std: Replace `for` with `do { .. }` expr in std::gc

Change all users of old-style for with internal iterators to using
`do`-loops.

The code in stackwalk.rs does not actually implement the
looping protocol (no break on return false).

The code in gc.rs does not use loop breaks, nor does any code using it.

We remove the capacity to break from the loops in std::gc and implement
the walks using `do { .. }` expressions.

No behavior change.

2 files changed, 52 insertions, 62 deletions

diff --git a/src/libstd/gc.rs b/src/libstd/gc.rs
index ee270b553e6..9a8db6990b6 100644
--- a/src/libstd/gc.rs
+++ b/src/libstd/gc.rs
@@ -121,11 +121,11 @@ unsafe fn is_safe_point(pc: *Word) -> Option<SafePoint> {
     return None;
 }
 
-type Visitor<'self> = &'self fn(root: **Word, tydesc: *TyDesc) -> bool;
+type Visitor<'self> = &'self fn(root: **Word, tydesc: *TyDesc);
 
 // Walks the list of roots for the given safe point, and calls visitor
 // on each root.
-unsafe fn _walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) -> bool {
+unsafe fn _walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) {
     let fp_bytes = fp as *u8;
     let sp_meta = sp.sp_meta as *u32;
 
@@ -151,7 +151,7 @@ unsafe fn _walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) -> bool {
             } else {
                 ptr::null()
             };
-            if !visitor(root, tydesc) { return false; }
+            visitor(root, tydesc);
         }
         sri += 1;
     }
@@ -164,10 +164,9 @@ unsafe fn _walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) -> bool {
         }
         rri += 1;
     }
-    return true;
 }
 
-unsafe fn walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) -> bool {
+unsafe fn walk_safe_point(fp: *Word, sp: SafePoint, visitor: Visitor) {
     _walk_safe_point(fp, sp, visitor)
 }
 
@@ -223,7 +222,7 @@ static need_cleanup:    Memory = exchange_heap | stack;
 
 // Walks stack, searching for roots of the requested type, and passes
 // each root to the visitor.
-unsafe fn _walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) -> bool {
+unsafe fn _walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) {
     let mut segment = rustrt::rust_get_stack_segment();
     let mut last_ret: *Word = ptr::null();
     // To avoid collecting memory used by the GC itself, skip stack
@@ -231,7 +230,7 @@ unsafe fn _walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) -> boo
     // frame is marked by a sentinel, which is a box pointer stored on
     // the stack.
     let mut reached_sentinel = ptr::is_null(sentinel);
-    for walk_stack |frame| {
+    do walk_stack |frame| {
         let pc = last_ret;
         let Segment {segment: next_segment, boundary: boundary} =
             find_segment_for_frame(frame.fp, segment);
@@ -248,53 +247,46 @@ unsafe fn _walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) -> boo
         let ret_offset = if boundary { 4 } else { 1 };
         last_ret = *ptr::offset(frame.fp, ret_offset as int) as *Word;
 
-        if ptr::is_null(pc) {
-            loop;
-        }
-
-        let mut delay_reached_sentinel = reached_sentinel;
-        let sp = is_safe_point(pc);
-        match sp {
-          Some(sp_info) => {
-            for walk_safe_point(frame.fp, sp_info) |root, tydesc| {
-                // Skip roots until we see the sentinel.
-                if !reached_sentinel {
-                    if root == sentinel {
-                        delay_reached_sentinel = true;
-                    }
-                    loop;
-                }
-
-                // Skip null pointers, which can occur when a
-                // unique pointer has already been freed.
-                if ptr::is_null(*root) {
-                    loop;
-                }
-
-                if ptr::is_null(tydesc) {
-                    // Root is a generic box.
-                    let refcount = **root;
-                    if mem | task_local_heap != 0 && refcount != -1 {
-                        if !visitor(root, tydesc) { return false; }
-                    } else if mem | exchange_heap != 0 && refcount == -1 {
-                        if !visitor(root, tydesc) { return false; }
-                    }
-                } else {
-                    // Root is a non-immediate.
-                    if mem | stack != 0 {
-                        if !visitor(root, tydesc) { return false; }
+        if !ptr::is_null(pc) {
+
+            let mut delay_reached_sentinel = reached_sentinel;
+            let sp = is_safe_point(pc);
+            match sp {
+                Some(sp_info) => {
+                    do walk_safe_point(frame.fp, sp_info) |root, tydesc| {
+                        // Skip roots until we see the sentinel.
+                        if !reached_sentinel && root == sentinel {
+                            delay_reached_sentinel = true;
+                        }
+
+                        // Skip null pointers, which can occur when a
+                        // unique pointer has already been freed.
+                        if reached_sentinel && !ptr::is_null(*root) {
+                            if ptr::is_null(tydesc) {
+                                // Root is a generic box.
+                                let refcount = **root;
+                                if mem | task_local_heap != 0 && refcount != -1 {
+                                    visitor(root, tydesc);
+                                } else if mem | exchange_heap != 0 && refcount == -1 {
+                                    visitor(root, tydesc);
+                                }
+                            } else {
+                                // Root is a non-immediate.
+                                if mem | stack != 0 {
+                                    visitor(root, tydesc);
+                                }
+                            }
+                        }
                     }
                 }
+                None => ()
             }
-          }
-          None => ()
+            reached_sentinel = delay_reached_sentinel;
         }
-        reached_sentinel = delay_reached_sentinel;
     }
-    return true;
 }
 
-unsafe fn walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) -> bool {
+unsafe fn walk_gc_roots(mem: Memory, sentinel: **Word, visitor: Visitor) {
     _walk_gc_roots(mem, sentinel, visitor)
 }
 pub fn gc() {
@@ -304,7 +296,7 @@ pub fn gc() {
             return;
         }
 
-        for walk_gc_roots(task_local_heap, ptr::null()) |_root, _tydesc| {
+        do walk_gc_roots(task_local_heap, ptr::null()) |_root, _tydesc| {
             // FIXME(#2997): Walk roots and mark them.
             io::stdout().write([46]); // .
         }
@@ -349,18 +341,17 @@ pub fn cleanup_stack_for_failure() {
         };
 
         let mut roots = HashSet::new();
-        for walk_gc_roots(need_cleanup, sentinel) |root, tydesc| {
+        do walk_gc_roots(need_cleanup, sentinel) |root, tydesc| {
             // Track roots to avoid double frees.
-            if roots.contains(&*root) {
-                loop;
-            }
-            roots.insert(*root);
+            if !roots.contains(&*root) {
+                roots.insert(*root);
 
-            if ptr::is_null(tydesc) {
-                // FIXME #4420: Destroy this box
-                // FIXME #4330: Destroy this box
-            } else {
-                ((*tydesc).drop_glue)(*root as *i8);
+                if ptr::is_null(tydesc) {
+                    // FIXME #4420: Destroy this box
+                    // FIXME #4330: Destroy this box
+                } else {
+                    ((*tydesc).drop_glue)(*root as *i8);
+                }
             }
         }
     }
diff --git a/src/libstd/stackwalk.rs b/src/libstd/stackwalk.rs
index c3e3ca57a8e..cc516fb559e 100644
--- a/src/libstd/stackwalk.rs
+++ b/src/libstd/stackwalk.rs
@@ -25,7 +25,7 @@ pub fn Frame(fp: *Word) -> Frame {
     }
 }
 
-pub fn walk_stack(visit: &fn(Frame) -> bool) -> bool {
+pub fn walk_stack(visit: &fn(Frame)) {
 
     debug!("beginning stack walk");
 
@@ -51,12 +51,11 @@ pub fn walk_stack(visit: &fn(Frame) -> bool) -> bool {
             }
         }
     }
-    return true;
 }
 
 #[test]
 fn test_simple() {
-    for walk_stack |_frame| {
+    do walk_stack |_frame| {
     }
 }
 
@@ -65,7 +64,7 @@ fn test_simple_deep() {
     fn run(i: int) {
         if i == 0 { return }
 
-        for walk_stack |_frame| {
+        do walk_stack |_frame| {
             // Would be nice to test something here...
         }
         run(i - 1);