Add export alias and export extern (#4878)

* export alias

* export extern

crates/nu-command/src/core_commands/export_alias.rs

@@ -0,0 +1,45 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+
+#[derive(Clone)]
+pub struct ExportAlias;
+
+impl Command for ExportAlias {
+    fn name(&self) -> &str {
+        "export alias"
+    }
+
+    fn usage(&self) -> &str {
+        "Define an alias and export it from a module"
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("export def")
+            .required("name", SyntaxShape::String, "name of the alias")
+            .required(
+                "initial_value",
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                "equals sign followed by value",
+            )
+            .category(Category::Core)
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        Ok(PipelineData::new(call.head))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "export an alias of ll to ls -l, from a module",
+            example: "export alias ll = ls -l",
+            result: None,
+        }]
+    }
+}

crates/nu-command/src/core_commands/export_extern.rs

@@ -0,0 +1,41 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+
+#[derive(Clone)]
+pub struct ExportExtern;
+
+impl Command for ExportExtern {
+    fn name(&self) -> &str {
+        "export extern"
+    }
+
+    fn usage(&self) -> &str {
+        "Define an extern and export it from a module"
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("export extern")
+            .required("def_name", SyntaxShape::String, "definition name")
+            .required("params", SyntaxShape::Signature, "parameters")
+            .category(Category::Core)
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        Ok(PipelineData::new(call.head))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Export the signature for an external command",
+            example: r#"export extern echo [text: string]"#,
+            result: None,
+        }]
+    }
+}

crates/nu-command/src/core_commands/mod.rs

@@ -7,9 +7,11 @@ mod do_;
 mod echo;
 mod error_make;
 mod export;
+mod export_alias;
 mod export_def;
 mod export_def_env;
 mod export_env;
+mod export_extern;
 mod extern_;
 mod for_;
 mod help;
@@ -34,9 +36,11 @@ pub use do_::Do;
 pub use echo::Echo;
 pub use error_make::ErrorMake;
 pub use export::ExportCommand;
+pub use export_alias::ExportAlias;
 pub use export_def::ExportDef;
 pub use export_def_env::ExportDefEnv;
 pub use export_env::ExportEnv;
+pub use export_extern::ExportExtern;
 pub use extern_::Extern;
 pub use for_::For;
 pub use help::Help;

crates/nu-command/src/core_commands/use_.rs

@@ -57,7 +57,7 @@ impl Command for Use {
 
                         if let Some(id) = overlay.get_env_var_id(name) {
                             output.push((name.clone(), id));
-                        } else if !overlay.has_decl(name) {
+                        } else if !overlay.has_decl(name) && !overlay.has_alias(name) {
                             return Err(ShellError::EnvVarNotFoundAtRuntime(
                                 String::from_utf8_lossy(name).into(),
                                 *span,

crates/nu-command/src/default_context.rs

@@ -34,10 +34,12 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             Du,
             Echo,
             ErrorMake,
+            ExportAlias,
             ExportCommand,
             ExportDef,
             ExportDefEnv,
             ExportEnv,
+            ExportExtern,
             Extern,
             For,
             Help,