Fix bug that caused extraneous assignment in IR generation

Previously we would expand optional record fields to assignments when
converting record patterns to "when" expressions. This resulted in
incorrect code being generated.

compiler/mono/src/ir.rs

@@ -1806,33 +1806,9 @@ fn patterns_to_when<'a>(
     for (pattern_var, pattern) in patterns.into_iter() {
         let context = crate::exhaustive::Context::BadArg;
         let mono_pattern = match from_can_pattern(env, layout_cache, &pattern.value) {
-            Ok((pat, assignments)) => {
+            Ok((pat, _assignments)) => {
-                for (symbol, variable, expr) in assignments.into_iter().rev() {
+                // Don't apply any assignments (e.g. to initialize optional variables) yet.
-                    if let Ok(old_body) = body {
+                // We'll take care of that later when expanding the new "when" branch.
-                        let def = roc_can::def::Def {
-                            annotation: None,
-                            expr_var: variable,
-                            loc_expr: Loc::at(pattern.region, expr),
-                            loc_pattern: Loc::at(
-                                pattern.region,
-                                roc_can::pattern::Pattern::Identifier(symbol),
-                            ),
-                            pattern_vars: std::iter::once((symbol, variable)).collect(),
-                        };
-                        let new_expr = roc_can::expr::Expr::LetNonRec(
-                            Box::new(def),
-                            Box::new(old_body),
-                            variable,
-                        );
-                        let new_body = Loc {
-                            region: pattern.region,
-                            value: new_expr,
-                        };
-
-                        body = Ok(new_body);
-                    }
-                }
-
                 pat
             }
             Err(runtime_error) => {

compiler/test_gen/src/gen_records.rs

@@ -587,9 +587,7 @@ fn optional_field_function_use_default() {
 
 #[test]
 #[cfg(any(feature = "gen-llvm"))]
-#[ignore]
 fn optional_field_function_no_use_default() {
-    // blocked on https://github.com/rtfeldman/roc/issues/786
     assert_evals_to!(
         indoc!(
             r#"
@@ -608,9 +606,7 @@ fn optional_field_function_no_use_default() {
 
 #[test]
 #[cfg(any(feature = "gen-llvm"))]
-#[ignore]
 fn optional_field_function_no_use_default_nested() {
-    // blocked on https://github.com/rtfeldman/roc/issues/786
     assert_evals_to!(
         indoc!(
             r#"

compiler/test_mono/generated/record_optional_field_function_no_use_default.txt

@@ -5,7 +5,6 @@ procedure Num.24 (#Attr.2, #Attr.3):
 procedure Test.1 (Test.4):
     let Test.2 = StructAtIndex 0 Test.4;
     let Test.3 = StructAtIndex 1 Test.4;
-    let Test.2 = 10i64;
     let Test.7 = CallByName Num.24 Test.2 Test.3;
     ret Test.7;
 

compiler/test_mono/generated/record_optional_field_function_use_default.txt

@@ -3,7 +3,6 @@ procedure Num.24 (#Attr.2, #Attr.3):
     ret Test.8;
 
 procedure Test.1 (Test.4):
-    let Test.2 = 10i64;
     let Test.2 = 10i64;
     let Test.7 = CallByName Num.24 Test.2 Test.4;
     ret Test.7;