diff --git a/src/libcore/unstable/lang.rs b/src/libcore/unstable/lang.rs
index 017fc4b7b63..d25147fcde1 100644
--- a/src/libcore/unstable/lang.rs
+++ b/src/libcore/unstable/lang.rs
@@ -133,7 +133,7 @@ pub unsafe fn exchange_malloc(td: *c_char, size: uintptr_t) -> *c_char {
 
 /// Because this code is so perf. sensitive, use a static constant so that
 /// debug printouts are compiled out most of the time.
-static ENABLE_DEBUG_PTR: bool = false;
+static ENABLE_DEBUG_PTR: bool = true;
 
 #[inline]
 pub fn debug_ptr<T>(tag: &'static str, p: *const T) {
diff --git a/src/libcore/util.rs b/src/libcore/util.rs
index a08e38c021f..43616ebfd30 100644
--- a/src/libcore/util.rs
+++ b/src/libcore/util.rs
@@ -26,19 +26,20 @@ pub fn ignore<T>(_x: T) { }
 
 /// Sets `*ptr` to `new_value`, invokes `op()`, and then restores the
 /// original value of `*ptr`.
+///
+/// NB: This function accepts `@mut T` and not `&mut T` to avoid
+/// an obvious borrowck hazard. Typically passing in `&mut T` will
+/// cause borrow check errors because it freezes whatever location
+/// that `&mut T` is stored in (either statically or dynamically).
 #[inline(always)]
-pub fn with<T:Copy,R>(
-    ptr: &mut T,
-    new_value: T,
+pub fn with<T,R>(
+    ptr: @mut T,
+    mut value: T,
     op: &fn() -> R) -> R
 {
-    // NDM: if swap operator were defined somewhat differently,
-    // we wouldn't need to copy...
-
-    let old_value = *ptr;
-    *ptr = new_value;
+    value <-> *ptr;
     let result = op();
-    *ptr = old_value;
+    *ptr = value;
     return result;
 }
 
diff --git a/src/librustc/middle/liveness.rs b/src/librustc/middle/liveness.rs
index 2e2c92abcdc..59a6e6469e2 100644
--- a/src/librustc/middle/liveness.rs
+++ b/src/librustc/middle/liveness.rs
@@ -112,7 +112,6 @@ use util::ppaux::ty_to_str;
 
 use core::cast::transmute;
 use core::hashmap::HashMap;
-use core::util::with;
 use syntax::ast::*;
 use syntax::codemap::span;
 use syntax::parse::token::special_idents;
@@ -343,9 +342,10 @@ pub impl IrMaps {
 }
 
 fn visit_item(item: @item, self: @mut IrMaps, v: vt<@mut IrMaps>) {
-    do with(&mut self.cur_item, item.id) {
-        visit::visit_item(item, self, v)
-    }
+    let old_cur_item = self.cur_item;
+    self.cur_item = item.id;
+    visit::visit_item(item, self, v);
+    self.cur_item = old_cur_item;
 }
 
 fn visit_fn(fk: &visit::fn_kind,
@@ -762,11 +762,13 @@ pub impl Liveness {
             None => {
                 // Vanilla 'break' or 'loop', so use the enclosing
                 // loop scope
-                let loop_scope = &mut *self.loop_scope;
-                if loop_scope.len() == 0 {
+                let len = { // FIXME(#5074) stage0
+                    let loop_scope = &mut *self.loop_scope;
+                    loop_scope.len()
+                };
+                if len == 0 {
                     self.tcx.sess.span_bug(sp, ~"break outside loop");
-                }
-                else {
+                } else {
                     // FIXME(#5275): this shouldn't have to be a method...
                     self.last_loop_scope()
                 }
diff --git a/src/librustc/middle/region.rs b/src/librustc/middle/region.rs
index ea21ab0527b..06eb2542235 100644
--- a/src/librustc/middle/region.rs
+++ b/src/librustc/middle/region.rs
@@ -949,7 +949,8 @@ pub fn determine_rp_in_crate(sess: Session,
         let cx = &mut *cx;
         while cx.worklist.len() != 0 {
             let c_id = cx.worklist.pop();
-            let c_variance = *cx.region_paramd_items.get(&c_id);
+            let c_variance = { *cx.region_paramd_items.get(&c_id) };
+            // NOTE cleanup scopes cause an exaggerated lock here
             debug!("popped %d from worklist", c_id);
             match cx.dep_map.find(&c_id) {
               None => {}
diff --git a/src/librustc/middle/resolve.rs b/src/librustc/middle/resolve.rs
index ff46abaf712..8e83ea7e32e 100644
--- a/src/librustc/middle/resolve.rs
+++ b/src/librustc/middle/resolve.rs
@@ -779,9 +779,9 @@ pub fn Resolver(session: Session,
         unresolved_imports: 0,
 
         current_module: current_module,
-        value_ribs: ~[],
-        type_ribs: ~[],
-        label_ribs: ~[],
+        value_ribs: @mut ~[],
+        type_ribs: @mut ~[],
+        label_ribs: @mut ~[],
 
         xray_context: NoXray,
         current_trait_refs: None,
@@ -830,13 +830,13 @@ pub struct Resolver {
 
     // The current set of local scopes, for values.
     // FIXME #4948: Reuse ribs to avoid allocation.
-    value_ribs: ~[@Rib],
+    value_ribs: @mut ~[@Rib],
 
     // The current set of local scopes, for types.
-    type_ribs: ~[@Rib],
+    type_ribs: @mut ~[@Rib],
 
     // The current set of local scopes, for labels.
-    label_ribs: ~[@Rib],
+    label_ribs: @mut ~[@Rib],
 
     // Whether the current context is an X-ray context. An X-ray context is
     // allowed to access private names of any module.
@@ -4313,19 +4313,18 @@ pub impl Resolver {
                 }
 
                 pat_struct(path, _, _) => {
-                    let structs: &mut HashSet<def_id> = &mut self.structs;
                     match self.resolve_path(path, TypeNS, false, visitor) {
                         Some(def_ty(class_id))
-                                if structs.contains(&class_id) => {
+                                if self.structs.contains(&class_id) => {
                             let class_def = def_struct(class_id);
                             self.record_def(pattern.id, class_def);
                         }
-                        Some(definition @ def_struct(class_id))
-                                if structs.contains(&class_id) => {
+                        Some(definition @ def_struct(class_id)) => {
+                            assert!(self.structs.contains(&class_id));
                             self.record_def(pattern.id, definition);
                         }
                         Some(definition @ def_variant(_, variant_id))
-                                if structs.contains(&variant_id) => {
+                                if self.structs.contains(&variant_id) => {
                             self.record_def(pattern.id, definition);
                         }
                         result => {
@@ -4627,12 +4626,12 @@ pub impl Resolver {
         let search_result;
         match namespace {
             ValueNS => {
-                search_result = self.search_ribs(&mut self.value_ribs, ident,
+                search_result = self.search_ribs(self.value_ribs, ident,
                                                  span,
                                                  DontAllowCapturingSelf);
             }
             TypeNS => {
-                search_result = self.search_ribs(&mut self.type_ribs, ident,
+                search_result = self.search_ribs(self.type_ribs, ident,
                                                  span, AllowCapturingSelf);
             }
         }
@@ -4822,15 +4821,14 @@ pub impl Resolver {
 
             expr_struct(path, _, _) => {
                 // Resolve the path to the structure it goes to.
-                let structs: &mut HashSet<def_id> = &mut self.structs;
                 match self.resolve_path(path, TypeNS, false, visitor) {
                     Some(def_ty(class_id)) | Some(def_struct(class_id))
-                            if structs.contains(&class_id) => {
+                            if self.structs.contains(&class_id) => {
                         let class_def = def_struct(class_id);
                         self.record_def(expr.id, class_def);
                     }
                     Some(definition @ def_variant(_, class_id))
-                            if structs.contains(&class_id) => {
+                            if self.structs.contains(&class_id) => {
                         self.record_def(expr.id, definition);
                     }
                     _ => {
@@ -4856,7 +4854,7 @@ pub impl Resolver {
             }
 
             expr_break(Some(label)) | expr_again(Some(label)) => {
-                match self.search_ribs(&mut self.label_ribs, label, expr.span,
+                match self.search_ribs(self.label_ribs, label, expr.span,
                                        DontAllowCapturingSelf) {
                     None =>
                         self.session.span_err(expr.span,
diff --git a/src/librustc/middle/typeck/check/vtable.rs b/src/librustc/middle/typeck/check/vtable.rs
index 44b62122612..c177d5ab0eb 100644
--- a/src/librustc/middle/typeck/check/vtable.rs
+++ b/src/librustc/middle/typeck/check/vtable.rs
@@ -244,11 +244,14 @@ fn lookup_vtable(vcx: &VtableContext,
                     // Nothing found. Continue.
                 }
                 Some(implementations) => {
-                    let implementations: &mut ~[@Impl] = *implementations;
+                    let len = { // FIXME(#5074): stage0 requires it
+                        let implementations: &mut ~[@Impl] = *implementations;
+                        implementations.len()
+                    };
 
                     // implementations is the list of all impls in scope for
                     // trait_ref. (Usually, there's just one.)
-                    for uint::range(0, implementations.len()) |i| {
+                    for uint::range(0, len) |i| {
                         let im = implementations[i];
 
                         // im is one specific impl of trait_ref.
diff --git a/src/librustc/middle/typeck/coherence.rs b/src/librustc/middle/typeck/coherence.rs
index 573e4bd5790..d779c20b3e8 100644
--- a/src/librustc/middle/typeck/coherence.rs
+++ b/src/librustc/middle/typeck/coherence.rs
@@ -240,8 +240,8 @@ pub impl CoherenceChecker {
 
     fn check_implementation(&self,
                             item: @item, associated_traits: ~[@trait_ref]) {
-        let self_type = self.crate_context.tcx.tcache.get(
-            &local_def(item.id));
+        let tcx = self.crate_context.tcx;
+        let self_type = ty::lookup_item_type(tcx, local_def(item.id));
 
         // If there are no traits, then this implementation must have a
         // base type.
@@ -452,10 +452,8 @@ pub impl CoherenceChecker {
     }
 
     fn check_implementation_coherence(&self) {
-        let coherence_info = &mut self.crate_context.coherence_info;
-        let extension_methods = &coherence_info.extension_methods;
-
-        for extension_methods.each_key |&trait_id| {
+        let coherence_info = self.crate_context.coherence_info;
+        for coherence_info.extension_methods.each_key |&trait_id| {
             self.check_implementation_coherence_of(trait_id);
         }
     }
@@ -514,13 +512,16 @@ pub impl CoherenceChecker {
     }
 
     fn iter_impls_of_trait(&self, trait_def_id: def_id, f: &fn(@Impl)) {
-        let coherence_info = &mut self.crate_context.coherence_info;
-        let extension_methods = &coherence_info.extension_methods;
+        let coherence_info = self.crate_context.coherence_info;
+        let extension_methods = &*coherence_info.extension_methods;
 
         match extension_methods.find(&trait_def_id) {
             Some(impls) => {
-                let impls: &mut ~[@Impl] = *impls;
-                for uint::range(0, impls.len()) |i| {
+                let len = { // FIXME(#5074) stage0 requires this
+                    let impls: &mut ~[@Impl] = *impls;
+                    impls.len()
+                };
+                for uint::range(0, len) |i| {
                     f(impls[i]);
                 }
             }
@@ -1014,7 +1015,7 @@ pub impl CoherenceChecker {
     //
 
     fn populate_destructor_table(&self) {
-        let coherence_info = &mut self.crate_context.coherence_info;
+        let coherence_info = self.crate_context.coherence_info;
         let tcx = self.crate_context.tcx;
         let drop_trait = tcx.lang_items.drop_trait();
         let impls_opt = coherence_info.extension_methods.find(&drop_trait);
diff --git a/src/librustc/middle/typeck/infer/glb.rs b/src/librustc/middle/typeck/infer/glb.rs
index 2bbcd24595c..b818f666444 100644
--- a/src/librustc/middle/typeck/infer/glb.rs
+++ b/src/librustc/middle/typeck/infer/glb.rs
@@ -16,6 +16,7 @@ use middle::typeck::infer::lub::Lub;
 use middle::typeck::infer::sub::Sub;
 use middle::typeck::infer::to_str::InferStr;
 use middle::typeck::infer::{cres, InferCtxt};
+use middle::typeck::infer::fold_regions_in_sig;
 use middle::typeck::isr_alist;
 use syntax::ast;
 use syntax::ast::{Many, Once, extern_fn, impure_fn, m_const, m_imm, m_mutbl};
@@ -188,7 +189,8 @@ impl Combine for Glb {
         let new_vars =
             self.infcx.region_vars.vars_created_since_snapshot(snapshot);
         let sig1 =
-            self.infcx.fold_regions_in_sig(
+            fold_regions_in_sig(
+                self.infcx.tcx,
                 &sig0,
                 |r, _in_fn| generalize_region(self, snapshot,
                                               new_vars, a_isr, a_vars, b_vars,
diff --git a/src/librustc/middle/typeck/infer/lub.rs b/src/librustc/middle/typeck/infer/lub.rs
index 85914338017..34e006c9615 100644
--- a/src/librustc/middle/typeck/infer/lub.rs
+++ b/src/librustc/middle/typeck/infer/lub.rs
@@ -16,6 +16,7 @@ use middle::typeck::infer::lattice::*;
 use middle::typeck::infer::sub::Sub;
 use middle::typeck::infer::to_str::InferStr;
 use middle::typeck::infer::{cres, InferCtxt};
+use middle::typeck::infer::fold_regions_in_sig;
 use middle::typeck::isr_alist;
 use util::common::indent;
 use util::ppaux::mt_to_str;
@@ -141,7 +142,8 @@ impl Combine for Lub {
         let new_vars =
             self.infcx.region_vars.vars_created_since_snapshot(snapshot);
         let sig1 =
-            self.infcx.fold_regions_in_sig(
+            fold_regions_in_sig(
+                self.infcx.tcx,
                 &sig0,
                 |r, _in_fn| generalize_region(self, snapshot, new_vars,
                                               a_isr, r));
diff --git a/src/librustc/middle/typeck/infer/mod.rs b/src/librustc/middle/typeck/infer/mod.rs
index 4491b04b382..899b8cfd7ed 100644
--- a/src/librustc/middle/typeck/infer/mod.rs
+++ b/src/librustc/middle/typeck/infer/mod.rs
@@ -574,7 +574,7 @@ pub impl InferCtxt {
     }
 
     /// Execute `f` and commit the bindings if successful
-    fn commit<T,E>(&mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
+    fn commit<T,E>(@mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
         assert!(!self.in_snapshot());
 
         debug!("commit()");
@@ -589,7 +589,7 @@ pub impl InferCtxt {
     }
 
     /// Execute `f`, unroll bindings on failure
-    fn try<T,E>(&mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
+    fn try<T,E>(@mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
         debug!("try()");
         do indent {
             let snapshot = self.start_snapshot();
@@ -603,7 +603,7 @@ pub impl InferCtxt {
     }
 
     /// Execute `f` then unroll any bindings it creates
-    fn probe<T,E>(&mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
+    fn probe<T,E>(@mut self, f: &fn() -> Result<T,E>) -> Result<T,E> {
         debug!("probe()");
         do indent {
             let snapshot = self.start_snapshot();
@@ -783,15 +783,14 @@ pub impl InferCtxt {
             });
         (fn_sig, isr)
     }
-
-    fn fold_regions_in_sig(
-        &mut self,
-        fn_sig: &ty::FnSig,
-        fldr: &fn(r: ty::Region, in_fn: bool) -> ty::Region) -> ty::FnSig
-    {
-        do ty::fold_sig(fn_sig) |t| {
-            ty::fold_regions(self.tcx, t, fldr)
-        }
-    }
-
 }
+
+pub fn fold_regions_in_sig(
+    tcx: ty::ctxt,
+    fn_sig: &ty::FnSig,
+    fldr: &fn(r: ty::Region, in_fn: bool) -> ty::Region) -> ty::FnSig
+{
+    do ty::fold_sig(fn_sig) |t| {
+        ty::fold_regions(tcx, t, fldr)
+    }
+}
\ No newline at end of file
diff --git a/src/librustc/rustc.rc b/src/librustc/rustc.rc
index 9e34d5c6177..1ecb38854c8 100644
--- a/src/librustc/rustc.rc
+++ b/src/librustc/rustc.rc
@@ -76,6 +76,9 @@ pub mod middle {
     }
     pub mod ty;
     pub mod subst;
+    #[cfg(stage0)] #[path = "resolve_stage0.rs"]
+    pub mod resolve;
+    #[cfg(not(stage0))]
     pub mod resolve;
     #[path = "typeck/mod.rs"]
     pub mod typeck;
diff --git a/src/libsyntax/codemap.rs b/src/libsyntax/codemap.rs
index 7facc181eff..1c822b520f6 100644
--- a/src/libsyntax/codemap.rs
+++ b/src/libsyntax/codemap.rs
@@ -355,7 +355,7 @@ pub impl CodeMap {
     }
 
     pub fn span_to_str(&self, sp: span) -> ~str {
-        let files = &mut *self.files;
+        let files = &*self.files;
         if files.len() == 0 && sp == dummy_sp() {
             return ~"no-location";
         }