Lowercase DOM getters at compile time, fixes #4728

The implementation was copied directly from
https://github.com/rust-lang/rust/pull/16636
and updated for rust changes, so the credit goes to @Manishearth

components/plugins/casing.rs

@@ -0,0 +1,60 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use syntax::ext::base::ExtCtxt;
+use syntax::ext::build::AstBuilder;
+use syntax::codemap::Span;
+use syntax::ast;
+use syntax::ext::base;
+use syntax::parse::token;
+
+pub fn expand_lower<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
+                        -> Box<base::MacResult + 'cx> {
+    expand_cased(cx, sp, tts, |c| { c.to_lowercase() })
+}
+
+pub fn expand_upper<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
+                        -> Box<base::MacResult + 'cx> {
+    expand_cased(cx, sp, tts, |c| { c.to_uppercase() })
+}
+
+fn expand_cased<'cx, T>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree], transform: T)
+                        -> Box<base::MacResult + 'cx>
+    where T: Fn(char) -> char
+{
+    let es = match base::get_exprs_from_tts(cx, sp, tts) {
+        Some(e) => e,
+        None => return base::DummyResult::expr(sp)
+    };
+
+    let mut it = es.iter();
+    let res = if let Some(expr) = it.next() {
+        if let ast::ExprLit(ref lit) = expr.node {
+            if let ast::LitStr(ref s, _) = lit.node {
+                Some((s, lit.span))
+            } else {
+                cx.span_err(expr.span, "expected a string literal");
+                None
+            }
+        } else {
+            cx.span_err(expr.span, "expected a string literal");
+            None
+        }
+    } else {
+        cx.span_err(sp, "expected 1 argument, found 0");
+        None
+    };
+    match (res, it.count()) {
+        (Some((s, span)), 0) => {
+            let new_s = s.get().chars().map(transform).collect::<String>();
+            base::MacExpr::new(cx.expr_str(span, token::intern_and_get_ident(new_s.as_slice())))
+        }
+        (_, rest) => {
+            if rest > 0 {
+                cx.span_err(sp, format!("expected 1 argument, found {}", rest+1).as_slice());
+            }
+            base::DummyResult::expr(sp)
+        }
+    }
+}

components/plugins/lib.rs

@@ -40,12 +40,15 @@ pub mod reflector;
 pub mod lints;
 /// Utilities for writing plugins
 pub mod utils;
+pub mod casing;
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_syntax_extension(intern("dom_struct"), Modifier(box jstraceable::expand_dom_struct));
     reg.register_syntax_extension(intern("jstraceable"), Decorator(box jstraceable::expand_jstraceable));
     reg.register_syntax_extension(intern("_generate_reflector"), Decorator(box reflector::expand_reflector));
+    reg.register_macro("to_lower", casing::expand_lower);
+    reg.register_macro("to_upper", casing::expand_upper);
     reg.register_lint_pass(box lints::transmute_type::TransmutePass as LintPassObject);
     reg.register_lint_pass(box lints::unrooted_must_root::UnrootedPass as LintPassObject);
     reg.register_lint_pass(box lints::privatize::PrivatizePass as LintPassObject);