implement `?return` expression

doc/codegen.md

@@ -0,0 +1,33 @@
+# Code Generation
+
+## Data Representation
+
+Every type has a "handle". If a type is a simple primitive type such as i32 or
+f64, the handle is "by value", meaning that we pass around the value itself when
+we refer to a value of that type.
+
+If a type is a container, error union, maybe type, slice, or array, then its
+handle is a pointer, and everywhere we refer to a value of this type we refer to
+a pointer.
+
+Parameters and return values are always passed as handles.
+
+Error union types are represented as:
+
+    struct {
+        error: u32,
+        payload: T,
+    }
+
+Maybe types are represented as:
+
+    struct {
+        payload: T,
+        is_non_null: u1,
+    }
+
+## Data Optimizations
+
+Maybe pointer types are special: the 0x0 pointer value is used to represent a
+null pointer. Thus, instead of the struct above, maybe pointer types are
+represented as a `usize` in codegen and the handle is by value.

src/analyze.cpp

@@ -6271,7 +6271,34 @@ static TypeTableEntry *analyze_return_expr(CodeGen *g, ImportTableEntry *import,
                 }
             }
         case ReturnKindMaybe:
-            zig_panic("TODO");
+            {
+                TypeTableEntry *expected_maybe_type;
+                if (expected_type) {
+                    expected_maybe_type = get_maybe_type(g, expected_type);
+                } else {
+                    expected_maybe_type = nullptr;
+                }
+                TypeTableEntry *resolved_type = analyze_expression(g, import, context, expected_maybe_type,
+                        node->data.return_expr.expr);
+                if (resolved_type->id == TypeTableEntryIdInvalid) {
+                    return resolved_type;
+                } else if (resolved_type->id == TypeTableEntryIdMaybe) {
+                    TypeTableEntry *return_type = context->fn_entry->type_entry->data.fn.fn_type_id.return_type;
+                    if (return_type->id != TypeTableEntryIdMaybe) {
+                        ErrorMsg *msg = add_node_error(g, node,
+                            buf_sprintf("?return statement in function with return type '%s'",
+                                buf_ptr(&return_type->name)));
+                        AstNode *return_type_node = context->fn_entry->fn_def_node->data.fn_def.fn_proto->data.fn_proto.return_type;
+                        add_error_note(g, msg, return_type_node, buf_sprintf("function return type here"));
+                    }
+
+                    return resolved_type->data.maybe.child_type;
+                } else {
+                    add_node_error(g, node->data.return_expr.expr,
+                        buf_sprintf("expected maybe type, got '%s'", buf_ptr(&resolved_type->name)));
+                    return g->builtin_types.entry_invalid;
+                }
+            }
     }
     zig_unreachable();
 }

src/codegen.cpp

@@ -2497,7 +2497,45 @@ static LLVMValueRef gen_return_expr(CodeGen *g, AstNode *node) {
                 }
             }
         case ReturnKindMaybe:
-            zig_panic("TODO");
+            {
+                assert(value_type->id == TypeTableEntryIdMaybe);
+                TypeTableEntry *child_type = value_type->data.maybe.child_type;
+
+                LLVMBasicBlockRef return_block = LLVMAppendBasicBlock(g->cur_fn->fn_value, "MaybeRetReturn");
+                LLVMBasicBlockRef continue_block = LLVMAppendBasicBlock(g->cur_fn->fn_value, "MaybeRetContinue");
+
+                set_debug_source_node(g, node);
+                LLVMValueRef maybe_val_ptr = LLVMBuildStructGEP(g->builder, value, 1, "");
+                LLVMValueRef is_non_null = LLVMBuildLoad(g->builder, maybe_val_ptr, "");
+
+                LLVMValueRef zero = LLVMConstNull(LLVMInt1Type());
+                LLVMValueRef cond_val = LLVMBuildICmp(g->builder, LLVMIntNE, is_non_null, zero, "");
+                LLVMBuildCondBr(g->builder, cond_val, continue_block, return_block);
+
+                LLVMPositionBuilderAtEnd(g->builder, return_block);
+                TypeTableEntry *return_type = g->cur_fn->type_entry->data.fn.fn_type_id.return_type;
+                assert(return_type->id == TypeTableEntryIdMaybe);
+                if (handle_is_ptr(return_type)) {
+                    assert(g->cur_ret_ptr);
+
+                    set_debug_source_node(g, node);
+                    LLVMValueRef maybe_bit_ptr = LLVMBuildStructGEP(g->builder, g->cur_ret_ptr, 1, "");
+                    LLVMBuildStore(g->builder, zero, maybe_bit_ptr);
+                    LLVMBuildRetVoid(g->builder);
+                } else {
+                    LLVMValueRef ret_zero_value = LLVMConstNull(return_type->type_ref);
+                    gen_return(g, node, ret_zero_value, ReturnKnowledgeKnownNull);
+                }
+
+                LLVMPositionBuilderAtEnd(g->builder, continue_block);
+                if (type_has_bits(child_type)) {
+                    set_debug_source_node(g, node);
+                    LLVMValueRef val_ptr = LLVMBuildStructGEP(g->builder, value, 0, "");
+                    return get_handle_value(g, node, val_ptr, child_type);
+                } else {
+                    return nullptr;
+                }
+            }
     }
     zig_unreachable();
 }

test/cases/return_type_type.zig

@@ -11,7 +11,7 @@ pub struct SmallList(inline T: type, inline STATIC_SIZE: usize) {
 }
 
 #attribute("test")
-fn function_with_return_type_type() {
+fn functionWithReturnTypeType() {
     var list: List(i32) = undefined;
     var list2: List(i32) = undefined;
     list.length = 10;

test/self_hosted.zig

@@ -6,6 +6,7 @@ const other = @import("other.zig");
 const test_return_type_type = @import("cases/return_type_type.zig");
 const test_zeroes = @import("cases/zeroes.zig");
 const test_sizeof_and_typeof = @import("cases/sizeof_and_typeof.zig");
+const test_maybe_return = @import("cases/maybe_return.zig");
 
 // normal comment
 /// this is a documentation comment