Make sure patterns continue to parse as such

crates/compiler/fmt/src/def.rs

@@ -454,7 +454,15 @@ impl<'a> Formattable for TypeDef<'a> {
 
                 let make_multiline = ann.is_multiline() || has_abilities_multiline;
 
-                fmt_general_def(header, buf, indent, ":=", &ann.value, newlines);
+                fmt_general_def(
+                    header,
+                    Parens::NotNeeded,
+                    buf,
+                    indent,
+                    ":=",
+                    &ann.value,
+                    newlines,
+                );
 
                 if let Some(has_abilities) = has_abilities {
                     buf.spaces(1);
@@ -824,8 +832,14 @@ impl<'a> Formattable for ValueDef<'a> {
         use roc_parse::ast::ValueDef::*;
         match self {
             Annotation(loc_pattern, loc_annotation) => {
+                let pat_parens = if ann_pattern_needs_parens(&loc_pattern.value) {
+                    Parens::InApply
+                } else {
+                    Parens::NotNeeded
+                };
                 fmt_general_def(
                     loc_pattern,
+                    pat_parens,
                     buf,
                     indent,
                     ":",
@@ -845,7 +859,20 @@ impl<'a> Formattable for ValueDef<'a> {
                 body_pattern,
                 body_expr,
             } => {
-                fmt_general_def(ann_pattern, buf, indent, ":", &ann_type.value, newlines);
+                let pat_parens = if ann_pattern_needs_parens(&ann_pattern.value) {
+                    Parens::InApply
+                } else {
+                    Parens::NotNeeded
+                };
+                fmt_general_def(
+                    ann_pattern,
+                    pat_parens,
+                    buf,
+                    indent,
+                    ":",
+                    &ann_type.value,
+                    newlines,
+                );
 
                 fmt_annotated_body_comment(buf, indent, lines_between);
 
@@ -860,8 +887,19 @@ impl<'a> Formattable for ValueDef<'a> {
     }
 }
 
+fn ann_pattern_needs_parens(value: &Pattern<'_>) -> bool {
+    match value.extract_spaces().item {
+        Pattern::Tag(_) => true,
+        Pattern::Apply(func, _args) if matches!(func.extract_spaces().item, Pattern::Tag(..)) => {
+            true
+        }
+        _ => false,
+    }
+}
+
 fn fmt_general_def<L: Formattable>(
     lhs: L,
+    lhs_parens: Parens,
     buf: &mut Buf,
 
     indent: u16,
@@ -869,7 +907,7 @@ fn fmt_general_def<L: Formattable>(
     rhs: &TypeAnnotation,
     newlines: Newlines,
 ) {
-    lhs.format(buf, indent);
+    lhs.format_with_options(buf, lhs_parens, Newlines::Yes, indent);
     buf.indent(indent);
 
     if rhs.is_multiline() {

crates/compiler/test_syntax/tests/snapshots/pass/annotated_tag_destructure.expr.formatted.roc

@@ -1,4 +1,4 @@
-UserId x : [UserId I64]
+(UserId x) : [UserId I64]
 (UserId x) = UserId 42
 
 x

crates/compiler/test_syntax/tests/snapshots/pass/nested_parens_in_pattern.expr.formatted.roc

@@ -0,0 +1,2 @@
+(J x) : i
+i

crates/compiler/test_syntax/tests/snapshots/pass/nested_parens_in_pattern.expr.result-ast

@@ -0,0 +1,44 @@
+Defs(
+    Defs {
+        tags: [
+            EitherIndex(2147483648),
+        ],
+        regions: [
+            @0-8,
+        ],
+        space_before: [
+            Slice<roc_parse::ast::CommentOrNewline> { start: 0, length: 0 },
+        ],
+        space_after: [
+            Slice<roc_parse::ast::CommentOrNewline> { start: 0, length: 0 },
+        ],
+        spaces: [],
+        type_defs: [],
+        value_defs: [
+            Annotation(
+                @1-5 Apply(
+                    @2-3 Tag(
+                        "J",
+                    ),
+                    [
+                        @4-5 Identifier {
+                            ident: "x",
+                        },
+                    ],
+                ),
+                @7-8 BoundVariable(
+                    "i",
+                ),
+            ),
+        ],
+    },
+    @9-10 SpaceBefore(
+        Var {
+            module_name: "",
+            ident: "i",
+        },
+        [
+            Newline,
+        ],
+    ),
+)

crates/compiler/test_syntax/tests/snapshots/pass/nested_parens_in_pattern.expr.roc

@@ -0,0 +1,2 @@
+((J)x):i
+i

crates/compiler/test_syntax/tests/test_snapshots.rs

@@ -480,6 +480,7 @@ mod test_snapshots {
         pass/nested_def_annotation.moduledefs,
         pass/nested_if.expr,
         pass/nested_list_comment_in_closure_arg.expr,
+        pass/nested_parens_in_pattern.expr,
         pass/newline_after_equals.expr, // Regression test for https://github.com/roc-lang/roc/issues/51
         pass/newline_after_mul.expr,
         pass/newline_after_opt_field.expr,