Hiding of environment variables (#362)

* Remember environment variables from previous scope

* Re-introduce env var hiding

Right now, hiding decls is broken

* Re-introduce hidden field of import patterns

All tests pass now.

* Remove/Address tests TODOs

* Fix test typo; Report hiding error

* Add a few more tests

* Fix wrong expected test result

crates/nu-protocol/src/ast/import_pattern.rs

@@ -1,4 +1,5 @@
 use crate::{span, Span};
+use std::collections::HashSet;
 
 #[derive(Debug, Clone)]
 pub enum ImportPatternMember {
@@ -17,6 +18,9 @@ pub struct ImportPatternHead {
 pub struct ImportPattern {
     pub head: ImportPatternHead,
     pub members: Vec<ImportPatternMember>,
+    // communicate to eval which decls/aliases were hidden during `parse_hide()` so it does not
+    // interpret these as env var names:
+    pub hidden: HashSet<Vec<u8>>,
 }
 
 impl ImportPattern {
@@ -37,4 +41,12 @@ impl ImportPattern {
 
         span(&spans)
     }
+
+    pub fn with_hidden(self, hidden: HashSet<Vec<u8>>) -> Self {
+        ImportPattern {
+            head: self.head,
+            members: self.members,
+            hidden,
+        }
+    }
 }

crates/nu-protocol/src/engine/stack.rs

@@ -21,8 +21,10 @@ use crate::{Config, ShellError, Value, VarId, CONFIG_VARIABLE_ID};
 /// use the Stack as a way of representing the local and closure-captured state.
 #[derive(Debug, Clone)]
 pub struct Stack {
+    /// Variables
     pub vars: HashMap<VarId, Value>,
-    pub env_vars: HashMap<String, String>,
+    /// Environment variables arranged as a stack to be able to recover values from parent scopes
+    pub env_vars: Vec<HashMap<String, String>>,
 }
 
 impl Default for Stack {
@@ -35,7 +37,7 @@ impl Stack {
     pub fn new() -> Stack {
         Stack {
             vars: HashMap::new(),
-            env_vars: HashMap::new(),
+            env_vars: vec![],
         }
     }
 
@@ -52,7 +54,11 @@ impl Stack {
     }
 
     pub fn add_env_var(&mut self, var: String, value: String) {
-        self.env_vars.insert(var, value);
+        if let Some(scope) = self.env_vars.last_mut() {
+            scope.insert(var, value);
+        } else {
+            self.env_vars.push(HashMap::from([(var, value)]));
+        }
     }
 
     pub fn collect_captures(&self, captures: &[VarId]) -> Stack {
@@ -68,6 +74,7 @@ impl Stack {
 
         // FIXME: this is probably slow
         output.env_vars = self.env_vars.clone();
+        output.env_vars.push(HashMap::new());
 
         let config = self
             .get_var(CONFIG_VARIABLE_ID)
@@ -77,19 +84,35 @@ impl Stack {
         output
     }
 
+    /// Flatten the env var scope frames into one frame
     pub fn get_env_vars(&self) -> HashMap<String, String> {
-        self.env_vars.clone()
+        let mut result = HashMap::new();
+
+        for scope in &self.env_vars {
+            result.extend(scope.clone());
+        }
+
+        result
     }
 
     pub fn get_env_var(&self, name: &str) -> Option<String> {
-        if let Some(v) = self.env_vars.get(name) {
-            return Some(v.to_string());
+        for scope in self.env_vars.iter().rev() {
+            if let Some(v) = scope.get(name) {
+                return Some(v.to_string());
+            }
         }
+
         None
     }
 
     pub fn remove_env_var(&mut self, name: &str) -> Option<String> {
-        self.env_vars.remove(name)
+        for scope in self.env_vars.iter_mut().rev() {
+            if let Some(v) = scope.remove(name) {
+                return Some(v);
+            }
+        }
+
+        None
     }
 
     pub fn get_config(&self) -> Result<Config, ShellError> {
@@ -109,9 +132,11 @@ impl Stack {
         for (var, val) in &self.vars {
             println!("  {}: {:?}", var, val);
         }
-        println!("env vars:");
-        for (var, val) in &self.env_vars {
-            println!("  {}: {:?}", var, val);
+        for (i, scope) in self.env_vars.iter().rev().enumerate() {
+            println!("env vars, scope {} (from the last);", i);
+            for (var, val) in scope {
+                println!("  {}: {:?}", var, val);
+            }
         }
     }
 }