Make unit assignment optional for suffixed defs

crates/compiler/fmt/Cargo.toml

@@ -12,5 +12,6 @@ roc_collections = { path = "../collections" }
 roc_module = { path = "../module" }
 roc_parse = { path = "../parse" }
 roc_region = { path = "../region" }
+roc_error_macros = { path = "../../error_macros" }
 
 bumpalo.workspace = true

crates/compiler/fmt/src/def.rs

@@ -357,9 +357,19 @@ pub fn fmt_defs(buf: &mut Buf, defs: &Defs, indent: u16) {
 }
 
 pub fn fmt_body<'a>(buf: &mut Buf, pattern: &'a Pattern<'a>, body: &'a Expr<'a>, indent: u16) {
-    pattern.format_with_options(buf, Parens::InApply, Newlines::No, indent);
-    buf.indent(indent);
-    buf.push_str(" =");
+    // Check if this is an assignment into the unit/empty record, format as a Statement
+    let is_statement = if let Pattern::RecordDestructure(collection) = pattern {
+        collection.is_empty()
+    } else {
+        false
+    };
+
+    // Don't format the `{} =` for defs with this pattern
+    if !is_statement {
+        pattern.format_with_options(buf, Parens::InApply, Newlines::No, indent);
+        buf.indent(indent);
+        buf.push_str(" =");
+    }
 
     if body.is_multiline() {
         match body {

crates/compiler/fmt/src/pattern.rs

@@ -2,6 +2,7 @@ use crate::annotation::{Formattable, Newlines, Parens};
 use crate::expr::{fmt_str_literal, format_sq_literal};
 use crate::spaces::{fmt_comments_only, fmt_spaces, NewlineAt, INDENT};
 use crate::Buf;
+use roc_error_macros::internal_error;
 use roc_parse::ast::{Base, CommentOrNewline, Pattern, PatternAs};
 
 pub fn fmt_pattern<'a>(buf: &mut Buf, pattern: &'a Pattern<'a>, indent: u16, parens: Parens) {
@@ -81,6 +82,7 @@ impl<'a> Formattable for Pattern<'a> {
             Pattern::Tuple(patterns) | Pattern::List(patterns) => {
                 patterns.iter().any(|p| p.is_multiline())
             }
+            Pattern::Stmt(_) => true,
         }
     }
 
@@ -279,6 +281,10 @@ impl<'a> Formattable for Pattern<'a> {
 
                 buf.push_str(ident);
             }
+
+            // Statement e.g. Suffixed with optional `{}=`
+            // e.g. `Stdout.line! "Hello World"`
+            Stmt(_) => internal_error!("should be desugared in parser into alternate pattern"),
         }
     }
 }

crates/compiler/fmt/src/spaces.rs

@@ -837,6 +837,7 @@ impl<'a> RemoveSpaces<'a> for Pattern<'a> {
                 opt_pattern_as
                     .map(|(_, pattern_as)| ([].as_ref(), pattern_as.remove_spaces(arena))),
             ),
+            Pattern::Stmt(a) => Pattern::Stmt(a),
         }
     }
 }