diff --git a/CHANGELOG.md b/CHANGELOG.md
index 58517e1..a6befcd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,8 @@
 
 -->
 
+# 0.13.0
+
 # 0.12.4
 
 - implement builtin map-module functions `map.deep-merge(..)` and `map.deep-remove(..)`
diff --git a/README.md b/README.md
index f8a0d6f..8921d0b 100644
--- a/README.md
+++ b/README.md
@@ -29,13 +29,6 @@ All known missing features and bugs are tracked in [#19](https://github.com/conn
 
 `grass` is benchmarked against `dart-sass` and `sassc` (`libsass`) [here](https://github.com/connorskees/sass-perf). In general, `grass` appears to be ~2x faster than `dart-sass` and ~1.7x faster than `sassc`.
 
-## Web Assembly
-
-`grass` experimentally releases a
-[WASM version of the library to npm](https://www.npmjs.com/package/@connorskees/grass),
-compiled using wasm-bindgen. To use `grass` in your JavaScript projects, run
-`npm install @connorskees/grass` to add it to your package.json. This version of grass is not currently well documented, but one can find example usage in the [`grassmeister` repository](https://github.com/connorskees/grassmeister).
-
 ## Cargo Features
 
 ### commandline
@@ -84,9 +77,9 @@ The spec runner does not work on Windows.
 Using a modified version of the spec runner that ignores warnings and error spans (but does include error messages), `grass` achieves the following results:
 
 ```
-2022-05-11
+2022-05-20
 PASSING: 6277
-FAILING: 596
+FAILING: 548
 TOTAL: 6905
 ```
 
diff --git a/crates/compiler/src/ast/stmt.rs b/crates/compiler/src/ast/stmt.rs
index 537b323..c9306f1 100644
--- a/crates/compiler/src/ast/stmt.rs
+++ b/crates/compiler/src/ast/stmt.rs
@@ -415,6 +415,13 @@ impl ConfiguredValue {
             configuration_span: Some(configuration_span),
         }
     }
+
+    pub fn implicit(value: Value) -> Self {
+        Self {
+            value,
+            configuration_span: None,
+        }
+    }
 }
 
 #[derive(Debug, Clone)]
diff --git a/crates/compiler/src/builtin/modules/mod.rs b/crates/compiler/src/builtin/modules/mod.rs
index 4b6ec66..0159e96 100644
--- a/crates/compiler/src/builtin/modules/mod.rs
+++ b/crates/compiler/src/builtin/modules/mod.rs
@@ -14,7 +14,9 @@ use crate::{
     error::SassResult,
     evaluate::{Environment, Visitor},
     selector::ExtensionStore,
-    utils::{BaseMapView, MapView, MergedMapView, PrefixedMapView, PublicMemberMapView},
+    utils::{
+        BaseMapView, LimitedMapView, MapView, MergedMapView, PrefixedMapView, PublicMemberMapView,
+    },
     value::{SassFunction, SassMap, Value},
 };
 
@@ -28,6 +30,82 @@ mod meta;
 mod selector;
 mod string;
 
+/// A [Module] that only exposes members that aren't shadowed by a given
+/// blocklist of member names.
+#[derive(Debug, Clone)]
+pub(crate) struct ShadowedModule {
+    #[allow(dead_code)]
+    inner: Arc<RefCell<Module>>,
+    scope: ModuleScope,
+}
+
+impl ShadowedModule {
+    pub fn new(
+        module: Arc<RefCell<Module>>,
+        variables: Option<&HashSet<Identifier>>,
+        functions: Option<&HashSet<Identifier>>,
+        mixins: Option<&HashSet<Identifier>>,
+    ) -> Self {
+        let module_scope = module.borrow().scope();
+
+        let variables = Self::shadowed_map(Arc::clone(&module_scope.variables), variables);
+        let functions = Self::shadowed_map(Arc::clone(&module_scope.functions), functions);
+        let mixins = Self::shadowed_map(Arc::clone(&module_scope.mixins), mixins);
+
+        let new_scope = ModuleScope {
+            variables,
+            functions,
+            mixins,
+        };
+
+        Self {
+            inner: module,
+            scope: new_scope,
+        }
+    }
+
+    fn needs_blocklist<V: fmt::Debug + Clone>(
+        map: Arc<dyn MapView<Value = V>>,
+        blocklist: Option<&HashSet<Identifier>>,
+    ) -> bool {
+        blocklist.is_some()
+            && !map.is_empty()
+            && blocklist.unwrap().iter().any(|key| map.contains_key(*key))
+    }
+
+    fn shadowed_map<V: fmt::Debug + Clone + 'static>(
+        map: Arc<dyn MapView<Value = V>>,
+        blocklist: Option<&HashSet<Identifier>>,
+    ) -> Arc<dyn MapView<Value = V>> {
+        match blocklist {
+            Some(..) if !Self::needs_blocklist(Arc::clone(&map), blocklist) => map,
+            Some(blocklist) => Arc::new(LimitedMapView::blocklist(map, blocklist)),
+            None => map,
+        }
+    }
+
+    pub fn if_necessary(
+        module: Arc<RefCell<Module>>,
+        variables: Option<&HashSet<Identifier>>,
+        functions: Option<&HashSet<Identifier>>,
+        mixins: Option<&HashSet<Identifier>>,
+    ) -> Option<Arc<RefCell<Module>>> {
+        let module_scope = module.borrow().scope();
+
+        let needs_blocklist = Self::needs_blocklist(Arc::clone(&module_scope.variables), variables)
+            || Self::needs_blocklist(Arc::clone(&module_scope.functions), functions)
+            || Self::needs_blocklist(Arc::clone(&module_scope.mixins), mixins);
+
+        if needs_blocklist {
+            Some(Arc::new(RefCell::new(Module::Shadowed(Self::new(
+                module, variables, functions, mixins,
+            )))))
+        } else {
+            None
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct ForwardedModule {
     inner: Arc<RefCell<Module>>,
@@ -149,10 +227,11 @@ pub(crate) enum Module {
         scope: ModuleScope,
     },
     Forwarded(ForwardedModule),
+    Shadowed(ShadowedModule),
 }
 
 #[derive(Debug, Clone)]
-pub(crate) struct Modules(BTreeMap<Identifier, Arc<RefCell<Module>>>);
+pub(crate) struct Modules(pub BTreeMap<Identifier, Arc<RefCell<Module>>>);
 
 impl Modules {
     pub fn new() -> Self {
@@ -279,16 +358,20 @@ impl Module {
         }
     }
 
-    fn scope(&self) -> ModuleScope {
+    pub(crate) fn scope(&self) -> ModuleScope {
         match self {
-            Self::Builtin { scope } | Self::Environment { scope, .. } => scope.clone(),
+            Self::Builtin { scope }
+            | Self::Environment { scope, .. }
+            | Self::Shadowed(ShadowedModule { scope, .. }) => scope.clone(),
             Self::Forwarded(forwarded) => (*forwarded.inner).borrow().scope(),
         }
     }
 
     fn set_scope(&mut self, new_scope: ModuleScope) {
         match self {
-            Self::Builtin { scope } | Self::Environment { scope, .. } => *scope = new_scope,
+            Self::Builtin { scope }
+            | Self::Environment { scope, .. }
+            | Self::Shadowed(ShadowedModule { scope, .. }) => *scope = new_scope,
             Self::Forwarded(forwarded) => (*forwarded.inner).borrow_mut().set_scope(new_scope),
         }
     }
@@ -319,7 +402,9 @@ impl Module {
             Self::Builtin { .. } => {
                 return Err(("Cannot modify built-in variable.", name.span).into())
             }
-            Self::Environment { scope, .. } => scope.clone(),
+            Self::Environment { scope, .. } | Self::Shadowed(ShadowedModule { scope, .. }) => {
+                scope.clone()
+            }
             Self::Forwarded(forwarded) => (*forwarded.inner).borrow_mut().scope(),
         };
 
diff --git a/crates/compiler/src/evaluate/env.rs b/crates/compiler/src/evaluate/env.rs
index 2e612c8..8b59abb 100644
--- a/crates/compiler/src/evaluate/env.rs
+++ b/crates/compiler/src/evaluate/env.rs
@@ -1,14 +1,20 @@
 use codemap::{Span, Spanned};
 
 use crate::{
-    ast::{AstForwardRule, Configuration, Mixin},
-    builtin::modules::{ForwardedModule, Module, ModuleScope, Modules},
+    ast::{AstForwardRule, Configuration, ConfiguredValue, Mixin},
+    builtin::modules::{ForwardedModule, Module, ModuleScope, Modules, ShadowedModule},
     common::Identifier,
     error::SassResult,
     selector::ExtensionStore,
     value::{SassFunction, Value},
 };
-use std::{cell::RefCell, collections::BTreeMap, sync::Arc};
+use std::{
+    cell::RefCell,
+    collections::{BTreeMap, HashSet},
+    sync::Arc,
+};
+
+type Mutable<T> = Arc<RefCell<T>>;
 
 use super::{scope::Scopes, visitor::CallableContentBlock};
 
@@ -19,6 +25,9 @@ pub(crate) struct Environment {
     pub global_modules: Vec<Arc<RefCell<Module>>>,
     pub content: Option<Arc<CallableContentBlock>>,
     pub forwarded_modules: Arc<RefCell<Vec<Arc<RefCell<Module>>>>>,
+    pub imported_modules: Arc<RefCell<Vec<Arc<RefCell<Module>>>>>,
+    #[allow(clippy::type_complexity)]
+    pub nested_forwarded_modules: Option<Mutable<Vec<Mutable<Vec<Mutable<Module>>>>>>,
 }
 
 impl Environment {
@@ -29,6 +38,8 @@ impl Environment {
             global_modules: Vec::new(),
             content: None,
             forwarded_modules: Arc::new(RefCell::new(Vec::new())),
+            imported_modules: Arc::new(RefCell::new(Vec::new())),
+            nested_forwarded_modules: None,
         }
     }
 
@@ -39,6 +50,8 @@ impl Environment {
             global_modules: self.global_modules.iter().map(Arc::clone).collect(),
             content: self.content.as_ref().map(Arc::clone),
             forwarded_modules: Arc::clone(&self.forwarded_modules),
+            imported_modules: Arc::clone(&self.imported_modules),
+            nested_forwarded_modules: self.nested_forwarded_modules.as_ref().map(Arc::clone),
         }
     }
 
@@ -49,6 +62,8 @@ impl Environment {
             global_modules: Vec::new(),
             content: self.content.as_ref().map(Arc::clone),
             forwarded_modules: Arc::clone(&self.forwarded_modules),
+            imported_modules: Arc::clone(&self.imported_modules),
+            nested_forwarded_modules: self.nested_forwarded_modules.as_ref().map(Arc::clone),
         }
     }
 
@@ -61,101 +76,167 @@ impl Environment {
         }
     }
 
+    /// Makes the members forwarded by [module] available in the current
+    /// environment.
+    ///
+    /// This is called when [module] is `@import`ed.
     pub fn import_forwards(&mut self, _env: Module) {
-        //      if (module is _EnvironmentModule) {
-        //   var forwarded = module._environment._forwardedModules;
-        //   if (forwarded == null) return;
+        if let Module::Environment { env, .. } = _env {
+            let mut forwarded = env.forwarded_modules;
 
-        //   // Omit modules from [forwarded] that are already globally available and
-        //   // forwarded in this module.
-        //   var forwardedModules = _forwardedModules;
-        //   if (forwardedModules != null) {
-        //     forwarded = {
-        //       for (var entry in forwarded.entries)
-        //         if (!forwardedModules.containsKey(entry.key) ||
-        //             !_globalModules.containsKey(entry.key))
-        //           entry.key: entry.value,
-        //     };
-        //   } else {
-        //     forwardedModules = _forwardedModules ??= {};
-        //   }
+            if (*forwarded).borrow().is_empty() {
+                return;
+            }
 
-        //   var forwardedVariableNames =
-        //       forwarded.keys.expand((module) => module.variables.keys).toSet();
-        //   var forwardedFunctionNames =
-        //       forwarded.keys.expand((module) => module.functions.keys).toSet();
-        //   var forwardedMixinNames =
-        //       forwarded.keys.expand((module) => module.mixins.keys).toSet();
+            // Omit modules from [forwarded] that are already globally available and
+            // forwarded in this module.
+            let forwarded_modules = Arc::clone(&self.forwarded_modules);
+            if !(*forwarded_modules).borrow().is_empty() {
+                // todo: intermediate name
+                let mut x = Vec::new();
+                for entry in (*forwarded).borrow().iter() {
+                    if !forwarded_modules
+                        .borrow()
+                        .iter()
+                        .any(|module| Arc::ptr_eq(module, entry))
+                        || !self
+                            .global_modules
+                            .iter()
+                            .any(|module| Arc::ptr_eq(module, entry))
+                    {
+                        x.push(Arc::clone(entry));
+                    }
+                }
 
-        //   if (atRoot) {
-        //     // Hide members from modules that have already been imported or
-        //     // forwarded that would otherwise conflict with the @imported members.
-        //     for (var entry in _importedModules.entries.toList()) {
-        //       var module = entry.key;
-        //       var shadowed = ShadowedModuleView.ifNecessary(module,
-        //           variables: forwardedVariableNames,
-        //           mixins: forwardedMixinNames,
-        //           functions: forwardedFunctionNames);
-        //       if (shadowed != null) {
-        //         _importedModules.remove(module);
-        //         if (!shadowed.isEmpty) _importedModules[shadowed] = entry.value;
-        //       }
-        //     }
+                forwarded = Arc::new(RefCell::new(x));
+            }
 
-        //     for (var entry in forwardedModules.entries.toList()) {
-        //       var module = entry.key;
-        //       var shadowed = ShadowedModuleView.ifNecessary(module,
-        //           variables: forwardedVariableNames,
-        //           mixins: forwardedMixinNames,
-        //           functions: forwardedFunctionNames);
-        //       if (shadowed != null) {
-        //         forwardedModules.remove(module);
-        //         if (!shadowed.isEmpty) forwardedModules[shadowed] = entry.value;
-        //       }
-        //     }
+            let forwarded_var_names = forwarded
+                .borrow()
+                .iter()
+                .flat_map(|module| (*module).borrow().scope().variables.keys())
+                .collect::<HashSet<Identifier>>();
+            let forwarded_fn_names = forwarded
+                .borrow()
+                .iter()
+                .flat_map(|module| (*module).borrow().scope().functions.keys())
+                .collect::<HashSet<Identifier>>();
+            let forwarded_mixin_names = forwarded
+                .borrow()
+                .iter()
+                .flat_map(|module| (*module).borrow().scope().mixins.keys())
+                .collect::<HashSet<Identifier>>();
 
-        //     _importedModules.addAll(forwarded);
-        //     forwardedModules.addAll(forwarded);
-        //   } else {
-        //     (_nestedForwardedModules ??=
-        //             List.generate(_variables.length - 1, (_) => []))
-        //         .last
-        //         .addAll(forwarded.keys);
-        //   }
+            if self.at_root() {
+                let mut to_remove = Vec::new();
 
-        //   // Remove existing member definitions that are now shadowed by the
-        //   // forwarded modules.
-        //   for (var variable in forwardedVariableNames) {
-        //     _variableIndices.remove(variable);
-        //     _variables.last.remove(variable);
-        //     _variableNodes.last.remove(variable);
-        //   }
-        //   for (var function in forwardedFunctionNames) {
-        //     _functionIndices.remove(function);
-        //     _functions.last.remove(function);
-        //   }
-        //   for (var mixin in forwardedMixinNames) {
-        //     _mixinIndices.remove(mixin);
-        //     _mixins.last.remove(mixin);
-        //   }
-        // }
-        // todo!()
+                // Hide members from modules that have already been imported or
+                // forwarded that would otherwise conflict with the @imported members.
+                for (idx, module) in (*self.imported_modules).borrow().iter().enumerate() {
+                    let shadowed = ShadowedModule::if_necessary(
+                        Arc::clone(module),
+                        Some(&forwarded_var_names),
+                        Some(&forwarded_fn_names),
+                        Some(&forwarded_mixin_names),
+                    );
+
+                    if shadowed.is_some() {
+                        to_remove.push(idx);
+                    }
+                }
+
+                let mut imported_modules = (*self.imported_modules).borrow_mut();
+
+                for &idx in to_remove.iter().rev() {
+                    imported_modules.remove(idx);
+                }
+
+                to_remove.clear();
+
+                for (idx, module) in (*self.forwarded_modules).borrow().iter().enumerate() {
+                    let shadowed = ShadowedModule::if_necessary(
+                        Arc::clone(module),
+                        Some(&forwarded_var_names),
+                        Some(&forwarded_fn_names),
+                        Some(&forwarded_mixin_names),
+                    );
+
+                    if shadowed.is_some() {
+                        to_remove.push(idx);
+                    }
+                }
+
+                let mut forwarded_modules = (*self.forwarded_modules).borrow_mut();
+
+                for &idx in to_remove.iter().rev() {
+                    forwarded_modules.remove(idx);
+                }
+
+                imported_modules.extend(forwarded.borrow().iter().map(Arc::clone));
+                forwarded_modules.extend(forwarded.borrow().iter().map(Arc::clone));
+            } else {
+                self.scopes.last_variable_index = None;
+                self.nested_forwarded_modules
+                    .get_or_insert_with(|| {
+                        Arc::new(RefCell::new(
+                            (0..self.scopes.len())
+                                .map(|_| Arc::new(RefCell::new(Vec::new())))
+                                .collect(),
+                        ))
+                    })
+                    .borrow_mut()
+                    .last_mut()
+                    .unwrap()
+                    .borrow_mut()
+                    .extend(forwarded.borrow().iter().map(Arc::clone));
+            }
+
+            // Remove existing member definitions that are now shadowed by the
+            // forwarded modules.
+            for variable in forwarded_var_names {
+                (*self.scopes.variables)
+                    .borrow_mut()
+                    .last_mut()
+                    .unwrap()
+                    .borrow_mut()
+                    .remove(&variable);
+            }
+            self.scopes.last_variable_index = None;
+
+            for func in forwarded_fn_names {
+                (*self.scopes.functions)
+                    .borrow_mut()
+                    .last_mut()
+                    .unwrap()
+                    .borrow_mut()
+                    .remove(&func);
+            }
+            for mixin in forwarded_mixin_names {
+                (*self.scopes.mixins)
+                    .borrow_mut()
+                    .last_mut()
+                    .unwrap()
+                    .borrow_mut()
+                    .remove(&mixin);
+            }
+        }
     }
 
     pub fn to_implicit_configuration(&self) -> Configuration {
-        //     var configuration = <String, ConfiguredValue>{};
-        // for (var i = 0; i < _variables.length; i++) {
-        //   var values = _variables[i];
-        //   var nodes = _variableNodes[i];
-        //   for (var entry in values.entries) {
-        //     // Implicit configurations are never invalid, making [configurationSpan]
-        //     // unnecessary, so we pass null here to avoid having to compute it.
-        //     configuration[entry.key] =
-        //         ConfiguredValue.implicit(entry.value, nodes[entry.key]!);
-        //   }
-        // }
-        // return Configuration.implicit(configuration);
-        todo!()
+        let mut configuration = BTreeMap::new();
+
+        let variables = (*self.scopes.variables).borrow();
+
+        for variables in variables.iter() {
+            let entries = (**variables).borrow();
+            for (key, value) in entries.iter() {
+                // Implicit configurations are never invalid, making [configurationSpan]
+                // unnecessary, so we pass null here to avoid having to compute it.
+                configuration.insert(*key, ConfiguredValue::implicit(value.clone()));
+            }
+        }
+
+        Configuration::implicit(configuration)
     }
 
     pub fn forward_module(&mut self, module: Arc<RefCell<Module>>, rule: AstForwardRule) {
@@ -274,24 +355,22 @@ impl Environment {
         }
 
         if is_global || self.at_root() {
-            //         // Don't set the index if there's already a variable with the given name,
-            //   // since local accesses should still return the local variable.
-            //   _variableIndices.putIfAbsent(name, () {
-            //     _lastVariableName = name;
-            //     _lastVariableIndex = 0;
-            //     return 0;
-            //   });
+            // If this module doesn't already contain a variable named [name], try
+            // setting it in a global module.
+            if !self.scopes.global_var_exists(name.node) {
+                let module_with_name = self.from_one_module(name.node, "variable", |module| {
+                    if module.borrow().var_exists(*name) {
+                        Some(Arc::clone(module))
+                    } else {
+                        None
+                    }
+                });
 
-            //   // If this module doesn't already contain a variable named [name], try
-            //   // setting it in a global module.
-            //   if (!_variables.first.containsKey(name)) {
-            //     var moduleWithName = _fromOneModule(name, "variable",
-            //         (module) => module.variables.containsKey(name) ? module : null);
-            //     if (moduleWithName != null) {
-            //       moduleWithName.setVariable(name, value, nodeWithSpan);
-            //       return;
-            //     }
-            //   }
+                if let Some(module_with_name) = module_with_name {
+                    module_with_name.borrow_mut().update_var(name, value)?;
+                    return Ok(());
+                }
+            }
 
             self.scopes.insert_var(0, name.node, value);
             return Ok(());
@@ -334,33 +413,19 @@ impl Environment {
     }
 
     fn get_variable_from_global_modules(&self, name: Identifier) -> Option<Value> {
-        for module in &self.global_modules {
-            if (**module).borrow().var_exists(name) {
-                return (**module).borrow().get_var_no_err(name);
-            }
-        }
-
-        None
+        self.from_one_module(name, "variable", |module| {
+            (**module).borrow().get_var_no_err(name)
+        })
     }
 
     fn get_function_from_global_modules(&self, name: Identifier) -> Option<SassFunction> {
-        for module in &self.global_modules {
-            if (**module).borrow().fn_exists(name) {
-                return (**module).borrow().get_fn(name);
-            }
-        }
-
-        None
+        self.from_one_module(name, "function", |module| (**module).borrow().get_fn(name))
     }
 
     fn get_mixin_from_global_modules(&self, name: Identifier) -> Option<Mixin> {
-        for module in &self.global_modules {
-            if (**module).borrow().mixin_exists(name) {
-                return (**module).borrow().get_mixin_no_err(name);
-            }
-        }
-
-        None
+        self.from_one_module(name, "mixin", |module| {
+            (**module).borrow().get_mixin_no_err(name)
+        })
     }
 
     pub fn add_module(
@@ -396,4 +461,61 @@ impl Environment {
 
         Arc::new(RefCell::new(Module::new_env(self, extension_store)))
     }
+
+    fn from_one_module<T>(
+        &self,
+        _name: Identifier,
+        _ty: &str,
+        callback: impl Fn(&Arc<RefCell<Module>>) -> Option<T>,
+    ) -> Option<T> {
+        if let Some(nested_forwarded_modules) = &self.nested_forwarded_modules {
+            for modules in nested_forwarded_modules.borrow().iter().rev() {
+                for module in modules.borrow().iter().rev() {
+                    if let Some(value) = callback(module) {
+                        return Some(value);
+                    }
+                }
+            }
+        }
+
+        for module in self.imported_modules.borrow().iter() {
+            if let Some(value) = callback(module) {
+                return Some(value);
+            }
+        }
+
+        let mut value: Option<T> = None;
+        //     Object? identity;
+
+        for module in self.global_modules.iter() {
+            let value_in_module = match callback(module) {
+                Some(v) => v,
+                None => continue,
+            };
+
+            value = Some(value_in_module);
+
+            //       Object? identityFromModule = valueInModule is AsyncCallable
+            //           ? valueInModule
+            //           : module.variableIdentity(name);
+            //       if (identityFromModule == identity) continue;
+
+            //       if (value != null) {
+            //         var spans = _globalModules.entries.map(
+            //             (entry) => callback(entry.key).andThen((_) => entry.value.span));
+
+            //         throw MultiSpanSassScriptException(
+            //             'This $type is available from multiple global modules.',
+            //             '$type use', {
+            //           for (var span in spans)
+            //             if (span != null) span: 'includes $type'
+            //         });
+            //       }
+
+            //       value = valueInModule;
+            //       identity = identityFromModule;
+        }
+
+        value
+    }
 }
diff --git a/crates/compiler/src/evaluate/scope.rs b/crates/compiler/src/evaluate/scope.rs
index 579b1c3..4e31471 100644
--- a/crates/compiler/src/evaluate/scope.rs
+++ b/crates/compiler/src/evaluate/scope.rs
@@ -17,9 +17,9 @@ use crate::{
 #[allow(clippy::type_complexity)]
 #[derive(Debug, Default, Clone)]
 pub(crate) struct Scopes {
-    variables: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, Value>>>>>>,
-    mixins: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, Mixin>>>>>>,
-    functions: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, SassFunction>>>>>>,
+    pub(crate) variables: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, Value>>>>>>,
+    pub(crate) mixins: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, Mixin>>>>>>,
+    pub(crate) functions: Arc<RefCell<Vec<Arc<RefCell<BTreeMap<Identifier, SassFunction>>>>>>,
     len: Arc<Cell<usize>>,
     pub last_variable_index: Option<(Identifier, usize)>,
 }
@@ -167,6 +167,10 @@ impl Scopes {
 
         false
     }
+
+    pub fn global_var_exists(&self, name: Identifier) -> bool {
+        self.global_variables().borrow().contains_key(&name)
+    }
 }
 
 /// Mixins
diff --git a/crates/compiler/src/evaluate/visitor.rs b/crates/compiler/src/evaluate/visitor.rs
index 2cfa22d..b163357 100644
--- a/crates/compiler/src/evaluate/visitor.rs
+++ b/crates/compiler/src/evaluate/visitor.rs
@@ -113,6 +113,8 @@ pub struct Visitor<'a> {
     pub(crate) extender: ExtensionStore,
     pub(crate) current_import_path: PathBuf,
     pub(crate) is_plain_css: bool,
+    pub(crate) modules: BTreeMap<PathBuf, Arc<RefCell<Module>>>,
+    pub(crate) active_modules: BTreeSet<PathBuf>,
     css_tree: CssTree,
     parent: Option<CssTreeIdx>,
     configuration: Arc<RefCell<Configuration>>,
@@ -157,6 +159,8 @@ impl<'a> Visitor<'a> {
             configuration: Arc::new(RefCell::new(Configuration::empty())),
             is_plain_css: false,
             import_nodes: Vec::new(),
+            modules: BTreeMap::new(),
+            active_modules: BTreeSet::new(),
             options,
             span_before,
             map,
@@ -166,6 +170,7 @@ impl<'a> Visitor<'a> {
     }
 
     pub(crate) fn visit_stylesheet(&mut self, mut style_sheet: StyleSheet) -> SassResult<()> {
+        self.active_modules.insert(style_sheet.url.clone());
         let was_in_plain_css = self.is_plain_css;
         self.is_plain_css = style_sheet.is_plain_css;
         mem::swap(&mut self.current_import_path, &mut style_sheet.url);
@@ -178,6 +183,8 @@ impl<'a> Visitor<'a> {
         mem::swap(&mut self.current_import_path, &mut style_sheet.url);
         self.is_plain_css = was_in_plain_css;
 
+        self.active_modules.remove(&style_sheet.url);
+
         Ok(())
     }
 
@@ -366,6 +373,8 @@ impl<'a> Visitor<'a> {
         )))
     }
 
+    /// Remove configured values from [upstream] that have been removed from
+    /// [downstream], unless they match a name in [except].
     fn remove_used_configuration(
         upstream: &Arc<RefCell<Configuration>>,
         downstream: &Arc<RefCell<Configuration>>,
@@ -534,6 +543,40 @@ impl<'a> Visitor<'a> {
         // todo: different errors based on this
         _names_in_errors: bool,
     ) -> SassResult<Arc<RefCell<Module>>> {
+        let url = stylesheet.url.clone();
+
+        // todo: use canonical url for modules
+        if let Some(already_loaded) = self.modules.get(&stylesheet.url) {
+            let current_configuration =
+                configuration.unwrap_or_else(|| Arc::clone(&self.configuration));
+
+            if !current_configuration.borrow().is_implicit() {
+                //   if (!_moduleConfigurations[url]!.sameOriginal(currentConfiguration) &&
+                //       currentConfiguration is ExplicitConfiguration) {
+                //     var message = namesInErrors
+                //         ? "${p.prettyUri(url)} was already loaded, so it can't be "
+                //             "configured using \"with\"."
+                //         : "This module was already loaded, so it can't be configured using "
+                //             "\"with\".";
+
+                //     var existingSpan = _moduleNodes[url]?.span;
+                //     var configurationSpan = configuration == null
+                //         ? currentConfiguration.nodeWithSpan.span
+                //         : null;
+                //     var secondarySpans = {
+                //       if (existingSpan != null) existingSpan: "original load",
+                //       if (configurationSpan != null) configurationSpan: "configuration"
+                //     };
+
+                //     throw secondarySpans.isEmpty
+                //         ? _exception(message)
+                //         : _multiSpanException(message, "new load", secondarySpans);
+                //   }
+            }
+
+            return Ok(Arc::clone(already_loaded));
+        }
+
         let env = Environment::new();
         let mut extension_store = ExtensionStore::new(self.span_before);
 
@@ -589,6 +632,8 @@ impl<'a> Visitor<'a> {
 
         let module = env.to_module(extension_store);
 
+        self.modules.insert(url, Arc::clone(&module));
+
         Ok(module)
     }
 
@@ -635,7 +680,7 @@ impl<'a> Visitor<'a> {
             callback(
                 self,
                 Arc::new(RefCell::new(builtin)),
-                StyleSheet::new(false, PathBuf::from("")),
+                StyleSheet::new(false, url.to_path_buf()),
             )?;
             return Ok(());
         }
@@ -643,8 +688,22 @@ impl<'a> Visitor<'a> {
         // todo: decide on naming convention for style_sheet vs stylesheet
         let stylesheet = self.load_style_sheet(url.to_string_lossy().as_ref(), false, span)?;
 
+        let canonical_url = self
+            .options
+            .fs
+            .canonicalize(&stylesheet.url)
+            .unwrap_or_else(|_| stylesheet.url.clone());
+
+        if self.active_modules.contains(&canonical_url) {
+            return Err(("Module loop: this module is already being loaded.", span).into());
+        }
+
+        self.active_modules.insert(canonical_url.clone());
+
         let module = self.execute(stylesheet.clone(), configuration, names_in_errors)?;
 
+        self.active_modules.remove(&canonical_url);
+
         callback(self, module, stylesheet)?;
 
         Ok(())
@@ -832,6 +891,7 @@ impl<'a> Visitor<'a> {
         span: Span,
     ) -> SassResult<StyleSheet> {
         if let Some(name) = self.find_import(url.as_ref()) {
+            let name = self.options.fs.canonicalize(&name).unwrap_or(name);
             if let Some(style_sheet) = self.import_cache.get(&name) {
                 return Ok(style_sheet.clone());
             }
@@ -876,6 +936,14 @@ impl<'a> Visitor<'a> {
     fn visit_dynamic_import_rule(&mut self, dynamic_import: &AstSassImport) -> SassResult<()> {
         let stylesheet = self.load_style_sheet(&dynamic_import.url, true, dynamic_import.span)?;
 
+        let url = stylesheet.url.clone();
+
+        if self.active_modules.contains(&url) {
+            return Err(("This file is already being loaded.", dynamic_import.span).into());
+        }
+
+        self.active_modules.insert(url.clone());
+
         // If the imported stylesheet doesn't use any modules, we can inject its
         // CSS directly into the current stylesheet. If it does use modules, we
         // need to put its CSS into an intermediate [ModifiableCssStylesheet] so
@@ -924,6 +992,7 @@ impl<'a> Visitor<'a> {
         self.env.import_forwards(module);
 
         if loads_user_defined_modules {
+            // todo:
             //     if (module.transitivelyContainsCss) {
             //       // If any transitively used module contains extensions, we need to
             //       // clone all modules' CSS. Otherwise, it's possible that they'll be
@@ -940,6 +1009,8 @@ impl<'a> Visitor<'a> {
             //     }
         }
 
+        self.active_modules.remove(&url);
+
         Ok(())
     }
 
diff --git a/crates/compiler/src/fs.rs b/crates/compiler/src/fs.rs
index 9dced3c..db58b51 100644
--- a/crates/compiler/src/fs.rs
+++ b/crates/compiler/src/fs.rs
@@ -1,6 +1,6 @@
 use std::{
     io::{self, Error, ErrorKind},
-    path::Path,
+    path::{Path, PathBuf},
 };
 
 /// A trait to allow replacing the file system lookup mechanisms.
@@ -18,6 +18,11 @@ pub trait Fs: std::fmt::Debug {
     fn is_file(&self, path: &Path) -> bool;
     /// Read the entire contents of a file into a bytes vector.
     fn read(&self, path: &Path) -> io::Result<Vec<u8>>;
+
+    /// Canonicalize a file path
+    fn canonicalize(&self, path: &Path) -> io::Result<PathBuf> {
+        Ok(path.to_path_buf())
+    }
 }
 
 /// Use [`std::fs`] to read any files from disk.
@@ -41,6 +46,11 @@ impl Fs for StdFs {
     fn read(&self, path: &Path) -> io::Result<Vec<u8>> {
         std::fs::read(path)
     }
+
+    #[inline]
+    fn canonicalize(&self, path: &Path) -> io::Result<PathBuf> {
+        std::fs::canonicalize(path)
+    }
 }
 
 /// A file system implementation that acts like it’s completely empty.
diff --git a/crates/compiler/src/parse/css.rs b/crates/compiler/src/parse/css.rs
index dc5fae5..81e4256 100644
--- a/crates/compiler/src/parse/css.rs
+++ b/crates/compiler/src/parse/css.rs
@@ -38,15 +38,15 @@ impl<'a> BaseParser<'a> for CssParser<'a> {
 }
 
 impl<'a> StylesheetParser<'a> for CssParser<'a> {
-    fn is_plain_css(&mut self) -> bool {
+    fn is_plain_css(&self) -> bool {
         true
     }
 
-    fn is_indented(&mut self) -> bool {
+    fn is_indented(&self) -> bool {
         false
     }
 
-    fn path(&mut self) -> &'a Path {
+    fn path(&self) -> &'a Path {
         self.path
     }
 
@@ -58,7 +58,7 @@ impl<'a> StylesheetParser<'a> for CssParser<'a> {
         self.options
     }
 
-    fn flags(&mut self) -> &ContextFlags {
+    fn flags(&self) -> &ContextFlags {
         &self.flags
     }
 
diff --git a/crates/compiler/src/parse/sass.rs b/crates/compiler/src/parse/sass.rs
index 2b1f1d3..1d1b997 100644
--- a/crates/compiler/src/parse/sass.rs
+++ b/crates/compiler/src/parse/sass.rs
@@ -71,15 +71,15 @@ impl<'a> BaseParser<'a> for SassParser<'a> {
 }
 
 impl<'a> StylesheetParser<'a> for SassParser<'a> {
-    fn is_plain_css(&mut self) -> bool {
+    fn is_plain_css(&self) -> bool {
         false
     }
 
-    fn is_indented(&mut self) -> bool {
+    fn is_indented(&self) -> bool {
         true
     }
 
-    fn path(&mut self) -> &'a Path {
+    fn path(&self) -> &'a Path {
         self.path
     }
 
@@ -91,7 +91,7 @@ impl<'a> StylesheetParser<'a> for SassParser<'a> {
         self.options
     }
 
-    fn flags(&mut self) -> &ContextFlags {
+    fn flags(&self) -> &ContextFlags {
         &self.flags
     }
 
diff --git a/crates/compiler/src/parse/scss.rs b/crates/compiler/src/parse/scss.rs
index 6be08cb..77c8d5a 100644
--- a/crates/compiler/src/parse/scss.rs
+++ b/crates/compiler/src/parse/scss.rs
@@ -50,15 +50,15 @@ impl<'a> BaseParser<'a> for ScssParser<'a> {
 }
 
 impl<'a> StylesheetParser<'a> for ScssParser<'a> {
-    fn is_plain_css(&mut self) -> bool {
+    fn is_plain_css(&self) -> bool {
         false
     }
 
-    fn is_indented(&mut self) -> bool {
+    fn is_indented(&self) -> bool {
         false
     }
 
-    fn path(&mut self) -> &'a Path {
+    fn path(&self) -> &'a Path {
         self.path
     }
 
@@ -74,7 +74,7 @@ impl<'a> StylesheetParser<'a> for ScssParser<'a> {
         0
     }
 
-    fn flags(&mut self) -> &ContextFlags {
+    fn flags(&self) -> &ContextFlags {
         &self.flags
     }
 
diff --git a/crates/compiler/src/parse/stylesheet.rs b/crates/compiler/src/parse/stylesheet.rs
index 6ac1562..0a137c6 100644
--- a/crates/compiler/src/parse/stylesheet.rs
+++ b/crates/compiler/src/parse/stylesheet.rs
@@ -28,15 +28,15 @@ use super::{
 /// SCSS share the behavior
 pub(crate) trait StylesheetParser<'a>: BaseParser<'a> + Sized {
     // todo: make constant?
-    fn is_plain_css(&mut self) -> bool;
+    fn is_plain_css(&self) -> bool;
     // todo: make constant?
-    fn is_indented(&mut self) -> bool;
+    fn is_indented(&self) -> bool;
     fn options(&self) -> &Options;
-    fn path(&mut self) -> &Path;
+    fn path(&self) -> &Path;
     fn map(&mut self) -> &mut CodeMap;
     fn span_before(&self) -> Span;
     fn current_indentation(&self) -> usize;
-    fn flags(&mut self) -> &ContextFlags;
+    fn flags(&self) -> &ContextFlags;
     fn flags_mut(&mut self) -> &mut ContextFlags;
 
     #[allow(clippy::type_complexity)]
@@ -185,7 +185,13 @@ pub(crate) trait StylesheetParser<'a>: BaseParser<'a> + Sized {
     }
 
     fn __parse(&mut self) -> SassResult<StyleSheet> {
-        let mut style_sheet = StyleSheet::new(self.is_plain_css(), self.path().to_path_buf());
+        let mut style_sheet = StyleSheet::new(
+            self.is_plain_css(),
+            self.options()
+                .fs
+                .canonicalize(self.path())
+                .unwrap_or_else(|_| self.path().to_path_buf()),
+        );
 
         // Allow a byte-order mark at the beginning of the document.
         self.scan_char('\u{feff}');
diff --git a/crates/compiler/src/utils/map_view.rs b/crates/compiler/src/utils/map_view.rs
index b75671f..b8b794d 100644
--- a/crates/compiler/src/utils/map_view.rs
+++ b/crates/compiler/src/utils/map_view.rs
@@ -180,6 +180,17 @@ impl<V: fmt::Debug + Clone, T: MapView<Value = V> + Clone> MapView for PrefixedM
     }
 }
 
+/// A mostly-unmodifiable view of a map that only allows certain keys to be
+/// accessed.
+///
+/// Whether or not the underlying map contains keys that aren't allowed, this
+/// view will behave as though it doesn't contain them.
+///
+/// The underlying map's values may change independently of this view, but its
+/// set of keys may not.
+///
+/// This is unmodifiable *except for the [remove] method*, which is used for
+/// `@used with` to mark configured variables as used.
 #[derive(Debug, Clone)]
 pub(crate) struct LimitedMapView<V: fmt::Debug + Clone, T: MapView<Value = V> + Clone>(
     pub T,
@@ -197,11 +208,11 @@ impl<V: fmt::Debug + Clone, T: MapView<Value = V> + Clone> LimitedMapView<V, T>
         Self(map, keys)
     }
 
-    pub fn blocklist(map: T, keys: &HashSet<Identifier>) -> Self {
-        let keys = keys
-            .iter()
-            .copied()
-            .filter(|key| !map.contains_key(*key))
+    pub fn blocklist(map: T, blocklist: &HashSet<Identifier>) -> Self {
+        let keys = map
+            .keys()
+            .into_iter()
+            .filter(|key| !blocklist.contains(key))
             .collect();
 
         Self(map, keys)
diff --git a/crates/lib/tests/forward.rs b/crates/lib/tests/forward.rs
index 86ac00c..b7c5702 100644
--- a/crates/lib/tests/forward.rs
+++ b/crates/lib/tests/forward.rs
@@ -334,6 +334,173 @@ fn forward_module_with_error() {
     );
 }
 
+#[test]
+fn use_with_multi_load_forward() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "_midstream.scss",
+        r#"
+        @forward "upstream";
+    "#,
+    );
+    fs.add_file(
+        "_upstream.scss",
+        r#"
+    $a: original !default;
+    "#,
+    );
+
+    let input = r#"
+        @use "upstream" with ($a: configured);
+        @use "midstream";
+        b {c: midstream.$a}
+    "#;
+
+    assert_eq!(
+        "b {\n  c: configured;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
+#[test]
+fn forward_member_import_precedence_nested() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "_midstream.scss",
+        r#"
+        @forward "upstream";
+    "#,
+    );
+    fs.add_file(
+        "_upstream.scss",
+        r#"
+        $a: in-upstream;
+    "#,
+    );
+
+    let input = r#"
+        b {
+            $a: in-input;
+        
+            @import "midstream";
+        
+            c: $a;
+        }
+    "#;
+
+    assert_eq!(
+        "b {\n  c: in-upstream;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
+#[test]
+fn forward_with_through_forward_hide() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "_downstream.scss",
+        r#"
+        @forward "midstream" with ($a: configured);
+    "#,
+    );
+    fs.add_file(
+        "_midstream.scss",
+        r#"
+        @forward "upstream" hide $b;
+    "#,
+    );
+    fs.add_file(
+        "_upstream.scss",
+        r#"
+        $a: original !default;
+        b {c: $a}
+    "#,
+    );
+
+    let input = r#"
+        @use "downstream";
+    "#;
+
+    assert_eq!(
+        "b {\n  c: configured;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
+#[test]
+fn forward_with_through_forward_show() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "_downstream.scss",
+        r#"
+        @forward "midstream" with ($a: configured);
+    "#,
+    );
+    fs.add_file(
+        "_midstream.scss",
+        r#"
+        @forward "upstream" show $a;
+    "#,
+    );
+    fs.add_file(
+        "_upstream.scss",
+        r#"
+        $a: original !default;
+        b {c: $a}
+    "#,
+    );
+
+    let input = r#"
+        @use "downstream";
+    "#;
+
+    assert_eq!(
+        "b {\n  c: configured;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
+#[test]
+#[ignore = "incorrectly thinks there's a module loop"]
+fn import_forwarded_first_no_use() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "first.scss",
+        r#"
+        $variable: value;
+    "#,
+    );
+    fs.add_file(
+        "first.import.scss",
+        r#"
+        @forward "first";
+    "#,
+    );
+    fs.add_file(
+        "second.scss",
+        r#"
+        a {
+            b: $variable;
+        }
+    "#,
+    );
+
+    let input = r#"
+        @import "first";
+        @import "second";
+    "#;
+
+    assert_eq!(
+        "a {\n  b: value;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
 error!(
     after_style_rule,
     r#"
diff --git a/crates/lib/tests/imports.rs b/crates/lib/tests/imports.rs
index 9a71dd4..97916c3 100644
--- a/crates/lib/tests/imports.rs
+++ b/crates/lib/tests/imports.rs
@@ -349,6 +349,22 @@ fn imports_same_file_thrice() {
         &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
     );
 }
+#[test]
+fn imports_self() {
+    let mut fs = TestFs::new();
+
+    fs.add_file("input.scss", r#"@import "input";"#);
+
+    let input = r#"
+        @import "input";
+    "#;
+
+    assert_err!(
+        input,
+        "Error: This file is already being loaded.",
+        &grass::Options::default().fs(&fs)
+    );
+}
 
 #[test]
 fn imports_explicit_file_extension() {
diff --git a/crates/lib/tests/use.rs b/crates/lib/tests/use.rs
index 8d5c6b9..27d1374 100644
--- a/crates/lib/tests/use.rs
+++ b/crates/lib/tests/use.rs
@@ -845,6 +845,37 @@ fn import_module_using_same_builtin_module_has_styles() {
     );
 }
 
+#[test]
+fn use_member_global_variable_assignment_toplevel() {
+    let mut fs = TestFs::new();
+
+    fs.add_file(
+        "other.scss",
+        r#"
+            $member: value;
+
+            @function get-member() {
+                @return $member
+            }
+    "#,
+    );
+
+    let input = r#"
+        @use "other" as *;
+
+        $member: new value;
+        
+        a {
+            b: get-member()
+        }
+    "#;
+
+    assert_eq!(
+        "a {\n  b: new value;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
 #[test]
 #[ignore = "we don't hermetically evaluate @extend"]
 fn use_module_with_extend() {