Suggest non-ambiguous comparison after cast

```
warning: `<` is interpreted as a start of generic arguments for `usize`, not comparison
  --> $DIR/issue-22644.rs:16:33
   |
16 |     println!("{}", a as usize < b);
   |                                 ^ expected one of `!`, `(`, `+`, `,`, `::`, or `>` here
   |
help: if you want to compare the casted value then write
   |     println!("{}", (a as usize) < b);
```

Author: estebank

src/librustc_errors/diagnostic.rs

@@ -248,6 +248,10 @@ impl Diagnostic {
         self.message.iter().map(|i| i.0.to_owned()).collect::<String>()
     }
 
+    pub fn set_message(&mut self, message: &str) {
+        self.message = vec![(message.to_owned(), Style::NoStyle)];
+    }
+
     pub fn styled_message(&self) -> &Vec<(String, Style)> {
         &self.message
     }

src/libsyntax/parse/parser.rs

@@ -42,7 +42,7 @@ use ast::RangeEnd;
 use {ast, attr};
 use codemap::{self, CodeMap, Spanned, respan};
 use syntax_pos::{self, Span, BytePos};
-use errors::{self, DiagnosticBuilder};
+use errors::{self, DiagnosticBuilder, Level};
 use parse::{self, classify, token};
 use parse::common::SeqSep;
 use parse::lexer::TokenAndSpan;
@@ -2840,7 +2840,24 @@ impl<'a> Parser<'a> {
                         let path = match self.parse_path_without_generics(PathStyle::Type) {
                             Ok(path) => {
                                 // Successfully parsed the type leaving a `<` yet to parse
-                                err.cancel();
+                                let codemap = self.sess.codemap();
+                                let suggestion_span = lhs_span.to(self.prev_span);
+                                let suggestion = match codemap.span_to_snippet(suggestion_span) {
+                                    Ok(lstring) => format!("({})", lstring),
+                                    _ => format!("(<expression>)")
+                                };
+                                let warn_message = match codemap.span_to_snippet(self.prev_span) {
+                                    Ok(lstring) => format!("`{}`", lstring),
+                                    _ => "a type".to_string(),
+                                };
+                                err.span_suggestion(suggestion_span,
+                                                    "if you want to compare the casted value then write",
+                                                    suggestion);
+                                err.level = Level::Warning;
+                                err.set_message(&format!("`<` is interpreted as a start of generic \
+                                                          arguments for {}, not a comparison",
+                                                          warn_message));
+                                err.emit();
                                 path
                             }
                             Err(mut path_err) => {

src/test/run-pass/issue-22644.rs renamed to src/test/ui/issue-22644.rs

@@ -0,0 +1,20 @@
+warning: `<` is interpreted as a start of generic arguments for `usize`, not comparison
+  --> $DIR/issue-22644.rs:16:33
+   |
+16 |     println!("{}", a as usize < b);
+   |                                 ^ expected one of `!`, `(`, `+`, `,`, `::`, or `>` here
+   |
+help: if you want to compare the casted value then write
+   |     println!("{}", (a as usize) < b);
+
+warning: `<` is interpreted as a start of generic arguments for `usize`, not comparison
+  --> $DIR/issue-22644.rs:17:33
+   |
+17 |     println!("{}", a as usize < 4);
+   |                                -^ unexpected token
+   |                                |
+   |                                expected one of `>`, identifier, lifetime, or type here
+   |
+help: if you want to compare the casted value then write
+   |     println!("{}", (a as usize) < 4);
+