array size inference from value

LANGUAGE.md

@@ -22,7 +22,7 @@ roadmap and milestone history.
 - target-dependent C scalar primitives from `c_char` through `c_longdouble`, kept semantically distinct from native scalars
 - contextual integer/float/character literals, backward type-demand inference through names and arithmetic, and compile-time folding for numeric constant expressions
 - strict numeric conversion by default, widening where valid, C scalar coercions at C boundaries, and explicit scalar keyword casts such as `i32(x)` / `c_float(x)`
-- arrays `[N]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`
+- 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
 - UTF-8 string literals as immutable pointers to static zero-terminated byte arrays, plus raw backtick multiline strings

compiler/checker/checker.odin

@@ -607,6 +607,25 @@ resolve_inferred_array :: proc(checker: ^Checker, value: types.Type, expr_id: as
 	return types.with_array_count(&checker.module.types, value, u64(len(expr.args)))
 }
 
+has_inferred_array_count :: proc(checker: ^Checker, value: types.Type) -> bool {
+	item, ok := types.node(&checker.module.types, value)
+	return ok && item.kind == .Array && item.inferred_count
+}
+
+resolve_inferred_array_from_type :: proc(checker: ^Checker, value, inferred: types.Type) -> types.Type {
+	item, ok := types.node(&checker.module.types, value)
+	actual, actual_ok := types.node(&checker.module.types, inferred)
+	if !ok || !actual_ok || item.kind != .Array || actual.kind != .Array || !item.inferred_count {
+		return value
+	}
+	if item.child != actual.child || item.mutable != actual.mutable ||
+	   item.has_sentinel != actual.has_sentinel ||
+	   (item.has_sentinel && item.sentinel != actual.sentinel) {
+		return value
+	}
+	return types.with_array_count(&checker.module.types, value, actual.count)
+}
+
 function_index_less :: proc(left, right: Function_Index_Entry) -> bool {
 	if left.scope != right.scope {
 		return left.scope < right.scope
@@ -2249,8 +2268,9 @@ infer_statements :: proc(
 			value_type := types.INVALID
 			if !is_undefined_expr(checker, statement.expr) {
 				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
+				declared_local = resolve_inferred_array_from_type(checker, declared_local, value_type)
 			}
-			if is_runtime_type(checker, declared_local) {
+			if is_runtime_type(checker, declared_local) && !has_inferred_array_count(checker, declared_local) {
 				value_type = declared_local
 			} else if types.is_constraint(declared_local) {
 				// Seed the binding in-family (INVALID on mismatch, which
@@ -2823,8 +2843,12 @@ infer_all :: proc(checker: ^Checker) {
 	// Demands accumulate in global_demands so the default never blocks a later
 	// cross-family demand (e.g. integer literal -> unsigned or float).
 	for global, index in checker.ast_module.globals {
-		declared := type_from_syntax(checker, global.type, global.pkg, global.file)
+		declared := resolve_inferred_array(
-		if is_runtime_type(checker, declared) {
+			checker,
+			type_from_syntax(checker, global.type, global.pkg, global.file),
+			global.expr,
+		)
+		if is_runtime_type(checker, declared) && !has_inferred_array_count(checker, declared) {
 			checker.global_types[index] = declared
 			continue
 		}
@@ -2871,7 +2895,20 @@ infer_all :: proc(checker: ^Checker) {
 				continue
 			}
 			inferred := infer_expr(checker, global.expr, nil, global.pkg, global.file)
-			if is_runtime_type(checker, type_from_syntax(checker, global.type, global.pkg, global.file)) {
+			declared := resolve_inferred_array(
+				checker,
+				type_from_syntax(checker, global.type, global.pkg, global.file),
+				global.expr,
+			)
+			if resolved := resolve_inferred_array_from_type(checker, declared, inferred);
+			   resolved != declared {
+				if !types.equal(checker.global_types[index], resolved) {
+					checker.global_types[index] = resolved
+					changed = true
+				}
+				continue
+			}
+			if is_runtime_type(checker, declared) && !has_inferred_array_count(checker, declared) {
 				continue
 			}
 			if is_runtime_type(checker, checker.global_demands[index]) {
@@ -4935,7 +4972,7 @@ build_block :: proc(
 			if statement_id != ast.INVALID_STMT && int(statement_id) < len(ctx.local_types) &&
 			   is_runtime_type(checker, ctx.local_types[statement_id]) &&
 			   (types.is_constraint(declared) || is_undefined_expr(checker, statement.expr) ||
-			    open_const_decl || numeric_arithmetic_decl) {
+			    open_const_decl || numeric_arithmetic_decl || has_inferred_array_count(checker, declared)) {
 				declared = ctx.local_types[statement_id]
 			}
 			// A still-unresolved constraint means the initializer's numeric
@@ -4974,7 +5011,7 @@ build_block :: proc(
 					ctx.problematic^ = true
 					continue
 				}
-				if !is_runtime_type(checker, declared) {
+				if !is_runtime_type(checker, declared) || has_inferred_array_count(checker, declared) {
 					id := source.addf(
 						checker.diagnostics,
 						statement.span,
@@ -7477,6 +7514,10 @@ build_globals :: proc(checker: ^Checker) {
 		calls: [dynamic]hir.Function_Id
 		calls.allocator = checker.allocator
 		declared := resolve_inferred_array(checker, type_from_syntax(checker, global.type, global.pkg, global.file), global.expr)
+		if has_inferred_array_count(checker, declared) &&
+		   is_runtime_type(checker, checker.global_types[global_index]) {
+			declared = checker.global_types[global_index]
+		}
 		expected := types.INVALID
 		if is_runtime_type(checker, declared) {
 			expected = declared

compiler_tests.odin

@@ -1981,12 +1981,14 @@ comptime_value_params_specialize_by_value_and_omit_runtime_args :: proc(t: ^test
 	return data
 }
 main func() void {
-	four [4]u8 :: make_array(4)
+	four [_]u8 :: make_array(4)
-	eight [8]u8 :: make_array(8)
+	eight [_]u8 :: make_array(8)
-	again [4]u8 :: make_array(4)
+	again [_]u8 :: make_array(4)
+	literal [_]u8 :: [1, 2, 3, 4]
 	_ = four
 	_ = eight
 	_ = again
+	_ = literal
 }
 `
 	source_file := source.Source{path="test.bro", text=text}

examples/programs/comptime_value_params/main.bro

@@ -8,7 +8,7 @@ value func($N usize) usize {
 }
 
 main func() i32 {
-	four [4]u8 :: make_array(4)
+	four [_]u8 :: make_array(4)
 	if four.len != 4 {
 		return 1
 	}
@@ -16,7 +16,7 @@ main func() i32 {
 		return 2
 	}
 
-	eight [8]u8 :: make_array(8)
+	eight [_]u8 :: make_array(8)
 	if eight.len != 8 {
 		return 3
 	}
@@ -24,7 +24,7 @@ main func() i32 {
 		return 4
 	}
 
-	again [4]u8 :: make_array(4)
+	again [_]u8 :: [1, 2, 3, 4]
 	if again.len != 4 {
 		return 5
 	}