implicit address-taking for array slices

TODO.md

@@ -144,6 +144,13 @@
 9. allow pointer field access pass-through (implemented)
   - having a pointer (`ptr`) to a struct, we should allow access through `ptr.field` as opposed to mandating `ptr^.field`
 
+10. make slice expressions on array variables implicitly address-taking (implemented)
+  - zig's slice expression on an array variable handles the address-taking implicitly (nice ergonomics)
+  - `arr[a..b]` on an array variable now slices without the explicit `&`; the
+    explicit `(&arr)[a..b]` pointer-to-array form keeps working unchanged
+  - array rvalues (e.g. a by-value array return) are materialized into a
+    temporary before slicing, matching the for-loop iterable lowering
+
 ## A word on multi-unwrap
 
 Unwrap multiple optionals with `and`. This **short-circuits**: if the first optional is none, subsequent expressions are not evaluated.

compiler/lower/lower.odin

@@ -244,9 +244,26 @@ lower_compound_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instructi
 			diagnostic=source.INVALID_DIAGNOSTIC,
 		})
 	case .Slice:
-		container := lower_nested_expr(state, expr.left)
-		if types.is_array(state.hir_module.exprs[expr.left].type, &state.hir_module.types) {
-			container = lower_location(state, expr.left)
+		left_type := state.hir_module.exprs[expr.left].type
+		container: ir.Instruction_Id
+		if types.is_array(left_type, &state.hir_module.types) {
+			if hir_expr_is_location(state, expr.left) {
+				container = lower_location(state, expr.left)
+			} else {
+				value := lower_nested_expr(state, expr.left)
+				container = append_instruction(state, ir.Instruction{
+					op=.Alloca, span=expr.span, type=left_type,
+					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+				append_instruction(state, ir.Instruction{
+					op=.Store, span=expr.span, type=left_type,
+					target=ir.INVALID_REF, a=container, b=value,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+			}
+		} else {
+			container = lower_nested_expr(state, expr.left)
 		}
 		args := make([]ir.Instruction_Id, len(expr.args), state.allocator)
 		for arg, index in expr.args {

compiler_tests.odin

@@ -1017,6 +1017,78 @@ immutable_pointer_and_slice_bindings_preserve_mutable_pointees :: proc(t: ^testi
 	testing.expect_value(t, len(diagnostics.items), 0)
 }
 
+@(test)
+slicing_an_array_variable_takes_its_address_implicitly :: proc(t: ^testing.T) {
+	// Milestone 10: `arr[a..b]` on an array variable slices without an explicit
+	// `&`. The slice operand must be a pointer to the array (getelementptr off a
+	// `ptr`), not the array value.
+	text := `sink :: func(s []i32) i32 {
+	return s[0]
+}
+main :: func() i32 {
+	arr [4]i32 = [10, 20, 30, 40]
+	full :: sink(arr[..])
+	part :: sink(arr[1..3])
+	return full + part
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	ir_module := lower.lower(&hir_module)
+	defer ir.destroy_module(&ir_module)
+	llvm_text := llvm.emit(&ir_module, &diagnostics, &symbols)
+	defer delete(llvm_text)
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, strings.contains(llvm_text, "getelementptr [4 x i32], ptr"))
+}
+
+@(test)
+slicing_an_array_rvalue_materializes_a_temporary :: proc(t: ^testing.T) {
+	// The checker accepts slicing a non-location array (here, a by-value array
+	// return). Lowering must store it into a temporary and slice that address;
+	// otherwise the slice operand is an array value, which is an invalid pointer.
+	text := `make_arr :: func() [4]i32 {
+	return [1, 2, 3, 4]
+}
+sink :: func(s []i32) i32 {
+	return s[0]
+}
+main :: func() i32 {
+	return sink(make_arr()[0..])
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	ir_module := lower.lower(&hir_module)
+	defer ir.destroy_module(&ir_module)
+	llvm_text := llvm.emit(&ir_module, &diagnostics, &symbols)
+	defer delete(llvm_text)
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+	// The materialization store of the array rvalue into its temporary.
+	testing.expect(t, strings.contains(llvm_text, "store [4 x i32]"))
+	testing.expect(t, strings.contains(llvm_text, "getelementptr [4 x i32], ptr"))
+}
+
 @(test)
 c_variadic_calls_promote_extras_and_emit_variadic_llvm :: proc(t: ^testing.T) {
 text := `variadic :: c_func(tag c_int, ...) c_int

examples/programs/for_loop/main.bro

@@ -17,7 +17,7 @@ main :: func() i32 {
 		item^ = item^ + 1
 	}
 
-	view []mut i32 = (&items)[..]
+	view []mut i32 = items[..]
 	for view |@item, index| {
 		item^ = item^ + 1
 		_ = index

testbed/main.bro

@@ -71,14 +71,14 @@ main :: func() i32 {
 		Player { id = PlayerID(4), name = "Grace", tier = .gold, score = 33, streak = 5, active = true },
 	]
 
-	apply_decay((&players)[..])
+	apply_decay(players[..])
 
 	for (&players) |@player, index| {
 		score :: projected_score(player^)
 		_ = printf("%d: %s projected=%d\n", index, player.name, score)
 	}
 
-	if best_player((&players)[..]) |winner : projected_score(winner^) >= 80| {
+	if best_player(players[..]) |winner : projected_score(winner^) >= 80| {
 		_ = printf("winner: %s\n", winner.name)
 		return 0
 	}