Use Cow to handle modifications of projection in preparation for interning

Author: spastorino

src/librustc/mir/visit.rs

@@ -792,26 +792,37 @@ macro_rules! visit_place_fns {
         ) {
             self.visit_place_base(&mut place.base, context, location);
 
-            place.projection = self.process_projection(&place.projection);
+            if let Some(new_projection) = self.process_projection(&place.projection) {
+                place.projection = new_projection;
+            }
         }
 
         fn process_projection(
             &mut self,
-            projection: &Box<[PlaceElem<'tcx>]>,
-        ) -> Box<[PlaceElem<'tcx>]> {
-            let new_projection: Vec<_> = projection.iter().map(|elem|
-                self.process_projection_elem(elem)
-            ).collect();
+            projection: &'a [PlaceElem<'tcx>],
+        ) -> Option<Box<[PlaceElem<'tcx>]>> {
+            let mut projection = Cow::Borrowed(projection);
+
+            for i in 0..projection.len() {
+                if let Some(elem) = projection.get(i) {
+                    if let Cow::Owned(elem) = self.process_projection_elem(Cow::Borrowed(elem)) {
+                        let vec = projection.to_mut();
+                        vec[i] = elem;
+                    }
+                }
+            }
 
-            new_projection.into_boxed_slice()
+            match projection {
+                Cow::Borrowed(_) => None,
+                Cow::Owned(vec) => Some(vec.into_boxed_slice()),
+            }
         }
 
         fn process_projection_elem(
             &mut self,
-            elem: &PlaceElem<'tcx>,
-        ) -> PlaceElem<'tcx> {
-            // FIXME: avoid cloning here
-            elem.clone()
+            elem: Cow<'a, PlaceElem<'tcx>>,
+        ) -> Cow<'a, PlaceElem<'tcx>> {
+            elem
         }
     );
 

src/librustc_mir/borrow_check/nll/renumber.rs

@@ -4,6 +4,7 @@ use rustc::mir::{Body, Location, PlaceElem, Promoted};
 use rustc::mir::visit::{MutVisitor, TyContext};
 use rustc::infer::{InferCtxt, NLLRegionVariableOrigin};
 use rustc_index::vec::IndexVec;
+use std::borrow::Cow;
 
 /// Replaces all free regions appearing in the MIR with fresh
 /// inference variables, returning the number of variables created.
@@ -64,12 +65,13 @@ impl<'a, 'tcx> MutVisitor<'tcx> for NLLVisitor<'a, 'tcx> {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
-        if let PlaceElem::Field(field, ty) = elem {
-            PlaceElem::Field(*field, self.renumber_regions(ty))
-        } else {
-            elem.clone()
+        elem: Cow<'b, PlaceElem<'tcx>>,
+    ) -> Cow<'b, PlaceElem<'tcx>> {
+        match elem {
+            Cow::Borrowed(PlaceElem::Field(field, ty)) => {
+                Cow::Owned(PlaceElem::Field(*field, self.renumber_regions(ty)))
+            }
+            _ => elem,
         }
     }
 

src/librustc_mir/transform/erase_regions.rs

@@ -9,6 +9,7 @@ use rustc::ty::{self, Ty, TyCtxt};
 use rustc::mir::*;
 use rustc::mir::visit::{MutVisitor, TyContext};
 use crate::transform::{MirPass, MirSource};
+use std::borrow::Cow;
 
 struct EraseRegionsVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
@@ -41,12 +42,13 @@ impl MutVisitor<'tcx> for EraseRegionsVisitor<'tcx> {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
-        if let PlaceElem::Field(field, ty) = elem {
-            PlaceElem::Field(*field, self.tcx.erase_regions(ty))
-        } else {
-            elem.clone()
+        elem: Cow<'a, PlaceElem<'tcx>>,
+    ) -> Cow<'a, PlaceElem<'tcx>> {
+        match elem {
+            Cow::Borrowed(PlaceElem::Field(field, ty)) => {
+                Cow::Owned(PlaceElem::Field(*field, self.tcx.erase_regions(ty)))
+            }
+            _ => elem,
         }
     }
 }

src/librustc_mir/transform/inline.rs

@@ -11,6 +11,7 @@ use rustc::mir::visit::*;
 use rustc::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
 use rustc::ty::subst::{Subst, SubstsRef};
 
+use std::borrow::Cow;
 use std::collections::VecDeque;
 use std::iter;
 use crate::transform::{MirPass, MirSource};
@@ -702,12 +703,13 @@ impl<'a, 'tcx> MutVisitor<'tcx> for Integrator<'a, 'tcx> {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
-        if let PlaceElem::Index(local) = elem {
-            PlaceElem::Index(self.make_integrate_local(local))
-        } else {
-            elem.clone()
+        elem: Cow<'b, PlaceElem<'tcx>>,
+    ) -> Cow<'b, PlaceElem<'tcx>> {
+        match elem {
+            Cow::Borrowed(PlaceElem::Index(local)) => {
+                Cow::Owned(PlaceElem::Index(self.make_integrate_local(local)))
+            }
+            _ => elem,
         }
     }
 

src/librustc_mir/transform/promote_consts.rs

@@ -23,6 +23,7 @@ use syntax_pos::Span;
 use rustc_index::vec::{IndexVec, Idx};
 
 use std::{iter, mem, usize};
+use std::borrow::Cow;
 
 /// State of a temporary during collection and promotion.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -407,13 +408,13 @@ impl<'a, 'tcx> MutVisitor<'tcx> for Promoter<'a, 'tcx> {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
+        elem: Cow<'b, PlaceElem<'tcx>>,
+    ) -> Cow<'b, PlaceElem<'tcx>> {
         match elem {
-            PlaceElem::Index(local) if self.is_temp_kind(*local) => {
-                PlaceElem::Index(self.promote_temp(*local))
+            Cow::Borrowed(PlaceElem::Index(local)) if self.is_temp_kind(*local) => {
+                Cow::Owned(PlaceElem::Index(self.promote_temp(*local)))
             }
-            _ => elem.clone(),
+            _ => elem,
         }
     }
 }

src/librustc_mir/transform/simplify.rs

@@ -373,12 +373,13 @@ impl<'tcx> MutVisitor<'tcx> for LocalUpdater {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
-        if let PlaceElem::Index(local) = elem {
-            PlaceElem::Index(self.map[*local].unwrap())
-        } else {
-            elem.clone()
+        elem: Cow<'a, PlaceElem<'tcx>>,
+    ) -> Cow<'a, PlaceElem<'tcx>> {
+        match elem {
+            Cow::Borrowed(PlaceElem::Index(local)) => {
+                Cow::Owned(PlaceElem::Index(self.map[*local].unwrap()))
+            }
+            _ => elem,
         }
     }
 }

src/librustc_mir/util/def_use.rs

@@ -3,6 +3,7 @@
 use rustc::mir::{Body, Local, Location, PlaceElem};
 use rustc::mir::visit::{PlaceContext, MutVisitor, Visitor};
 use rustc_index::vec::IndexVec;
+use std::borrow::Cow;
 use std::mem;
 
 pub struct DefUseAnalysis {
@@ -140,13 +141,13 @@ impl MutVisitor<'_> for MutateUseVisitor {
 
     fn process_projection_elem(
         &mut self,
-        elem: &PlaceElem<'tcx>,
-    ) -> PlaceElem<'tcx> {
+        elem: Cow<'a, PlaceElem<'tcx>>,
+    ) -> Cow<'a, PlaceElem<'tcx>> {
         match elem {
-            PlaceElem::Index(local) if *local == self.query => {
-                PlaceElem::Index(self.new_local)
+            Cow::Borrowed(PlaceElem::Index(local)) if *local == self.query => {
+                Cow::Owned(PlaceElem::Index(self.new_local))
             }
-            _ => elem.clone(),
+            _ => elem,
         }
     }
 }