feat: Add default docs for aliases, generated from the command they point to (#10825)

crates/nu-command/tests/commands/alias.rs

@@ -155,3 +155,11 @@ fn alias_ordering() {
     let actual = nu!(r#"alias bar = echo; def echo [] { 'dummy echo' }; bar 'foo'"#);
     assert_eq!(actual.out, "foo");
 }
+
+#[test]
+fn alias_default_help() {
+    let actual = nu!("alias teapot = echo 'I am a beautiful teapot'; help teapot");
+    // There must be at least one line of help
+    let first_help_line = actual.out.lines().next().unwrap();
+    assert!(first_help_line.starts_with("Alias for `echo 'I am a beautiful teapot'`"));
+}

crates/nu-parser/src/parse_keywords.rs

@@ -856,7 +856,6 @@ pub fn parse_alias(
 
     if let Some(decl_id) = working_set.find_decl(b"alias") {
         let (command_spans, rest_spans) = spans.split_at(split_id);
-        let (usage, extra_usage) = working_set.build_usage(&lite_command.comments);
 
         let original_starting_error_count = working_set.parse_errors.len();
 
@@ -865,6 +864,7 @@ pub fn parse_alias(
             output,
             ..
         } = parse_internal_call(working_set, span(command_spans), rest_spans, decl_id);
+
         working_set
             .parse_errors
             .truncate(original_starting_error_count);
@@ -1010,6 +1010,27 @@ pub fn parse_alias(
                 }
             };
 
+            // Tries to build a useful usage string
+            let (usage, extra_usage) = match lite_command.comments.is_empty() {
+                // First from comments, if any are present
+                false => working_set.build_usage(&lite_command.comments),
+                // Then from the command itself
+                true => match alias_call.arguments.get(1) {
+                    Some(Argument::Positional(Expression {
+                        expr: Expr::Keyword(.., expr),
+                        ..
+                    })) => {
+                        let aliased = working_set.get_span_contents(expr.span);
+                        (
+                            format!("Alias for `{}`", String::from_utf8_lossy(aliased)),
+                            String::new(),
+                        )
+                    }
+                    // Then with a default.
+                    _ => ("User declared alias".into(), String::new()),
+                },
+            };
+
             let decl = Alias {
                 name: alias_name,
                 command,