Preserve comments before header

Closes #4212

crates/compiler/fmt/src/module.rs

@@ -1,7 +1,7 @@
 use crate::annotation::{Formattable, Newlines};
 use crate::collection::{fmt_collection, Braces};
 use crate::expr::fmt_str_literal;
-use crate::spaces::{fmt_default_spaces, fmt_spaces, INDENT};
+use crate::spaces::{fmt_comments_only, fmt_default_spaces, fmt_spaces, NewlineAt, INDENT};
 use crate::Buf;
 use roc_parse::ast::{Collection, Module, Spaced};
 use roc_parse::header::{
@@ -31,6 +31,8 @@ pub fn fmt_module<'a>(buf: &mut Buf<'_>, module: &'a Module<'a>) {
 pub fn fmt_interface_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a InterfaceHeader<'a>) {
     let indent = INDENT;
 
+    fmt_comments_only(buf, header.before_header.iter(), NewlineAt::Bottom, indent);
+
     buf.indent(0);
     buf.push_str("interface");
 
@@ -56,6 +58,8 @@ pub fn fmt_interface_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a Interface
 pub fn fmt_hosted_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a HostedHeader<'a>) {
     let indent = INDENT;
 
+    fmt_comments_only(buf, header.before_header.iter(), NewlineAt::Bottom, indent);
+
     buf.indent(0);
     buf.push_str("hosted");
 
@@ -94,6 +98,9 @@ pub fn fmt_hosted_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a HostedHeader
 
 pub fn fmt_app_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a AppHeader<'a>) {
     let indent = INDENT;
+
+    fmt_comments_only(buf, header.before_header.iter(), NewlineAt::Bottom, indent);
+
     buf.indent(0);
     buf.push_str("app");
 
@@ -130,6 +137,8 @@ pub fn fmt_app_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a AppHeader<'a>)
 pub fn fmt_platform_header<'a, 'buf>(buf: &mut Buf<'buf>, header: &'a PlatformHeader<'a>) {
     let indent = INDENT;
 
+    fmt_comments_only(buf, header.before_header.iter(), NewlineAt::Bottom, indent);
+
     buf.indent(0);
     buf.push_str("platform");
 

crates/compiler/fmt/tests/test_fmt.rs

@@ -5615,6 +5615,37 @@ mod test_fmt {
         ));
     }
 
+    #[test]
+    fn leading_comments_preserved() {
+        module_formats_same(indoc!(
+            r#"
+            # hello world
+            interface Foo
+                exposes []
+                imports []
+            "#
+        ));
+
+        module_formats_same(indoc!(
+            r#"
+            # hello world
+            app "test" packages {} imports [] provides [] to "./platform"
+            "#
+        ));
+
+        module_formats_same(indoc!(
+            r#"
+            # hello world
+            platform "hello-world"
+                requires {} { main : Str }
+                exposes []
+                packages {}
+                imports []
+                provides [mainForHost]
+            "#
+        ));
+    }
+
     // this is a parse error atm
     //    #[test]
     //    fn multiline_apply() {