auto merge of #6670 : cmr/rust/static_assert, r=graydon

This verifies that a static item evaluates to true, at compile time.

doc/rust.md

@@ -1426,6 +1426,7 @@ names are effectively reserved. Some significant attributes include:
 by the compiler can be found via `rustc -W help`.
 * The `deriving` attribute, for automatically generating
   implementations of certain traits.
+* The `static_assert` attribute, for asserting that a static bool is true at compiletime
 
 Other attributes may be added or removed during development of the language.
 

src/librustc/middle/trans/base.rs

@@ -2147,7 +2147,26 @@ pub fn trans_item(ccx: @CrateContext, item: &ast::item) {
             trans_enum_def(ccx, enum_definition, item.id, vi, &mut i);
         }
       }
-      ast::item_const(_, expr) => consts::trans_const(ccx, expr, item.id),
+      ast::item_const(_, expr) => {
+          consts::trans_const(ccx, expr, item.id);
+          // Do static_assert checking. It can't really be done much earlier because we need to get
+          // the value of the bool out of LLVM
+          for item.attrs.each |attr| {
+              match attr.node.value.node {
+                  ast::meta_word(x) => {
+                      if x.slice(0, x.len()) == "static_assert" {
+                          let v = ccx.const_values.get_copy(&item.id);
+                          unsafe {
+                              if !(llvm::LLVMConstIntGetZExtValue(v) as bool) {
+                                  ccx.sess.span_fatal(expr.span, "static assertion failed");
+                              }
+                          }
+                      }
+                  },
+                  _ => ()
+              }
+          }
+      },
       ast::item_foreign_mod(ref foreign_mod) => {
         foreign::trans_foreign_mod(ccx, path, foreign_mod);
       }

src/test/compile-fail/static-assert.rs

@@ -0,0 +1,5 @@
+#[static_assert]
+static a: bool = false; //~ ERROR static assertion failed
+
+fn main() {
+}

src/test/compile-fail/static-assert2.rs

@@ -0,0 +1,4 @@
+#[static_assert]
+static e: bool = 1 == 2; //~ ERROR static assertion failed
+
+fn main() {}

src/test/run-pass/static-assert.rs

@@ -0,0 +1,14 @@
+#[static_assert]
+static b: bool = true;
+
+#[static_assert]
+static c: bool = 1 == 1;
+
+#[static_assert]
+static d: bool = 1 != 2;
+
+#[static_assert]
+static f: bool = (4/2) == 2;
+
+fn main() {
+}