Compile lambdas that have an empty lambda set

Closes #3224

This should also fix a number of other issues often observed when a
lambda is never actually called, but we fail to codegen because it has
an empty lambda set.

crates/compiler/debug_flags/src/lib.rs

@@ -104,6 +104,9 @@ flags! {
     /// Prints debug information during the alias analysis pass.
     ROC_DEBUG_ALIAS_ANALYSIS
 
+    /// Print to stderr when a runtime error function is generated.
+    ROC_PRINT_RUNTIME_ERROR_GEN
+
     // ===LLVM Gen===
 
     /// Prints LLVM function verification output.

crates/compiler/mono/src/ir.rs

@@ -15,6 +15,7 @@ use roc_debug_flags::dbg_do;
 #[cfg(debug_assertions)]
 use roc_debug_flags::{
     ROC_PRINT_IR_AFTER_REFCOUNT, ROC_PRINT_IR_AFTER_RESET_REUSE, ROC_PRINT_IR_AFTER_SPECIALIZATION,
+    ROC_PRINT_RUNTIME_ERROR_GEN,
 };
 use roc_derive_key::GlobalDerivedSymbols;
 use roc_error_macros::{internal_error, todo_abilities};
@@ -2807,10 +2808,12 @@ fn generate_runtime_error_function<'a>(
     )
     .unwrap();
 
-    eprintln!(
-        "emitted runtime error function {:?} for layout {:?}",
-        &msg, layout
-    );
+    dbg_do!(ROC_PRINT_RUNTIME_ERROR_GEN, {
+        eprintln!(
+            "emitted runtime error function {:?} for layout {:?}",
+            &msg, layout
+        );
+    });
 
     let runtime_error = Stmt::RuntimeError(msg.into_bump_str());
 
@@ -2878,6 +2881,15 @@ fn specialize_external<'a>(
     let specialized =
         build_specialized_proc_from_var(env, layout_cache, lambda_name, pattern_symbols, fn_var)?;
 
+    let recursivity = if partial_proc.is_self_recursive {
+        SelfRecursive::SelfRecursive(JoinPointId(env.unique_symbol()))
+    } else {
+        SelfRecursive::NotSelfRecursive
+    };
+
+    let body = partial_proc.body.clone();
+    let body_var = partial_proc.body_var;
+
     // determine the layout of aliases/rigids exposed to the host
     let host_exposed_layouts = if host_exposed_variables.is_empty() {
         HostExposedLayouts::NotHostExposed
@@ -2922,6 +2934,7 @@ fn specialize_external<'a>(
 
                     let body = match_on_lambda_set(
                         env,
+                        procs,
                         lambda_set,
                         Symbol::ARG_CLOSURE,
                         argument_symbols.into_bump_slice(),
@@ -2965,15 +2978,7 @@ fn specialize_external<'a>(
         }
     };
 
-    let recursivity = if partial_proc.is_self_recursive {
-        SelfRecursive::SelfRecursive(JoinPointId(env.unique_symbol()))
-    } else {
-        SelfRecursive::NotSelfRecursive
-    };
-
-    let body = partial_proc.body.clone();
-
-    let mut specialized_body = from_can(env, partial_proc.body_var, body, procs, layout_cache);
+    let mut specialized_body = from_can(env, body_var, body, procs, layout_cache);
 
     match specialized {
         SpecializedLayout::FunctionPointerBody {
@@ -4712,6 +4717,7 @@ pub fn with_hole<'a>(
 
                                     result = match_on_lambda_set(
                                         env,
+                                        procs,
                                         lambda_set,
                                         closure_data_symbol,
                                         arg_symbols,
@@ -4752,6 +4758,7 @@ pub fn with_hole<'a>(
 
                                     result = match_on_lambda_set(
                                         env,
+                                        procs,
                                         lambda_set,
                                         closure_data_symbol,
                                         arg_symbols,
@@ -4809,6 +4816,7 @@ pub fn with_hole<'a>(
 
                                     result = match_on_lambda_set(
                                         env,
+                                        procs,
                                         lambda_set,
                                         closure_data_symbol,
                                         arg_symbols,
@@ -7485,6 +7493,7 @@ fn call_by_name<'a>(
 
                     let result = match_on_lambda_set(
                         env,
+                        procs,
                         lambda_set,
                         closure_data_symbol,
                         arg_symbols,
@@ -8045,6 +8054,7 @@ fn call_specialized_proc<'a>(
 
                 let new_hole = match_on_lambda_set(
                     env,
+                    procs,
                     lambda_set,
                     closure_data_symbol,
                     field_symbols,
@@ -9118,6 +9128,7 @@ where
 #[allow(clippy::too_many_arguments)]
 fn match_on_lambda_set<'a>(
     env: &mut Env<'a, '_>,
+    procs: &mut Procs<'a>,
     lambda_set: LambdaSet<'a>,
     closure_data_symbol: Symbol,
     argument_symbols: &'a [Symbol],
@@ -9161,7 +9172,22 @@ fn match_on_lambda_set<'a>(
             field_layouts,
             field_order_hash,
         } => {
-            let function_symbol = lambda_set.iter_set().next().unwrap();
+            let function_symbol = match lambda_set.iter_set().next() {
+                Some(function_symbol) => function_symbol,
+                None => {
+                    // Lambda set is empty, so this function is never called; synthesize a function
+                    // that always yields a runtime error.
+                    let name = env.unique_symbol();
+                    let function_layout =
+                        RawFunctionLayout::Function(argument_layouts, lambda_set, return_layout);
+                    let proc = generate_runtime_error_function(env, name, function_layout);
+                    let top_level =
+                        ProcLayout::from_raw(env.arena, function_layout, CapturesNiche::no_niche());
+
+                    procs.specialized.insert_specialized(name, top_level, proc);
+                    LambdaName::no_niche(name)
+                }
+            };
 
             let closure_info = match field_layouts {
                 [] => ClosureInfo::DoesNotCapture,

crates/compiler/test_gen/src/gen_list.rs

@@ -2958,3 +2958,19 @@ fn with_capacity() {
         RocList<u64>
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn call_function_in_empty_list() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            lst : List ({} -> {})
+            lst = []
+            List.map lst \f -> f {}
+            "#
+        ),
+        RocList::from_slice(&[]),
+        RocList<()>
+    )
+}

crates/compiler/test_mono/generated/call_function_in_empty_list.txt

@@ -0,0 +1,18 @@
+procedure List.5 (#Attr.2, #Attr.3):
+    let List.279 : List {} = lowlevel ListMap { xs: `#Attr.#arg1` } #Attr.2 Test.2 #Attr.3;
+    decref #Attr.2;
+    ret List.279;
+
+procedure Test.2 (Test.3):
+    let Test.7 : {} = Struct {};
+    let Test.6 : {} = CallByName Test.8 Test.7;
+    ret Test.6;
+
+procedure Test.8 (Test.9, #Attr.12):
+    Error The `#UserApp.IdentId(8)` function could not be generated, likely due to a type error.
+
+procedure Test.0 ():
+    let Test.1 : List {} = Array [];
+    let Test.5 : {} = Struct {};
+    let Test.4 : List {} = CallByName List.5 Test.1 Test.5;
+    ret Test.4;

crates/compiler/test_mono/src/tests.rs

@@ -1682,3 +1682,14 @@ fn choose_u128_layout() {
         "#
     )
 }
+
+#[mono_test]
+fn call_function_in_empty_list() {
+    indoc!(
+        r#"
+        lst : List ({} -> {})
+        lst = []
+        List.map lst \f -> f {}
+        "#
+    )
+}