Allow modules to use other modules (#6162)

* Allow private imports inside modules

Can call `use ...` inside modules now.

* Add more tests

* Add a leak test

* Refactor exportables; Prepare for 'export use'

* Fix description

* Implement 'export use' command

This allows re-exporting module's commands and aliases from another
module.

* Add more tests; Fix import pattern list strings

The import pattern strings didn't trim the surrounding quotes.

* Add ignored test

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

@@ -18,7 +18,7 @@ impl Command for ExportCommand {
     }
 
     fn usage(&self) -> &str {
-        "Export custom commands or environment variables from a module."
+        "Export definitions or environment variables from a module."
     }
 
     fn extra_usage(&self) -> &str {

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

@@ -0,0 +1,56 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+
+#[derive(Clone)]
+pub struct ExportUse;
+
+impl Command for ExportUse {
+    fn name(&self) -> &str {
+        "export use"
+    }
+
+    fn usage(&self) -> &str {
+        "Use definitions from a module and export them from this module"
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("export use")
+            .required("pattern", SyntaxShape::ImportPattern, "import pattern")
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nushell.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    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: "Re-export a command from another module",
+            example: r#"module spam { export def foo [] { "foo" } }
+    module eggs { export use spam foo }
+    use eggs foo
+    foo
+            "#,
+            result: Some(Value::String {
+                val: "foo".to_string(),
+                span: Span::test_data(),
+            }),
+        }]
+    }
+}

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

@@ -12,6 +12,7 @@ mod export_def;
 mod export_def_env;
 mod export_env;
 mod export_extern;
+mod export_use;
 mod extern_;
 mod for_;
 pub mod help;
@@ -40,6 +41,7 @@ pub use export_def::ExportDef;
 pub use export_def_env::ExportDefEnv;
 pub use export_env::ExportEnv;
 pub use export_extern::ExportExtern;
+pub use export_use::ExportUse;
 pub use extern_::Extern;
 pub use for_::For;
 pub use help::Help;