try im-rs

Cargo.toml

@@ -59,6 +59,7 @@ url = "1.1"
 clap = "2.31.2"
 unicode-width = "0.1.5"
 openssl = { version = '0.10.11', optional = true }
+im-rc = "12.1.0"
 
 # A noop dependency that changes in the Rust repository, it's a bit of a hack.
 # See the `src/tools/rustc-workspace-hack/README.md` file in `rust-lang/rust`

src/cargo/core/resolver/context.rs

@@ -5,6 +5,7 @@ use core::interning::InternedString;
 use core::{Dependency, FeatureValue, PackageId, SourceId, Summary};
 use util::CargoResult;
 use util::Graph;
+use im_rc;
 
 use super::errors::ActivateResult;
 use super::types::{ConflictReason, DepInfo, GraphNode, Method, RcList, RegistryQueryer};
@@ -19,12 +20,9 @@ pub use super::resolve::Resolve;
 // possible.
 #[derive(Clone)]
 pub struct Context {
-    // TODO: Both this and the two maps below are super expensive to clone. We should
-    //       switch to persistent hash maps if we can at some point or otherwise
-    //       make these much cheaper to clone in general.
     pub activations: Activations,
-    pub resolve_features: HashMap<PackageId, Rc<HashSet<InternedString>>>,
-    pub links: HashMap<InternedString, PackageId>,
+    pub resolve_features: im_rc::HashMap<PackageId, Rc<HashSet<InternedString>>>,
+    pub links: im_rc::HashMap<InternedString, PackageId>,
 
     // These are two cheaply-cloneable lists (O(1) clone) which are effectively
     // hash maps but are built up as "construction lists". We'll iterate these
@@ -36,16 +34,16 @@ pub struct Context {
     pub warnings: RcList<String>,
 }
 
-pub type Activations = HashMap<(InternedString, SourceId), Rc<Vec<Summary>>>;
+pub type Activations = im_rc::HashMap<(InternedString, SourceId), Rc<Vec<Summary>>>;
 
 impl Context {
     pub fn new() -> Context {
         Context {
             resolve_graph: RcList::new(),
-            resolve_features: HashMap::new(),
-            links: HashMap::new(),
+            resolve_features: im_rc::HashMap::new(),
+            links: im_rc::HashMap::new(),
             resolve_replacements: RcList::new(),
-            activations: HashMap::new(),
+            activations: im_rc::HashMap::new(),
             warnings: RcList::new(),
         }
     }

src/cargo/lib.rs

@@ -62,6 +62,7 @@ extern crate termcolor;
 extern crate toml;
 extern crate unicode_width;
 extern crate url;
+extern crate im_rc;
 
 use std::fmt;