feat: add support for alias command formatting and preserve comment spacing

src/formatting/calls.rs

@@ -19,6 +19,7 @@ pub(super) const BLOCK_COMMANDS: &[&str] = &["for", "while", "loop", "module"];
 pub(super) const CONDITIONAL_COMMANDS: &[&str] = &["if", "try"];
 pub(super) const DEF_COMMANDS: &[&str] = &["def", "def-env", "export def"];
 pub(super) const EXTERN_COMMANDS: &[&str] = &["extern", "export extern"];
+pub(super) const ALIAS_COMMANDS: &[&str] = &["alias", "export alias"];
 pub(super) const LET_COMMANDS: &[&str] = &["let", "let-env", "mut", "const", "export const"];
 
 impl<'a> Formatter<'a> {
@@ -47,6 +48,11 @@ impl<'a> Formatter<'a> {
             return;
         }
 
+        if matches!(cmd_type, CommandType::Alias) {
+            self.format_alias_call(call);
+            return;
+        }
+
         if decl_name == "for" {
             self.format_for_call(call);
             return;
@@ -270,12 +276,90 @@ impl<'a> Formatter<'a> {
         }
     }
 
+    /// Format alias definitions while preserving the literal right-hand side.
+    ///
+    /// The parser resolves alias references semantically, which can expand the
+    /// RHS if it is re-rendered from the AST. Preserve the original source text
+    /// after `=` to keep alias definitions idempotent.
+    pub(super) fn format_alias_call(&mut self, call: &nu_protocol::ast::Call) {
+        let positional: Vec<&Expression> = call
+            .arguments
+            .iter()
+            .filter_map(|arg| match arg {
+                Argument::Positional(expr) | Argument::Unknown(expr) => Some(expr),
+                _ => None,
+            })
+            .collect();
+
+        let Some(name) = positional.first() else {
+            for arg in &call.arguments {
+                self.format_call_argument(arg, &CommandType::Alias);
+            }
+            return;
+        };
+
+        self.space();
+        self.format_expression(name);
+
+        let rhs_end = call
+            .arguments
+            .iter()
+            .filter_map(|arg| match arg {
+                Argument::Positional(expr) | Argument::Unknown(expr) | Argument::Spread(expr) => {
+                    Some(expr.span.end)
+                }
+                Argument::Named(named) => named
+                    .2
+                    .as_ref()
+                    .map_or(Some(named.0.span.end), |value| Some(value.span.end)),
+            })
+            .max();
+
+        let Some(rhs_end) = rhs_end else {
+            return;
+        };
+
+        if name.span.end >= rhs_end || rhs_end > self.source.len() {
+            self.format_regular_arguments(&call.arguments[1..]);
+            return;
+        }
+
+        let between = &self.source[name.span.end..rhs_end];
+        let Some(eq_offset) = between.iter().position(|byte| *byte == b'=') else {
+            self.format_regular_arguments(&call.arguments[1..]);
+            return;
+        };
+
+        let rhs_start = between[eq_offset + 1..]
+            .iter()
+            .position(|byte| !byte.is_ascii_whitespace())
+            .map(|offset| name.span.end + eq_offset + 1 + offset);
+
+        let Some(rhs_start) = rhs_start else {
+            self.write(" =");
+            return;
+        };
+
+        // Keep the exact user-authored alias RHS to avoid semantic expansion
+        // when aliases reference other aliases.
+        self.write(" = ");
+        self.write_bytes(&self.source[rhs_start..rhs_end]);
+    }
+
+    fn format_regular_arguments(&mut self, args: &[Argument]) {
+        for arg in args {
+            self.format_call_argument(arg, &CommandType::Regular);
+        }
+    }
+
     /// Classify a command name into a [`CommandType`] for formatting purposes.
     pub(super) fn classify_command(name: &str) -> CommandType {
         if DEF_COMMANDS.contains(&name) {
             CommandType::Def
         } else if EXTERN_COMMANDS.contains(&name) {
             CommandType::Extern
+        } else if ALIAS_COMMANDS.contains(&name) {
+            CommandType::Alias
         } else if CONDITIONAL_COMMANDS.contains(&name) {
             CommandType::Conditional
         } else if LET_COMMANDS.contains(&name) {
@@ -336,6 +420,7 @@ impl<'a> Formatter<'a> {
         match cmd_type {
             CommandType::Def => self.format_def_argument(positional),
             CommandType::Extern => self.format_extern_argument(positional),
+            CommandType::Alias => self.format_expression(positional),
             CommandType::Conditional => {
                 self.conditional_context_depth += 1;
                 self.format_block_or_expr(positional);

src/formatting/collections.rs

@@ -258,17 +258,30 @@ impl<'a> Formatter<'a> {
             self.write("{");
             self.newline();
             self.indent_level += 1;
+
+            let closing_brace_pos = span.end.saturating_sub(1);
             for item in items {
+                let item_start = match item {
+                    RecordItem::Pair(key, _) => key.span.start,
+                    RecordItem::Spread(_, expr) => expr.span.start,
+                };
+                self.write_comments_before(item_start);
                 self.write_indent();
                 self.format_record_item(item, preserve_compact);
+
                 // Capture inline comments on the same line as the record item.
                 let item_end = match item {
                     RecordItem::Pair(_, v) => v.span.end,
                     RecordItem::Spread(_, expr) => expr.span.end,
                 };
-                self.write_inline_comment_bounded(item_end, None);
+                self.write_inline_comment_bounded(item_end, Some(closing_brace_pos));
+                // Advance beyond the current item so inter-item comments are
+                // searched from the correct source window.
+                self.last_pos = self.last_pos.max(item_end);
                 self.newline();
             }
+
+            self.write_comments_before(closing_brace_pos);
             self.indent_level -= 1;
             self.write_indent();
             self.write("}");

src/formatting/comments.rs

@@ -60,6 +60,55 @@ pub(super) fn extract_comments(source: &[u8]) -> Vec<(Span, Vec<u8>)> {
 // ─────────────────────────────────────────────────────────────────────────────
 
 impl<'a> Formatter<'a> {
+    /// Return true when the source slice contains at least one blank line
+    /// (two newline boundaries with only whitespace between them).
+    fn source_has_blank_line(&self, start: usize, end: usize) -> bool {
+        if start >= end || end > self.source.len() {
+            return false;
+        }
+
+        let mut previous_newline: Option<usize> = None;
+        for (offset, byte) in self.source[start..end].iter().enumerate() {
+            if *byte != b'\n' {
+                continue;
+            }
+
+            if let Some(prev) = previous_newline {
+                let gap_start = start + prev + 1;
+                let gap_end = start + offset;
+                if self.source[gap_start..gap_end]
+                    .iter()
+                    .all(|b| b.is_ascii_whitespace())
+                {
+                    return true;
+                }
+            }
+
+            previous_newline = Some(offset);
+        }
+
+        false
+    }
+
+    fn ensure_trailing_newlines(&mut self, min_newlines: usize) {
+        if self.output.is_empty() || min_newlines == 0 {
+            return;
+        }
+
+        // Count contiguous trailing newlines so we can top up to the requested
+        // separation without over-emitting line breaks.
+        let existing = self
+            .output
+            .iter()
+            .rev()
+            .take_while(|&&byte| byte == b'\n')
+            .count();
+
+        for _ in existing..min_newlines {
+            self.newline();
+        }
+    }
+
     /// Emit all comments that fall between `last_pos` and `pos`, each on its
     /// own line with the current indentation.
     pub(super) fn write_comments_before(&mut self, pos: usize) {
@@ -75,24 +124,29 @@ impl<'a> Formatter<'a> {
 
         comments_to_write.sort_by_key(|(_, start, _)| *start);
 
+        let Some((_, first_start, _)) = comments_to_write.first() else {
+            return;
+        };
+
+        let leading_newlines = if self.source_has_blank_line(self.last_pos, *first_start) {
+            2
+        } else {
+            1
+        };
+        // Preserve spacing before a standalone comment group.
+        self.ensure_trailing_newlines(leading_newlines);
+
         let mut prev_comment_end: Option<usize> = None;
         for (idx, start, content) in &comments_to_write {
             self.written_comments[*idx] = true;
 
-            // Preserve blank lines between comment groups by checking
-            // whether the original source has a blank line between the
-            // previous comment and this one.
             if let Some(prev_end) = prev_comment_end {
-                let between = &self.source[prev_end..*start];
-                let newline_count = between.iter().filter(|&&b| b == b'\n').count();
-                if newline_count >= 2 {
-                    // There was at least one blank line in the source
-                    // between the two comments — emit one now.
-                    if !self.at_line_start {
-                        self.newline();
-                    }
-                    self.newline();
-                }
+                let between_newlines = if self.source_has_blank_line(prev_end, *start) {
+                    2
+                } else {
+                    1
+                };
+                self.ensure_trailing_newlines(between_newlines);
             }
 
             if !self.at_line_start {
@@ -108,6 +162,15 @@ impl<'a> Formatter<'a> {
 
             prev_comment_end = Some(start + content.len());
         }
+
+        if let Some(last_comment_end) = prev_comment_end {
+            self.last_pos = last_comment_end;
+            if self.source_has_blank_line(last_comment_end, pos) {
+                // Preserve a blank separator when comments are followed by a
+                // spaced-apart statement group.
+                self.ensure_trailing_newlines(2);
+            }
+        }
     }
 
     /// Emit an inline comment (on the same line) that appears after `after_pos`,

src/formatting/mod.rs

@@ -80,6 +80,7 @@ pub(crate) struct Formatter<'a> {
 pub(crate) enum CommandType {
     Def,
     Extern,
+    Alias,
     Conditional,
     Let,
     Block,

tests/fixtures/expected/alias_references_do_not_duplicate_rhs_issue167.nu

@@ -0,0 +1,3 @@
+alias fdr = fd --follow --hidden --no-ignore
+alias fdf = fdr --follow --hidden --no-ignore --fixed-strings
+alias fdg = fdr --follow --hidden --no-ignore --glob

tests/fixtures/expected/empty_lines_between_comments_and_blocks_preserved_issue165.nu

@@ -0,0 +1,10 @@
+#!/usr/bin/env nu
+
+let x = 1
+
+# This is a nice comment.
+#
+# It's not associated with any particular block. It just describes things about
+# the contents of this file, or the broader section of code.
+
+let y = 2

tests/fixtures/expected/multiline_record_comments_preserved_issue168.nu

@@ -0,0 +1,5 @@
+let a = {
+    my_key: "my_key"
+    # a comment
+    my_other_key: "my_other_key"
+}

tests/fixtures/input/alias_references_do_not_duplicate_rhs_issue167.nu

@@ -0,0 +1,3 @@
+alias fdr = fd --follow --hidden --no-ignore
+alias fdf = fdr --follow --hidden --no-ignore --fixed-strings
+alias fdg = fdr --follow --hidden --no-ignore --glob

tests/fixtures/input/empty_lines_between_comments_and_blocks_preserved_issue165.nu

@@ -0,0 +1,10 @@
+#!/usr/bin/env nu
+
+let x = 1
+
+# This is a nice comment.
+#
+# It's not associated with any particular block. It just describes things about
+# the contents of this file, or the broader section of code.
+
+let y = 2

tests/fixtures/input/multiline_record_comments_preserved_issue168.nu

@@ -0,0 +1,5 @@
+let a = {
+  my_key: "my_key"
+  # a comment
+  my_other_key: "my_other_key"
+}

tests/ground_truth.rs

@@ -610,4 +610,19 @@ fixture_tests!(
         ground_truth_parens_stripping_boolean_exprs_issue162,
         idempotency_parens_stripping_boolean_exprs_issue162
     ),
+    (
+        "empty_lines_between_comments_and_blocks_preserved_issue165",
+        ground_truth_empty_lines_between_comments_and_blocks_preserved_issue165,
+        idempotency_empty_lines_between_comments_and_blocks_preserved_issue165
+    ),
+    (
+        "alias_references_do_not_duplicate_rhs_issue167",
+        ground_truth_alias_references_do_not_duplicate_rhs_issue167,
+        idempotency_alias_references_do_not_duplicate_rhs_issue167
+    ),
+    (
+        "multiline_record_comments_preserved_issue168",
+        ground_truth_multiline_record_comments_preserved_issue168,
+        idempotency_multiline_record_comments_preserved_issue168
+    ),
 );

tests/main.rs

@@ -4,8 +4,7 @@ use std::{fs, io::Write, path::PathBuf, process::Command};
 use tempfile::tempdir;
 
 const INVALID: &str = "# beginning of script comment
-
-let one = 1
+let   one   =   1
 ";
 const VALID: &str = "# beginning of script comment
 let one = 1