Implement the notion of a "generated unsafe block"

This way syntax extensions can generate unsafe blocks without worrying about
them generating unnecessary unsafe warnings. Perhaps a special keyword could be
added to be used in macros, but I don't think that's the best solution.

src/librustc/middle/effect.rs

@@ -102,8 +102,10 @@ impl Visitor<()> for EffectCheckVisitor {
     fn visit_block(&mut self, block:&Block, _:()) {
 
             let old_unsafe_context = self.context.unsafe_context;
-            if block.rules == ast::UnsafeBlock &&
-                    self.context.unsafe_context == SafeContext {
+            let is_unsafe = match block.rules {
+                ast::UnsafeBlock(*) => true, ast::DefaultBlock => false
+            };
+            if is_unsafe && self.context.unsafe_context == SafeContext {
                 self.context.unsafe_context = UnsafeBlock(block.id)
             }
 

src/librustc/middle/lint.rs

@@ -1131,7 +1131,9 @@ impl Visitor<@mut Context> for UnusedUnsafeLintVisitor {
     fn visit_expr(&mut self, e:@ast::Expr, cx:@mut Context) {
 
             match e.node {
-                ast::ExprBlock(ref blk) if blk.rules == ast::UnsafeBlock => {
+                // Don't warn about generated blocks, that'll just pollute the
+                // output.
+                ast::ExprBlock(ref blk) if blk.rules == ast::UnsafeBlock(false) => {
                     if !cx.tcx.used_unsafe.contains(&blk.id) {
                         cx.span_lint(unused_unsafe, blk.span,
                                      "unnecessary `unsafe` block");

src/librustc/middle/typeck/check/mod.rs

@@ -200,7 +200,7 @@ impl PurityState {
 
             purity => {
                 let (purity, def) = match blk.rules {
-                    ast::UnsafeBlock => (ast::unsafe_fn, blk.id),
+                    ast::UnsafeBlock(*) => (ast::unsafe_fn, blk.id),
                     ast::DefaultBlock => (purity, self.def),
                 };
                 PurityState{ def: def,

src/libsyntax/ast.rs

@@ -479,7 +479,7 @@ pub struct Field {
 #[deriving(Clone, Eq, Encodable, Decodable, IterBytes)]
 pub enum BlockCheckMode {
     DefaultBlock,
-    UnsafeBlock,
+    UnsafeBlock(/* generated internally */ bool),
 }
 
 #[deriving(Clone, Eq, Encodable, Decodable,IterBytes)]

src/libsyntax/parse/parser.rs

@@ -1792,7 +1792,7 @@ impl Parser {
         } else if self.eat_keyword(keywords::Match) {
             return self.parse_match_expr();
         } else if self.eat_keyword(keywords::Unsafe) {
-            return self.parse_block_expr(lo, UnsafeBlock);
+            return self.parse_block_expr(lo, UnsafeBlock(false));
         } else if *self.token == token::LBRACKET {
             self.bump();
             let mutbl = self.parse_mutability();

src/libsyntax/print/pprust.rs

@@ -951,7 +951,7 @@ pub fn print_possibly_embedded_block_(s: @ps,
                                       attrs: &[ast::Attribute],
                                       close_box: bool) {
     match blk.rules {
-      ast::UnsafeBlock => word_space(s, "unsafe"),
+      ast::UnsafeBlock(*) => word_space(s, "unsafe"),
       ast::DefaultBlock => ()
     }
     maybe_print_comment(s, blk.span.lo);