feat: support checked explicit sentinel slices

LANGUAGE.md

@@ -34,7 +34,7 @@ roadmap and milestone history.
 - compile-time `minval!(T)` and `maxval!(T)` bounds for concrete native, C, and scalar-backed distinct integer types; the result retains `T`
 - arrays `[N]T`, inferred-count arrays `[_]T`, sentinel arrays `[N;S]T`, compile-time expression array counts, slices `[]T` / `[;S]T`, single-item pointers `@T`, many-item pointers `*T`, and sentinel many-item pointers `[*;S]T`
 - pointer mutability via `mut`, optional pointers as nullable pointers, pointer arithmetic for many-item pointers, postfix dereference `^`, and trapping optional unwrap `?`
-- pointer-to-array `.len`, indexing, slicing, `.ptr` on slices and pointers-to-arrays, implicit address-taking for array-variable slices, and pointer/slice sentinel weakening
+- pointer-to-array `.len`, indexing, slicing, `.ptr` on slices and pointers-to-arrays, implicit address-taking for array-variable slices, pointer/slice sentinel weakening, and checked explicit sentinel slicing `base[start..end;S]`
 - `ptrcast!(ptr)` infers its complete pointer result from context; `ptrcast!(P, ptr)` supplies `P` explicitly; both change the child type, then permit only ordinary implicit pointer conversions to the requested target
 - unsafe `constcast!(value)` for restoring mutability to pointers, optional pointers, and slices without changing their child type or shape
 - UTF-8 string literals as immutable pointers to static zero-terminated byte arrays, supporting `\\`, `\"`, `\n`, `\r`, `\t`, `\0`, and `\xNN` escapes, plus raw backtick multiline strings
@@ -48,6 +48,28 @@ roadmap and milestone history.
 - fallible channel types `T ! E`, where `E` is a native enum, native struct, tagged union, or supported sum composition; `void ! E` functions complete successfully on fallthrough, and void-success `catch` handlers may fall through without `yield`
 - bodyful local functions and root `main` may write `T!` to infer a specialization-local error channel from propagated `try` expressions and concretely typed error returns; inference composes only existing named error types, never synthesizes variants, and requires at least one inferred error
 
+#### checked sentinel slicing
+
+`base[start..end;S]` constructs a sentinel slice `[;S]T` without copying elements.
+The sentinel `S` must be a compile-time integer representable by the source element type `T`;
+the result retains the source's element mutability. Bounds use `usize`, as in ordinary slicing.
+`start` defaults to zero; `end` may be omitted for arrays, pointers-to-arrays, and slices, where
+it defaults to the logical length. Many-item pointers, including sentinel pointers, always
+require an explicit end: `pointer[..count;0]`.
+
+The result contains elements `[start, end)` and excludes the sentinel at `end`.
+Before constructing it, runtime checks require ordered in-range bounds, a readable endpoint,
+and `base[end] == S`; failure traps. For a bounded source without a sentinel, `end` must be
+strictly less than its length. A sentinel source permits `end == len`, reading its stored
+sentinel. A many-item pointer has no length to check: the caller must provide valid memory
+through the endpoint, and the endpoint value is still checked. Comptime evaluation enforces
+the same endpoint and sentinel requirements and diagnoses failures.
+
+Ordinary `base[start..end]` slicing keeps its existing bounds rules and preserves a source
+sentinel only when the end bound is omitted. Explicit sentinel slicing can assert a different
+sentinel only when the actual endpoint matches it; it never adds a terminator or restores
+mutability.
+
 #### distinct types
 
 `Name :: distinct T` creates a nominal identity and reuses `T`'s runtime representation. When

compiler/checker/checker.odin

@@ -400,6 +400,9 @@ block_reads_name :: proc(checker: ^Checker, statements: []ast.Stmt_Id, name: sym
 		case .Call, .Array, .Struct_Literal, .Slice:
 			append(&expr_stack, ..expr.args)
 			append(&expr_stack, expr.left)
+			if expr.kind == .Slice {
+				append(&expr_stack, expr.right)
+			}
 		case .Negate, .Not, .Bit_Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal, .Cast:
 			append(&expr_stack, expr.left)
 		case .Comptime:
@@ -743,12 +746,18 @@ comptime_slice_bytes :: proc(
 	pkg: ast.Package_Id,
 	file: ast.File_Id,
 ) -> (string, bool) {
+	if expr.right != ast.INVALID_EXPR {
+		return "", false
+	}
 	state := ct_state_make(checker, pkg, file, diagnose=false)
 	defer ct_state_destroy(&state)
 	value, flow, ok := ct_eval_slice_expr(&state, expr, 0)
 	if !ok || flow.kind != .Normal {
 		return "", false
 	}
+	if item, item_ok := types.container(state.values[value].type, &checker.module.types); item_ok && item.has_sentinel {
+		return "", false
+	}
 	return ct_value_bytes(&state, value)
 }
 
@@ -2753,6 +2762,15 @@ match_inferred_type_pattern :: proc(
 				values, bound, span, diagnose, depth+1,
 			)
 		}
+		if actual_ok && actual_item.kind == .Slice {
+			if pattern_item.has_sentinel && (!actual_item.has_sentinel || pattern_item.sentinel != actual_item.sentinel) {
+				return false
+			}
+			return match_inferred_type_pattern(
+				checker, function, prefix, pattern_item.child, actual_item.child,
+				values, bound, span, diagnose, depth+1,
+			)
+		}
 	}
 	if !actual_ok || pattern_item.kind != actual_item.kind {
 		resolved := type_from_syntax(checker, pattern, function.pkg, function.file)
@@ -3786,6 +3804,9 @@ mark_expr_imports_used :: proc(checker: ^Checker, expr_id: ast.Expr_Id, file: as
 			if expr.left != ast.INVALID_EXPR {
 				append(&stack, expr.left)
 			}
+			if expr.kind == .Slice && expr.right != ast.INVALID_EXPR {
+				append(&stack, expr.right)
+			}
 		case .Negate, .Not, .Bit_Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal, .Cast:
 			append(&stack, expr.left)
 		case .Comptime:
@@ -5168,6 +5189,15 @@ infer_compound_expr :: proc(
 				_ = record_demand(checker, bound, types.USIZE, locals, local_types, pkg, file)
 			}
 		}
+		if expr.right != ast.INVALID_EXPR {
+			state := ct_state_make(checker, pkg, file, diagnose=false)
+			defer ct_state_destroy(&state)
+			sentinel, sentinel_ok := checked_slice_sentinel(&state, expr.right, item.child)
+			if !sentinel_ok {
+				return types.INVALID
+			}
+			return types.slice(store, item.child, item.mutable, true, u64(sentinel))
+		}
 		preserve := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
 		return types.slice(store, item.child, item.mutable, preserve, item.sentinel)
 	case .Field:
@@ -8773,6 +8803,22 @@ build_compound_expr :: proc(
 			id := source.add(checker.diagnostics, expr.span, "many-item pointer slicing requires an explicit end bound")
 			return invalid_hir_expr(checker, expr.span, id)
 		}
+		sentinel_expr := hir.INVALID_EXPR
+		sentinel := item.sentinel
+		if expr.right != ast.INVALID_EXPR {
+			state := ct_state_make(checker, pkg, file)
+			defer ct_state_destroy(&state)
+			constant, sentinel_ok := checked_slice_sentinel(&state, expr.right, item.child)
+			if !sentinel_ok {
+				return invalid_hir_expr(checker, expr.span, state.diagnostic)
+			}
+			sentinel = u64(constant)
+			sentinel_expr = add_hir_expr(checker, hir.Expr{
+				kind=.Integer, span=checker.ast_module.exprs[expr.right].span, type=item.child, integer=i64(sentinel),
+				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
+				diagnostic=source.INVALID_DIAGNOSTIC,
+			})
+		}
 		bounds := make([]hir.Expr_Id, 2, checker.allocator)
 		bounds[0] = hir.INVALID_EXPR
 		bounds[1] = hir.INVALID_EXPR
@@ -8791,11 +8837,11 @@ build_compound_expr :: proc(
 				}
 			}
 		}
-		preserve_sentinel := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
+		preserve_sentinel := expr.right != ast.INVALID_EXPR || item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
-		result_type := types.slice(store, item.child, item.mutable, preserve_sentinel, item.sentinel)
+		result_type := types.slice(store, item.child, item.mutable, preserve_sentinel, sentinel)
 		return add_hir_expr(checker, hir.Expr{
 			kind=.Slice, span=expr.span, type=result_type, args=bounds, left=container,
-			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
+			target=hir.INVALID_REF, right=sentinel_expr, diagnostic=source.INVALID_DIAGNOSTIC,
 		})
 	case .Field:
 		if enum_type, enum_ok := enum_type_from_field_expr(checker, expr, pkg, file, true); enum_ok {

compiler/checker/comptime.odin

@@ -792,6 +792,17 @@ ct_place_get :: proc(state: ^Ct_State, place_id: Ct_Place_Id) -> (Ct_Value_Id, b
 		#partial switch elem.kind {
 		case .Index:
 			index := int(elem.index)
+			if value.kind == .Array {
+				item, ok := types.node(&state.checker.module.types, value.type)
+				if ok && item.has_sentinel && u64(index) == item.count {
+					integer := i128(item.sentinel)
+					if types.is_signed(types.resolve_alias(item.child, &state.checker.module.types), state.checker.target) {
+						integer = i128(i64(item.sentinel))
+					}
+					current = ct_add_value(state, Ct_Value{kind=.Integer, type=item.child, integer=integer})
+					continue
+				}
+			}
 			if value.kind != .Array || index < 0 || index >= len(children) {
 				return INVALID_CT_VALUE, false
 			}
@@ -963,7 +974,7 @@ ct_value_can_materialize :: proc(state: ^Ct_State, id: Ct_Value_Id, depth := 0)
 		store := &state.checker.module.types
 		slice, slice_ok := types.node(store, value.type)
 		place_id := Ct_Place_Id(value.index)
-		if !slice_ok || slice.kind != .Slice || slice.mutable || value.start != 0 ||
+		if !slice_ok || slice.kind != .Slice || slice.mutable ||
 		   place_id == INVALID_CT_PLACE || int(place_id) >= len(state.places) {
 			return false
 		}
@@ -977,7 +988,7 @@ ct_value_can_materialize :: proc(state: ^Ct_State, id: Ct_Value_Id, depth := 0)
 		}
 		root := state.values[root_id]
 		array, array_ok := types.node(store, root.type)
-		return array_ok && array.kind == .Array && u64(value.count) == array.count &&
+		return array_ok && array.kind == .Array && u64(value.start)+u64(value.count) <= array.count &&
 		       types.equal(slice.child, array.child) && ct_value_can_materialize(state, root_id, depth+1)
 	case .Range, .Array, .Struct, .Optional_Some, .Fallible:
 		for child in ct_child_slice(state, value) {
@@ -1065,7 +1076,7 @@ ct_materialize_array_slice :: proc(
 	store := &state.checker.module.types
 	slice, slice_ok := types.node(store, value.type)
 	place_id := Ct_Place_Id(value.index)
-	if !slice_ok || slice.kind != .Slice || slice.mutable || value.start != 0 ||
+	if !slice_ok || slice.kind != .Slice || slice.mutable ||
 	   place_id == INVALID_CT_PLACE || int(place_id) >= len(state.places) {
 		return hir.INVALID_EXPR, false
 	}
@@ -1079,7 +1090,7 @@ ct_materialize_array_slice :: proc(
 	}
 	root := state.values[root_id]
 	array, array_ok := types.node(store, root.type)
-	if !array_ok || array.kind != .Array || u64(value.count) != array.count ||
+	if !array_ok || array.kind != .Array || u64(value.start)+u64(value.count) > array.count ||
 	   !types.equal(slice.child, array.child) {
 		return hir.INVALID_EXPR, false
 	}
@@ -1093,7 +1104,28 @@ ct_materialize_array_slice :: proc(
 	if !ok {
 		return hir.INVALID_EXPR, false
 	}
-	return coerce_expr(state.checker, address, value.type, span), true
+	checker := state.checker
+	bounds := make([]hir.Expr_Id, 2, checker.allocator)
+	endpoints := [2]u32{value.start, value.start+value.count}
+	for integer, index in endpoints {
+		bounds[index] = add_hir_expr(checker, hir.Expr{
+			kind=.Integer, span=span, type=types.USIZE, integer=i64(integer),
+			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
+	}
+	sentinel_expr := hir.INVALID_EXPR
+	if slice.has_sentinel {
+		sentinel_expr = add_hir_expr(checker, hir.Expr{
+			kind=.Integer, span=span, type=slice.child, integer=i64(slice.sentinel),
+			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
+	}
+	return add_hir_expr(checker, hir.Expr{
+		kind=.Slice, span=span, type=value.type, args=bounds, left=address, right=sentinel_expr,
+		target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
+	}), true
 }
 
 ct_materialize_value :: proc(
@@ -1223,7 +1255,7 @@ ct_materialize_value :: proc(
 			return result
 		}
 		if state.diagnostic == source.INVALID_DIAGNOSTIC {
-			state.diagnostic = source.add(checker.diagnostics, span, "only immutable full-array comptime slices can materialize as runtime memory")
+			state.diagnostic = source.add(checker.diagnostics, span, "only immutable array-backed comptime slices can materialize as runtime memory")
 		}
 		return invalid_hir_expr(checker, span, state.diagnostic, value.type)
 	case .Function:
@@ -1948,6 +1980,22 @@ ct_eval_struct_expr :: proc(state: ^Ct_State, expr: ast.Expr, expected: types.Ty
 	return ct_add_value(state, Ct_Value{kind=.Struct, type=struct_type, start=start, count=u32(len(values)), active=active_field}), ct_flow(.Normal), true
 }
 
+checked_slice_sentinel :: proc(state: ^Ct_State, id: ast.Expr_Id, child: types.Type) -> (i128, bool) {
+	checker := state.checker
+	span := checker.ast_module.exprs[id].span
+	previous_error := state.error
+	constant := eval_integer_constant_in_state(state, id, allow_bool=false)
+	state.error = previous_error
+	if constant.kind != .Value {
+		return 0, ct_fail(state, .Not_Comptime, span, "slice sentinel must be a compile-time integer")
+	}
+	representation := types.resolve_alias(child, &checker.module.types)
+	if !types.is_concrete_integer(representation) || !fits_integer_type(constant.value, representation, checker.target) {
+		return 0, ct_failf(state, .Not_Comptime, span, "slice sentinel value does not fit element type %s", type_label(checker, child))
+	}
+	return constant.value, true
+}
+
 ct_eval_slice_expr :: proc(state: ^Ct_State, expr: ast.Expr, depth: int) -> (Ct_Value_Id, Ct_Flow, bool) {
 	checker := state.checker
 	store := &checker.module.types
@@ -1958,6 +2006,7 @@ ct_eval_slice_expr :: proc(state: ^Ct_State, expr: ast.Expr, depth: int) -> (Ct_
 	container_place := INVALID_CT_PLACE
 	container_start := 0
 	container_len := 0
+	bounded := true
 	item: types.Node
 	item_ok := false
 	if base_value != INVALID_CT_VALUE && int(base_value) < len(state.values) {
@@ -1967,6 +2016,25 @@ ct_eval_slice_expr :: proc(state: ^Ct_State, expr: ast.Expr, depth: int) -> (Ct_
 			container_start = int(base.start)
 			container_len = int(base.count)
 			item, item_ok = types.node(store, base.type)
+		} else if base.kind == .Array && checker.ast_module.exprs[expr.left].kind == .Array && expr.right != ast.INVALID_EXPR {
+			item, item_ok = types.node(store, base.type)
+			container_place = ct_add_place(state, ct_add_cell(state, base_value, false), base.type, false)
+			container_len = int(base.count)
+			item.mutable = false
+		} else if base.kind == .String && expr.right != ast.INVALID_EXPR && base.index < u64(len(checker.ast_module.strings)) {
+			text := checker.ast_module.strings[base.index]
+			array_type := types.array(store, types.U8, u64(len(text)), false, true, 0)
+			children := make([]Ct_Value_Id, len(text), checker.allocator)
+			defer delete(children, checker.allocator)
+			for byte, index in transmute([]u8)text {
+				children[index] = ct_add_value(state, Ct_Value{kind=.Integer, type=types.U8, integer=i128(byte)})
+			}
+			child_start := u32(len(state.children))
+			append(&state.children, ..children)
+			array_value := ct_add_value(state, Ct_Value{kind=.Array, type=array_type, start=child_start, count=u32(len(text))})
+			container_place = ct_add_place(state, ct_add_cell(state, array_value, false), array_type, false)
+			container_len = len(text)
+			item, item_ok = types.container(base.type, store)
 		} else if base.kind == .Pointer {
 			pointer_item, pointer_ok := types.node(store, base.type)
 			array_item: types.Node
@@ -1980,6 +2048,15 @@ ct_eval_slice_expr :: proc(state: ^Ct_State, expr: ast.Expr, depth: int) -> (Ct_
 				item = array_item
 				item.mutable = pointer_item.mutable && array_item.mutable
 				item_ok = true
+			} else if pointer_ok && pointer_item.many {
+				if expr.args[1] == ast.INVALID_EXPR {
+					return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "many-item pointer slicing requires an explicit end bound")
+				}
+				container_place = Ct_Place_Id(base.index)
+				container_start = int(base.active)
+				item = pointer_item
+				item_ok = true
+				bounded = false
 			}
 		}
 	}
@@ -2031,11 +2108,31 @@ ct_eval_slice_expr :: proc(state: ^Ct_State, expr: ast.Expr, depth: int) -> (Ct_
 			end = int(integer)
 		}
 	}
-	if start < 0 || end < start || end > container_len {
+	if start < 0 || end < start || bounded && end > container_len || container_start < 0 {
 		return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "comptime slice bounds out of range")
 	}
-	preserve_sentinel := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
+	sentinel := item.sentinel
-	result_type := types.slice(store, item.child, item.mutable, preserve_sentinel, item.sentinel)
+	if expr.right != ast.INVALID_EXPR {
+		constant, sentinel_ok := checked_slice_sentinel(state, expr.right, item.child)
+		if !sentinel_ok {
+			return INVALID_CT_VALUE, ct_flow(.Normal), false
+		}
+		if bounded && (end == container_len && !item.has_sentinel) {
+			return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "comptime sentinel slice endpoint out of range")
+		}
+		endpoint := ct_extend_place(state, container_place, Ct_Path_Elem{kind=.Index, index=u32(container_start+end)}, item.child, false)
+		value, value_ok := ct_place_get(state, endpoint)
+		if !value_ok {
+			return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "comptime sentinel slice endpoint out of range")
+		}
+		actual, actual_ok := ct_integer_value(state, value)
+		if !actual_ok || actual != constant {
+			return INVALID_CT_VALUE, ct_flow(.Normal), ct_fail(state, .Not_Comptime, expr.span, "comptime slice sentinel mismatch")
+		}
+		sentinel = u64(constant)
+	}
+	preserve_sentinel := expr.right != ast.INVALID_EXPR || item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
+	result_type := types.slice(store, item.child, item.mutable, preserve_sentinel, sentinel)
 	return ct_add_value(state, Ct_Value{
 		kind=.Slice, type=result_type, index=u64(container_place),
 		start=u32(container_start+start), count=u32(end-start),
@@ -5973,7 +6070,7 @@ eval_integer_constant_in_context :: proc(
 	return eval_integer_constant_in_state(&state, expr_id, depth)
 }
 
-eval_integer_constant_in_state :: proc(state: ^Ct_State, expr_id: ast.Expr_Id, depth := 0) -> Constant {
+eval_integer_constant_in_state :: proc(state: ^Ct_State, expr_id: ast.Expr_Id, depth := 0, allow_bool := true) -> Constant {
 	if depth > 64 || expr_id == ast.INVALID_EXPR || int(expr_id) >= len(state.checker.ast_module.exprs) {
 		return Constant{kind=.Not_Constant}
 	}
@@ -5994,6 +6091,9 @@ eval_integer_constant_in_state :: proc(state: ^Ct_State, expr_id: ast.Expr_Id, d
 		}
 		return Constant{kind=.Not_Constant}
 	}
+	if !allow_bool && state.values[value].kind != .Integer {
+		return Constant{kind=.Not_Constant}
+	}
 	integer, integer_ok := ct_integer_value(state, value)
 	if !integer_ok {
 		return Constant{kind=.Not_Constant}

compiler/llvm/llvm.odin

@@ -1556,6 +1556,11 @@ emit_instruction_stream :: proc(
 				emit_recovery_value(emitter, instruction_index, instruction, "invalid slice container")
 				continue
 			}
+			check_sentinel := instruction.b != ir.INVALID_INSTRUCTION
+			if check_sentinel && !valid_value(instructions, instruction.b, item.child, &emitter.module.types) {
+				emit_recovery_value(emitter, instruction_index, instruction, "invalid slice sentinel")
+				continue
+			}
 			pointer_name := fmt.tprintf("%%v%d", instruction.a)
 			length_name := fmt.tprintf("%d", item.count)
 			if item.kind == .Array {
@@ -1597,13 +1602,25 @@ emit_instruction_stream :: proc(
 			if item.kind == .Pointer {
 				fmt.sbprintf(&emitter.builder, "  %%slice_ok%d = or i1 false, %%slice_order%d\n", instruction_index, instruction_index)
 			} else {
-				fmt.sbprintf(&emitter.builder, "  %%slice_end_ok%d = icmp ule i64 %s, %s\n", instruction_index, end_name, length_name)
+				comparison := "ult" if check_sentinel && !item.has_sentinel else "ule"
+				fmt.sbprintf(&emitter.builder, "  %%slice_end_ok%d = icmp %s i64 %s, %s\n", instruction_index, comparison, end_name, length_name)
 				fmt.sbprintf(&emitter.builder, "  %%slice_ok%d = and i1 %%slice_order%d, %%slice_end_ok%d\n", instruction_index, instruction_index, instruction_index)
 			}
 			fmt.sbprintf(&emitter.builder, "  br i1 %%slice_ok%d, label %%slice_continue%d, label %%slice_trap%d\nslice_trap%d:\n", instruction_index, instruction_index, instruction_index, instruction_index)
 			message := diagnostic_message(emitter, source.INVALID_DIAGNOSTIC, instruction.span, "slice bounds out of range")
 			emit_trap_call(emitter, message)
 			fmt.sbprintf(&emitter.builder, "  unreachable\nslice_continue%d:\n", instruction_index)
+			if check_sentinel {
+				element_type := llvm_type(item.child, &emitter.module.types)
+				fmt.sbprintf(&emitter.builder, "  %%slice_sentinel_ptr%d = getelementptr %s, ptr %s, i64 %s\n", instruction_index, element_type, pointer_name, end_name)
+				fmt.sbprintf(&emitter.builder, "  %%slice_sentinel_value%d = load %s, ptr %%slice_sentinel_ptr%d\n", instruction_index, element_type, instruction_index)
+				fmt.sbprintf(&emitter.builder, "  %%slice_sentinel_ok%d = icmp eq %s %%slice_sentinel_value%d, ", instruction_index, element_type, instruction_index)
+				write_operand(&emitter.builder, instructions, instruction.b, item.child, &emitter.module.types)
+				fmt.sbprintf(&emitter.builder, "\n  br i1 %%slice_sentinel_ok%d, label %%slice_sentinel_continue%d, label %%slice_sentinel_trap%d\nslice_sentinel_trap%d:\n", instruction_index, instruction_index, instruction_index, instruction_index)
+				sentinel_message := diagnostic_message(emitter, source.INVALID_DIAGNOSTIC, instruction.span, "slice sentinel mismatch")
+				emit_trap_call(emitter, sentinel_message)
+				fmt.sbprintf(&emitter.builder, "  unreachable\nslice_sentinel_continue%d:\n", instruction_index)
+			}
 			fmt.sbprintf(&emitter.builder, "  %%slice_start%d = getelementptr %s, ptr %s, i64 %s\n", instruction_index, llvm_type(item.child, &emitter.module.types), pointer_name, start_name)
 			fmt.sbprintf(&emitter.builder, "  %%slice_result%d = insertvalue %s poison, ptr %%slice_start%d, 0\n", instruction_index, llvm_type(instruction.type, &emitter.module.types), instruction_index)
 			fmt.sbprintf(&emitter.builder, "  %%slice_result_len%d = sub i64 %s, %s\n", instruction_index, end_name, start_name)

compiler/lower/lower.odin

@@ -306,9 +306,13 @@ lower_compound_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instructi
 				args[index] = lower_nested_expr(state, arg)
 			}
 		}
+		checked_sentinel := ir.INVALID_INSTRUCTION
+		if expr.right != hir.INVALID_EXPR {
+			checked_sentinel = lower_nested_expr(state, expr.right)
+		}
 		return append_instruction(state, ir.Instruction{
 			op=.Slice, span=expr.span, type=expr.type, args=args,
-			target=ir.INVALID_REF, a=container, b=ir.INVALID_INSTRUCTION,
+			target=ir.INVALID_REF, a=container, b=checked_sentinel,
 			diagnostic=source.INVALID_DIAGNOSTIC,
 		})
 	case .Unwrap:

compiler/parser/parser.odin

@@ -1450,6 +1450,7 @@ parse_expression_bp :: proc(parser: ^Parser, minimum_binding_power, nesting: int
 			skip_newlines(parser)
 			start_expr := ast.INVALID_EXPR
 			end_expr := ast.INVALID_EXPR
+			sentinel_expr := ast.INVALID_EXPR
 			slicing := false
 			if _, ok := allow(parser, .Range); ok {
 				slicing = true
@@ -1463,12 +1464,19 @@ parse_expression_bp :: proc(parser: ^Parser, minimum_binding_power, nesting: int
 				}
 			}
 			skip_newlines(parser)
-			if slicing && current(parser).kind != .Right_Bracket {
+			if slicing && current(parser).kind != .Right_Bracket && current(parser).kind != .Semicolon {
 				parser.range_disabled += 1
 				end_expr = parse_expression_bp(parser, 0, nesting+1)
 				parser.range_disabled -= 1
 				skip_newlines(parser)
 			}
+			if slicing {
+				if _, ok := allow(parser, .Semicolon); ok {
+					skip_newlines(parser)
+					sentinel_expr = parse_expression_bp(parser, 0, nesting+1)
+					skip_newlines(parser)
+				}
+			}
 			end_token, ok := allow(parser, .Right_Bracket)
 			if !ok {
 				source.add(parser.diagnostics, current(parser).span, "expected ']' after index or slice")
@@ -1485,7 +1493,7 @@ parse_expression_bp :: proc(parser: ^Parser, minimum_binding_power, nesting: int
 					span=span_from(left_expr.span, end_token.span),
 					args=args,
 					left=left,
-					right=ast.INVALID_EXPR,
+					right=sentinel_expr,
 					diagnostic=source.INVALID_DIAGNOSTIC,
 				})
 			} else {

compiler_tests.odin

@@ -2252,6 +2252,256 @@ main func() i32 {
 	testing.expect(t, strings.contains(llvm_text, "getelementptr [4 x i32], ptr"))
 }
 
+@(test)
+explicit_sentinel_slices_execute_without_copying :: proc(t: ^testing.T) {
+	directory := "/tmp/brolang-test-explicit-sentinel-slices"
+	main_path := "/tmp/brolang-test-explicit-sentinel-slices/main.bro"
+	output := "/tmp/brolang-test-explicit-sentinel-slices-output"
+	text := `pointer func(value *mut u8, count usize) [;0]mut u8 { return value[..count;0] }
+bounded func(value []u8, count usize) [;0]u8 { return value[..count;0] }
+array func(value @mut [3]mut u8, start usize, count usize) [;0]mut u8 { return value[start..count;0] }
+terminated func(value [;0]u8) [;0]u8 { return value[..;0] }
+nonzero func(value [*;9]u8, count usize) [;9]u8 { return value[1..count;9] }
+signed func(value *i16, count usize) [;-7]i16 { return value[..count;-7] }
+argument func(value [*;0]u8, present bool) ?[;0]u8 {
+	if !present { return null }
+	length usize := 0
+	while value[length] != 0 { length += 1 }
+	return value[..length;0]
+}
+static_text func() [;0]u8 { return "hello"[..;0] }
+static_partial func() [;7]u16 {
+	values [4]u16 := [11, 22, 7, 44]
+	return values[1..2;7]
+}
+comptime_text [;0]u8 :: $static_text()
+comptime_partial [;7]u16 :: $static_partial()
+equal func($T type, left, right []T) bool {
+	if left.len != right.len { return false }
+	for 0..left.len |index| {
+		if left[index] != right[index] { return false }
+	}
+	return true
+}
+main func() i32 {
+	values [3]mut u8 := [11, 22, 0]
+	view :: pointer((&values).ptr, 2)
+	if view.len != 2 { return 1 }
+	view[0] = 33
+	if values[0] != 33 { return 2 }
+	all :: values[..]
+	if all.len != 3 { return 3 }
+	part :: bounded(all, 2)
+	if part.len != 2 or part[1] != 22 { return 4 }
+	suffix :: array(&values, 1, 2)
+	suffix[0] = 44
+	if suffix.len != 1 or values[1] != 44 { return 5 }
+	empty :: pointer((&values).ptr + 2, 0)
+	if empty.len != 0 { return 6 }
+	zeros [2;0]mut u8 := [1, 2]
+	full :: terminated(zeros[..])
+	if full.len != 2 or full[1] != 2 { return 7 }
+	nines [2;9]u8 := [3, 4]
+	nine :: nonzero((&nines).ptr, 2)
+	if nine.len != 1 or nine[0] != 4 { return 8 }
+	wide [2]i16 := [123, -7]
+	negative :: signed((&wide).ptr, 1)
+	if negative.len != 1 or negative[0] != 123 { return 9 }
+	direct :: zeros[..;0]
+	if direct.len != 2 { return 10 }
+	if argument((&zeros).ptr, true) |arg| {
+		zeros[0] = 55
+		if arg.len != 2 or arg[0] != 55 { return 11 }
+	} else { return 12 }
+	if argument((&zeros).ptr, false) |_| { return 13 }
+	if comptime_text.len != 5 or comptime_text[0] != 'h' or comptime_text[4] != 'o' { return 14 }
+	if comptime_partial.len != 1 or comptime_partial[0] != 22 { return 15 }
+	if !equal(comptime_text, "hello") or equal(comptime_text, "hellx") { return 16 }
+	if !equal(direct, "7\x02") { return 17 }
+	return 0
+}
+`
+	_ = os2.remove_all(directory)
+	defer _ = os2.remove_all(directory)
+	defer _ = os.remove(output)
+	testing.expect(t, os.make_directory(directory) == nil)
+	testing.expect(t, os.write_entire_file(main_path, transmute([]byte)text))
+	testing.expect_value(t, compiler_core.compile_package(directory, output), 0)
+	state := run_executable(output)
+	testing.expect_value(t, state.exit_code, 0)
+}
+
+@(test)
+explicit_sentinel_slice_runtime_guards_trap :: proc(t: ^testing.T) {
+	cases := [?]struct {
+		text, message: string,
+	}{
+		{
+			`check func(value *u8, count usize) void { _ = value[..count;0] }
+main func() void {
+	values [2]u8 := [1, 2]
+	check((&values).ptr, 1)
+}
+`, "slice sentinel mismatch",
+		},
+		{
+			`check func(value @[2]u8, count usize) void { _ = value[..count;0] }
+main func() void {
+	values [2]u8 := [1, 0]
+	check(&values, 2)
+}
+`, "slice bounds out of range",
+		},
+		{
+			`check func(value []u8, count usize) void { _ = value[..count;0] }
+main func() void {
+	values [2;0]u8 := [1, 2]
+	check(values[..2], 2)
+}
+`, "slice bounds out of range",
+		},
+		{
+			`check func(value [;0]u8, count usize) void { _ = value[..count;0] }
+main func() void {
+	values [2;0]u8 := [1, 2]
+	check(values[..], 3)
+}
+`, "slice bounds out of range",
+		},
+		{
+			`check func(value *u8, start usize, count usize) void { _ = value[start..count;0] }
+main func() void {
+	values [2]u8 := [1, 0]
+	check((&values).ptr, 2, 1)
+}
+`, "slice bounds out of range",
+		},
+		{
+			`check func(value [;0]u8, count usize) void { _ = value[..count;9] }
+main func() void {
+	values [2;0]u8 := [1, 2]
+	check(values[..], 2)
+}
+`, "slice sentinel mismatch",
+		},
+	}
+	for test_case, index in cases {
+		directory := fmt.tprintf("/tmp/brolang-test-sentinel-slice-trap-%d", index)
+		main_path := fmt.tprintf("%s/main.bro", directory)
+		output := fmt.tprintf("%s-output", directory)
+		_ = os2.remove_all(directory)
+		defer _ = os2.remove_all(directory)
+		defer _ = os.remove(output)
+		testing.expect(t, os.make_directory(directory) == nil)
+		testing.expect(t, os.write_entire_file(main_path, transmute([]byte)test_case.text))
+		testing.expect_value(t, compiler_core.compile_package(directory, output), 0)
+		state, stdout, stderr, err := os2.process_exec(
+			os2.Process_Desc{command=[]string{output}}, context.allocator,
+		)
+		defer delete(stdout)
+		defer delete(stderr)
+		testing.expect(t, err == nil)
+		testing.expect(t, !state.success)
+		testing.expect(t, strings.contains(string(stderr), test_case.message))
+	}
+}
+
+@(test)
+explicit_sentinel_slice_semantic_errors_are_diagnosed :: proc(t: ^testing.T) {
+	cases := [?]struct {
+		text, message: string,
+	}{
+		{
+			`bad func(value *u8, count usize, mark u8) void { _ = value[..count;mark] }
+main func() void {
+	values [2]u8 := [1, 0]
+	bad((&values).ptr, 1, 0)
+}
+`, "slice sentinel must be a compile-time integer",
+		},
+		{
+			`bad func(value *u8, count usize) void { _ = value[..count;true] }
+main func() void {
+	values [2]u8 := [1, 0]
+	bad((&values).ptr, 1)
+}
+`, "slice sentinel must be a compile-time integer",
+		},
+		{
+			`bad func(value *u8, count usize) void { _ = value[..count;256] }
+main func() void {
+	values [2]u8 := [1, 0]
+	bad((&values).ptr, 1)
+}
+`, "slice sentinel value does not fit element type",
+		},
+		{
+			`bad func(value *u8, count usize) void { _ = value[..count;-1] }
+main func() void {
+	values [2]u8 := [1, 0]
+	bad((&values).ptr, 1)
+}
+`, "slice sentinel value does not fit element type",
+		},
+		{
+			`bad func(value [*;0]u8) void { _ = value[..;0] }
+main func() void { bad("text") }
+`, "many-item pointer slicing requires an explicit end bound",
+		},
+		{
+			`bad func(value *u8, count usize) [;0]mut u8 { return value[..count;0] }
+main func() void {
+	values [2]u8 := [1, 0]
+	_ = bad((&values).ptr, 1)
+}
+`, "cannot implicitly convert",
+		},
+		{
+			`bad func(count usize) usize {
+	values [2]u8 := [1, 0]
+	return values[..count;0].len
+}
+result :: $bad(2)
+main func() void { _ = result }
+`, "comptime sentinel slice endpoint out of range",
+		},
+		{
+			`bad func(count usize) usize {
+	values [2]u8 := [1, 2]
+	return values[..count;0].len
+}
+result :: $bad(1)
+main func() void { _ = result }
+`, "comptime slice sentinel mismatch",
+		},
+		{
+			`bad func(value []u8) usize { return value[..;0].len }
+result :: $bad("text")
+main func() void { _ = result }
+`, "comptime sentinel slice endpoint out of range",
+		},
+	}
+	for test_case in cases {
+		source_file := source.Source{path="invalid_sentinel_slice.bro", text=test_case.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)
+
+		found := false
+		for diagnostic in diagnostics.items {
+			found = found || strings.contains(diagnostic.message, test_case.message)
+		}
+		testing.expect(t, found, test_case.message)
+	}
+}
+
 @(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
@@ -5848,16 +6098,6 @@ main func() void {
 		values [2]mut i32 := [1, 2]
 		yield &values[0]
 	}
-	_ = ${
-		values [2]mut i32 := [1, 2]
-		view []mut i32 := values[..]
-		yield view
-	}
-	_ = ${
-		values [2]mut i32 := [1, 2]
-		view []i32 := values[1..]
-		yield view
-	}
 	_ = $spin()
 	_ = $missing()
 	_ = ${
@@ -5880,7 +6120,6 @@ main func() void {
 	found_runtime := false
 	runtime_only_count := 0
 	pointer_errors := 0
-	slice_errors := 0
 	found_quota := false
 	found_missing := false
 	found_yield := false
@@ -5889,7 +6128,6 @@ main func() void {
 		found_runtime = found_runtime || strings.contains(message, "unresolved comptime value 'runtime'")
 		runtime_only_count += 1 if strings.contains(message, "runtime-only") else 0
 		pointer_errors += 1 if strings.contains(message, "only immutable pointers to whole comptime arrays can materialize as runtime memory") else 0
-		slice_errors += 1 if strings.contains(message, "only immutable full-array comptime slices can materialize as runtime memory") else 0
 		found_quota = found_quota || strings.contains(message, "comptime evaluation exceeded the step quota")
 		found_missing = found_missing || strings.contains(message, "did not return a value")
 		found_yield = found_yield || strings.contains(message, "a value block must end with an explicit 'yield'")
@@ -5897,7 +6135,6 @@ main func() void {
 	testing.expect(t, found_runtime)
 	testing.expect(t, runtime_only_count >= 2)
 	testing.expect(t, pointer_errors >= 4)
-	testing.expect(t, slice_errors >= 2)
 	testing.expect(t, found_quota)
 	testing.expect(t, found_missing)
 	testing.expect(t, found_yield)