Mark allowed dead code and lang items as live

Dead code pass now explicitly checks for `#[allow(dead_code)]` and
`#[lang=".."]` attributes on items and marks them as live if they have
those attributes. The former is done so that if we want to suppress
warnings for a group of dead functions, we only have to annotate the
"root" of the call chain.

3 files changed, 80 insertions, 13 deletions

diff --git a/src/librustc/middle/dead.rs b/src/librustc/middle/dead.rs
index c4feeabe516..f6cb635d01e 100644
--- a/src/librustc/middle/dead.rs
+++ b/src/librustc/middle/dead.rs
@@ -12,20 +12,23 @@
 // closely. The idea is that all reachable symbols are live, codes called
 // from live codes are live, and everything else is dead.
 
+use middle::lint::{allow, contains_lint, DeadCode};
+use middle::privacy;
 use middle::ty;
 use middle::typeck;
-use middle::privacy;
-use middle::lint::DeadCode;
 
 use std::hashmap::HashSet;
 use syntax::ast;
 use syntax::ast_map;
 use syntax::ast_util::{local_def, def_id_of_def, is_local};
+use syntax::attr;
 use syntax::codemap;
 use syntax::parse::token;
 use syntax::visit::Visitor;
 use syntax::visit;
 
+pub static DEAD_CODE_LINT_STR: &'static str = "dead_code";
+
 // Any local node that may call something in its body block should be
 // explored. For example, if it's a live NodeItem that is a
 // function, then we should explore its block to check for codes that
@@ -196,26 +199,57 @@ impl Visitor<()> for MarkSymbolVisitor {
     }
 }
 
-// This visitor is used to mark the implemented methods of a trait. Since we
-// can not be sure if such methods are live or dead, we simply mark them
-// as live.
-struct TraitMethodSeeder {
+fn has_allow_dead_code_or_lang_attr(attrs: &[ast::Attribute]) -> bool {
+    contains_lint(attrs, allow, DEAD_CODE_LINT_STR)
+    || attr::contains_name(attrs, "lang")
+}
+
+// This visitor seeds items that
+//   1) We want to explicitly consider as live:
+//     * Item annotated with #[allow(dead_code)]
+//         - This is done so that if we want to suppress warnings for a
+//           group of dead functions, we only have to annotate the "root".
+//           For example, if both `f` and `g` are dead and `f` calls `g`,
+//           then annotating `f` with `#[allow(dead_code)]` will suppress
+//           warning for both `f` and `g`.
+//     * Item annotated with #[lang=".."]
+//         - This is because lang items are always callable from elsewhere.
+//   or
+//   2) We are not sure to be live or not
+//     * Implementation of a trait method
+struct LifeSeeder {
     worklist: ~[ast::NodeId],
 }
 
-impl Visitor<()> for TraitMethodSeeder {
+impl Visitor<()> for LifeSeeder {
     fn visit_item(&mut self, item: &ast::Item, _: ()) {
+        if has_allow_dead_code_or_lang_attr(item.attrs) {
+            self.worklist.push(item.id);
+        }
         match item.node {
             ast::ItemImpl(_, Some(ref _trait_ref), _, ref methods) => {
                 for method in methods.iter() {
                     self.worklist.push(method.id);
                 }
             }
-            ast::ItemMod(..) | ast::ItemFn(..) => {
-                visit::walk_item(self, item, ());
+            _ => ()
+        }
+        visit::walk_item(self, item, ());
+    }
+
+    fn visit_fn(&mut self, fk: &visit::FnKind,
+                _: &ast::FnDecl, block: &ast::Block,
+                _: codemap::Span, id: ast::NodeId, _: ()) {
+        // Check for method here because methods are not ast::Item
+        match *fk {
+            visit::FkMethod(_, _, method) => {
+                if has_allow_dead_code_or_lang_attr(method.attrs) {
+                    self.worklist.push(id);
+                }
             }
             _ => ()
         }
+        visit::walk_block(self, block, ());
     }
 }
 
@@ -244,12 +278,12 @@ fn create_and_seed_worklist(tcx: ty::ctxt,
     }
 
     // Seed implemeneted trait methods
-    let mut trait_method_seeder = TraitMethodSeeder {
+    let mut life_seeder = LifeSeeder {
         worklist: worklist
     };
-    visit::walk_crate(&mut trait_method_seeder, crate, ());
+    visit::walk_crate(&mut life_seeder, crate, ());
 
-    return trait_method_seeder.worklist;
+    return life_seeder.worklist;
 }
 
 fn find_live(tcx: ty::ctxt,
diff --git a/src/librustc/middle/lint.rs b/src/librustc/middle/lint.rs
index 55cb9f87bcd..046ec557a87 100644
--- a/src/librustc/middle/lint.rs
+++ b/src/librustc/middle/lint.rs
@@ -34,6 +34,7 @@
 //! Context itself, span_lint should be used instead of add_lint.
 
 use driver::session;
+use middle::dead::DEAD_CODE_LINT_STR;
 use middle::privacy;
 use middle::trans::adt; // for `adt::is_ffi_safe`
 use middle::ty;
@@ -293,7 +294,7 @@ static lint_table: &'static [(&'static str, LintSpec)] = &[
         default: warn
     }),
 
-    ("dead_code",
+    (DEAD_CODE_LINT_STR,
      LintSpec {
         lint: DeadCode,
         desc: "detect piece of code that will never be used",
@@ -531,6 +532,8 @@ impl<'a> Context<'a> {
     }
 }
 
+// Check that every lint from the list of attributes satisfies `f`.
+// Return true if that's the case. Otherwise return false.
 pub fn each_lint(sess: session::Session,
                  attrs: &[ast::Attribute],
                  f: |@ast::MetaItem, level, @str| -> bool)
@@ -564,6 +567,25 @@ pub fn each_lint(sess: session::Session,
     true
 }
 
+// Check from a list of attributes if it contains the appropriate
+// `#[level(lintname)]` attribute (e.g. `#[allow(dead_code)]).
+pub fn contains_lint(attrs: &[ast::Attribute],
+                    level: level, lintname: &'static str) -> bool {
+    let level_name = level_to_str(level);
+    for attr in attrs.iter().filter(|m| level_name == m.name()) {
+        if attr.meta_item_list().is_none() {
+            continue
+        }
+        let list = attr.meta_item_list().unwrap();
+        for meta_item in list.iter() {
+            if lintname == meta_item.name() {
+                return true;
+            }
+        }
+    }
+    false
+}
+
 fn check_while_true_expr(cx: &Context, e: &ast::Expr) {
     match e.node {
         ast::ExprWhile(cond, _) => {
diff --git a/src/test/compile-fail/lint-dead-code-1.rs b/src/test/compile-fail/lint-dead-code-1.rs
index b029866060d..8a9397b0227 100644
--- a/src/test/compile-fail/lint-dead-code-1.rs
+++ b/src/test/compile-fail/lint-dead-code-1.rs
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[no_std];
 #[allow(unused_variable)];
 #[deny(dead_code)];
 
@@ -85,3 +86,13 @@ fn foo() { //~ ERROR: code is never used
 fn bar() { //~ ERROR: code is never used
     foo();
 }
+
+// Code with #[allow(dead_code)] should be marked live (and thus anything it
+// calls is marked live)
+#[allow(dead_code)]
+fn g() { h(); }
+fn h() {}
+
+// Similarly, lang items are live
+#[lang="fail_"]
+fn fail(_: *u8, _: *u8, _: uint) -> ! { loop {} }