake case name such as `render_html`
 }
 
 trait X {
     fn ABC();
-    //~^ ERROR trait method `ABC` should have a snake case name such as `a_b_c`
+    //~^ ERROR trait method `ABC` should have a snake case name such as `abc`
 
     fn a_b_C(&self) {}
     //~^ ERROR trait method `a_b_C` should have a snake case name such as `a_b_c`