parse and format pattern as

crates/compiler/fmt/src/lib.rs

@@ -69,7 +69,11 @@ impl<'a> Buf<'a> {
     }
 
     pub fn push_str_allow_spaces(&mut self, s: &str) {
-        debug_assert!(!self.beginning_of_line);
+        debug_assert!(
+            !self.beginning_of_line,
+            "push_str: `{s}` with text:\n{}",
+            self.text
+        );
 
         self.flush_spaces();
 
@@ -77,8 +81,13 @@ impl<'a> Buf<'a> {
     }
 
     pub fn push_str(&mut self, s: &str) {
-        debug_assert!(!self.beginning_of_line);
-        debug_assert!(!s.contains('\n') && !s.ends_with(' '));
+        debug_assert!(
+            !self.beginning_of_line,
+            "push_str: `{s}` with text:\n{}",
+            self.text
+        );
+        debug_assert!(!s.contains('\n'));
+        debug_assert!(!s.ends_with(' '));
 
         if !s.is_empty() {
             self.flush_spaces();

crates/compiler/fmt/src/pattern.rs

@@ -1,6 +1,6 @@
 use crate::annotation::{Formattable, Newlines, Parens};
 use crate::expr::{fmt_str_literal, format_sq_literal};
-use crate::spaces::{fmt_comments_only, fmt_spaces, NewlineAt};
+use crate::spaces::{fmt_comments_only, fmt_spaces, NewlineAt, INDENT};
 use crate::Buf;
 use roc_parse::ast::{Base, CommentOrNewline, Pattern};
 
@@ -28,6 +28,10 @@ impl<'a> Formattable for Pattern<'a> {
 
             Pattern::OptionalField(_, expr) => expr.is_multiline(),
 
+            Pattern::As(pattern, spaces, _identifier) => {
+                pattern.is_multiline() || spaces.iter().any(|s| s.is_comment())
+            }
+
             Pattern::Identifier(_)
             | Pattern::Tag(_)
             | Pattern::OpaqueRef(_)
@@ -199,6 +203,27 @@ impl<'a> Formattable for Pattern<'a> {
                 buf.push_str("..");
             }
 
+            As(pattern, spaces, identifier) => {
+                fmt_pattern(buf, &pattern.value, indent, parens);
+
+                let as_indent = indent + INDENT;
+
+                buf.indent(as_indent);
+
+                if !buf.ends_with_space() {
+                    buf.spaces(1);
+                }
+
+                buf.push_str("as");
+                buf.spaces(1);
+
+                // these spaces "belong" to the identifier, which can never be multiline
+                fmt_comments_only(buf, spaces.iter(), NewlineAt::Bottom, indent);
+
+                buf.indent(as_indent);
+                buf.push_str(identifier.value);
+            }
+
             // Space
             SpaceBefore(sub_pattern, spaces) => {
                 if !sub_pattern.is_multiline() {

crates/compiler/fmt/src/spaces.rs

@@ -766,6 +766,11 @@ impl<'a> RemoveSpaces<'a> for Pattern<'a> {
             Pattern::OptionalField(a, b) => {
                 Pattern::OptionalField(a, arena.alloc(b.remove_spaces(arena)))
             }
+            Pattern::As(pattern, spaces, identifier) => Pattern::As(
+                arena.alloc(pattern.remove_spaces(arena)),
+                spaces,
+                identifier,
+            ),
             Pattern::NumLiteral(a) => Pattern::NumLiteral(a),
             Pattern::NonBase10Literal {
                 string,