Add a concept of "global exports".

This adds a feature which allows one to look up an export by name
without knowing what module it's in -- `lookup_global_export` on an
`InstanceContents`.

The main expected use for this is to support APIs where module A
imports a function from module B, and module B needs to access module
A's memory. B can't import it from A in the normal way, because that
would create a dependency cycle. So for now, allow B to look up A's
exported memory dynamically with `lookup_global_export`.

In the future, with reference types and possibly host bindings, we'll be
able to pass references to memory as arguments, which will obviate the
need for this mechanism.

lib/environ/Cargo.toml

@@ -18,12 +18,12 @@ cranelift-wasm = "0.26.0"
 cast = { version = "0.2.2", default-features = false }
 failure = { version = "0.1.3", default-features = false }
 failure_derive = { version = "0.1.3", default-features = false }
-hashbrown = { version = "0.1.8", optional = true }
+indexmap = "1.0.2"
 
 [features]
 default = ["std"]
 std = ["cranelift-codegen/std", "cranelift-wasm/std"]
-core = ["hashbrown/nightly", "cranelift-codegen/core", "cranelift-wasm/core"]
+core = ["cranelift-codegen/core", "cranelift-wasm/core"]
 
 [badges]
 maintenance = { status = "experimental" }

lib/environ/src/lib.rs

@@ -34,11 +34,6 @@ extern crate alloc as std;
 #[macro_use]
 extern crate std;
 
-#[cfg(not(feature = "std"))]
-use hashbrown::HashMap;
-#[cfg(feature = "std")]
-use std::collections::HashMap;
-
 #[macro_use]
 extern crate failure_derive;
 

lib/environ/src/module.rs

@@ -1,6 +1,5 @@
 //! Data structures for representing decoded wasm modules.
 
-use super::HashMap;
 use crate::tunables::Tunables;
 use cranelift_codegen::ir;
 use cranelift_entity::{EntityRef, PrimaryMap};
@@ -8,6 +7,7 @@ use cranelift_wasm::{
     DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, Global,
     GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table, TableIndex,
 };
+use indexmap::IndexMap;
 use std::string::String;
 use std::vec::Vec;
 
@@ -162,7 +162,7 @@ pub struct Module {
     pub globals: PrimaryMap<GlobalIndex, Global>,
 
     /// Exported entities.
-    pub exports: HashMap<String, Export>,
+    pub exports: IndexMap<String, Export>,
 
     /// The module "start" function, if present.
     pub start_func: Option<FuncIndex>,
@@ -184,7 +184,7 @@ impl Module {
             table_plans: PrimaryMap::new(),
             memory_plans: PrimaryMap::new(),
             globals: PrimaryMap::new(),
-            exports: HashMap::new(),
+            exports: IndexMap::new(),
             start_func: None,
             table_elements: Vec::new(),
         }