diagnostics upgrade

compiler/ast/ast.odin

@@ -63,6 +63,14 @@ index :: proc(id: $T, invalid: T, count: int) -> (int, bool) {
 
 Type_Syntax :: types.Type
 
+Type_Use :: struct {
+	type: Type_Syntax,
+	span: source.Span,
+	pkg:  Package_Id,
+	file: File_Id,
+	diagnostic: source.Diagnostic_Id,
+}
+
 Expr_Kind :: enum u8 {
 	Invalid,
 	Integer,
@@ -325,6 +333,7 @@ Module :: struct {
 	c_trampolines: [dynamic]Trampoline,
 	strings:    [dynamic]string,
 	type_fields: [dynamic]types.Field,
+	type_uses:   [dynamic]Type_Use,
 	type_store: types.Store,
 	allocator:  mem.Allocator,
 }
@@ -345,6 +354,7 @@ init_module :: proc(allocator := context.allocator) -> Module {
 	module.c_trampolines.allocator = allocator
 	module.strings.allocator = allocator
 	module.type_fields.allocator = allocator
+	module.type_uses.allocator = allocator
 	return module
 }
 
@@ -397,5 +407,6 @@ destroy_module :: proc(module: ^Module) {
 	delete(module.c_trampolines)
 	delete(module.strings)
 	delete(module.type_fields)
+	delete(module.type_uses)
 	types.destroy_store(&module.type_store)
 }

compiler/checker/checker.odin

@@ -208,6 +208,7 @@ Checker :: struct {
 	record_field_conflicts:   []types.Type,
 	record_field_conflict_spans: []source.Span,
 	record_field_demands_dirty: bool,
+	poisoned_packages: []bool,
 	external_global_canonical: []ast.Global_Id,
 	external_global_diagnostics: []source.Diagnostic_Id,
 	constants:            []Constant,
@@ -1496,16 +1497,30 @@ expr_lookup_file :: proc(expr: ast.Expr, file: ast.File_Id) -> ast.File_Id {
 	return ast.INVALID_FILE if symbol.is_valid(expr.qualifier) else file
 }
 
+expr_symbol_span :: proc(checker: ^Checker, expr: ast.Expr, name: symbol.Id) -> source.Span {
+	span := expr.span
+	span.end = span.start+source.Offset(len(symbol_text(checker, name)))
+	return span
+}
+
 add_package_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, file: ast.File_Id) -> source.Diagnostic_Id {
 	if find_import(checker, file, expr.qualifier) == ast.INVALID_IMPORT {
-		return source.addf(checker.diagnostics, expr.span, "unknown package alias '%s'", symbol_text(checker, expr.qualifier))
+		span := expr_symbol_span(checker, expr, expr.qualifier)
+		id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.qualifier))
+		source.set_primary_label(checker.diagnostics, id, "unknown symbol")
+		return id
 	}
 	return source.addf(checker.diagnostics, expr.span, "unavailable imported package '%s'", symbol_text(checker, expr.qualifier))
 }
 
 add_name_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target_pkg: ast.Package_Id, file: ast.File_Id) -> source.Diagnostic_Id {
 	if find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file)) != ast.INVALID_FUNCTION {
-		return source.addf(checker.diagnostics, expr.span, "'%s' is a function, not a global value", symbol_text(checker, expr.name))
+		template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
+		id := source.addf(checker.diagnostics, expr_symbol_span(checker, expr, expr.name), "'%s' is a function, not a global value", symbol_text(checker, expr.name))
+		if !checker.ast_module.functions[template].imported {
+			source.add_secondary_label(checker.diagnostics, id, checker.ast_module.functions[template].span, "function declared here")
+		}
+		return id
 	}
 	if symbol.is_valid(expr.qualifier) {
 		return source.addf(
@@ -1516,12 +1531,20 @@ add_name_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target
 			symbol_text(checker, expr.name),
 		)
 	}
-	return source.addf(checker.diagnostics, expr.span, "unresolved global '%s'", symbol_text(checker, expr.name))
+	span := expr_symbol_span(checker, expr, expr.name)
+	id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.name))
+	source.set_primary_label(checker.diagnostics, id, "unknown symbol")
+	return id
 }
 
 add_call_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target_pkg: ast.Package_Id, file: ast.File_Id) -> source.Diagnostic_Id {
 	if find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file)) != ast.INVALID_GLOBAL {
-		return source.addf(checker.diagnostics, expr.span, "'%s' is a global, not a function", symbol_text(checker, expr.name))
+		global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
+		id := source.addf(checker.diagnostics, expr_symbol_span(checker, expr, expr.name), "'%s' is a global, not a function", symbol_text(checker, expr.name))
+		if !checker.ast_module.globals[global].external {
+			source.add_secondary_label(checker.diagnostics, id, checker.ast_module.globals[global].span, "global declared here")
+		}
+		return id
 	}
 	if symbol.is_valid(expr.qualifier) {
 		return source.addf(
@@ -1532,7 +1555,10 @@ add_call_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target
 			symbol_text(checker, expr.name),
 		)
 	}
-	return source.addf(checker.diagnostics, expr.span, "unresolved function '%s'", symbol_text(checker, expr.name))
+	span := expr_symbol_span(checker, expr, expr.name)
+	id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.name))
+	source.set_primary_label(checker.diagnostics, id, "unknown symbol")
+	return id
 }
 
 find_unsupported :: proc(checker: ^Checker, pkg: ast.Package_Id, name: symbol.Id) -> (ast.Unsupported, bool) {
@@ -2649,6 +2675,7 @@ resolve_type_factory_call :: proc(checker: ^Checker, expr_id: ast.Expr_Id, pkg:
 	}
 	target_pkg, available := expr_package(checker, expr, pkg, file, true)
 	if !available {
+		_ = add_package_resolution_diagnostic(checker, expr, file)
 		return types.INVALID
 	}
 	template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
@@ -3145,6 +3172,7 @@ validate_declarations :: proc(checker: ^Checker) {
 			continue
 		}
 		has_comptime := function_has_comptime_params(function)
+		signature_poisoned := function.diagnostic != source.INVALID_DIAGNOSTIC
 		locals: [dynamic]symbol.Id
 		locals.allocator = checker.allocator
 		for param in function.params {
@@ -3152,7 +3180,7 @@ validate_declarations :: proc(checker: ^Checker) {
 			if param.comptime_value || !has_comptime {
 				param_type = type_from_syntax(checker, param.type, function.pkg, function.file)
 			}
-			if param.comptime_value {
+			if param.comptime_value && !signature_poisoned {
 				if function.c_abi {
 					checker.template_diagnostics[function_id] = source.add(
 						checker.diagnostics,
@@ -3169,7 +3197,7 @@ validate_declarations :: proc(checker: ^Checker) {
 						symbol_text(checker, param.name),
 					)
 				}
-			} else if !has_comptime {
+			} else if !has_comptime && !signature_poisoned {
 				if diagnostic := add_unsupported_type_diagnostic(checker, param.span, param_type);
 				   diagnostic != source.INVALID_DIAGNOSTIC {
 					checker.template_diagnostics[function_id] = diagnostic
@@ -3197,7 +3225,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 			append(&locals, param.name)
-			if !has_comptime && types.contains_c_struct_by_value(param_type, &checker.module.types) {
+			if !has_comptime && !signature_poisoned && types.contains_c_struct_by_value(param_type, &checker.module.types) {
 				checker.template_diagnostics[function_id] = source.addf(
 					checker.diagnostics,
 					param.span,
@@ -3206,7 +3234,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 		}
-		if !has_comptime {
+		if !has_comptime && !signature_poisoned {
 			result_type := type_from_syntax(checker, function.result, function.pkg, function.file)
 			if diagnostic := add_unsupported_type_diagnostic(checker, function.span, result_type);
 			   diagnostic != source.INVALID_DIAGNOSTIC {
@@ -3221,7 +3249,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 		}
-		if types.is_valid(function.error) {
+		if types.is_valid(function.error) && !signature_poisoned {
 			error_type := type_from_syntax(checker, function.error, function.pkg, function.file)
 			error_sum := types.is_enum(error_type, &checker.module.types) ||
 			             types.is_tagged_union(error_type, &checker.module.types)
@@ -3239,7 +3267,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 		}
-		if !function.has_body && !function.c_abi {
+		if !function.has_body && !function.c_abi && !signature_poisoned {
 			checker.template_diagnostics[function_id] = source.addf(
 				checker.diagnostics,
 				function.span,
@@ -3247,7 +3275,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				symbol_text(checker, function.name),
 			)
 		}
-		if function.variadic && (!function.c_abi || function.has_body) {
+		if function.variadic && (!function.c_abi || function.has_body) && !signature_poisoned {
 			checker.template_diagnostics[function_id] = source.addf(
 				checker.diagnostics,
 				function.span,
@@ -3255,7 +3283,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				symbol_text(checker, function.name),
 			)
 		}
-		if !function.has_body && function.c_abi {
+		if !function.has_body && function.c_abi && !signature_poisoned {
 			for param in function.params {
 				param_type := type_from_syntax(checker, param.type, function.pkg, function.file)
 				if add_unsupported_type_diagnostic(checker, param.span, param_type) !=
@@ -3351,7 +3379,7 @@ is_inferred_record_field :: proc(checker: ^Checker, slot: int) -> bool {
 	       types.is_constraint(checker.record_field_constraints[slot])
 }
 
-record_field_owner :: proc(checker: ^Checker, slot: int) -> (symbol.Id, symbol.Id, bool) {
+record_field_owner :: proc(checker: ^Checker, slot: int) -> (symbol.Id, symbol.Id, ast.Package_Id, bool) {
 	for item in checker.module.types.nodes {
 		if !(item.declared && (item.kind == .Struct || item.kind == .Union) &&
 		     !item.c_layout && symbol.is_valid(symbol.Id(item.name))) {
@@ -3360,10 +3388,10 @@ record_field_owner :: proc(checker: ^Checker, slot: int) -> (symbol.Id, symbol.I
 		start := int(item.field_start)
 		if slot >= start && slot < start+int(item.field_count) &&
 		   slot >= 0 && slot < len(checker.module.types.fields) {
-			return symbol.Id(item.name), symbol.Id(checker.module.types.fields[slot].name), true
+			return symbol.Id(item.name), symbol.Id(checker.module.types.fields[slot].name), ast.Package_Id(item.pkg), true
 		}
 	}
-	return symbol.INVALID, symbol.INVALID, false
+	return symbol.INVALID, symbol.INVALID, ast.INVALID_PACKAGE, false
 }
 
 record_field_conflict :: proc(checker: ^Checker, slot: int, actual: types.Type, span: source.Span) {
@@ -3484,7 +3512,7 @@ finalize_record_field_inference :: proc(checker: ^Checker) {
 		if !types.is_constraint(constraint) {
 			continue
 		}
-		record_name, field_name, ok := record_field_owner(checker, slot)
+		record_name, field_name, record_pkg, ok := record_field_owner(checker, slot)
 		if !ok {
 			continue
 		}
@@ -3509,7 +3537,8 @@ finalize_record_field_inference :: proc(checker: ^Checker) {
 					symbol_text(checker, record_name), symbol_text(checker, field_name),
 				)
 			}
-		} else if types.is_constraint(current) {
+		} else if types.is_constraint(current) &&
+		          !(int(record_pkg) < len(checker.poisoned_packages) && checker.poisoned_packages[record_pkg]) {
 			source.addf(
 				checker.diagnostics,
 				source.Span{},
@@ -5622,6 +5651,10 @@ 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 {
+		if global.diagnostic != source.INVALID_DIAGNOSTIC {
+			checker.global_types[index] = types.I64
+			continue
+		}
 		declared := resolve_inferred_array(
 			checker,
 			type_from_syntax(checker, global.type, global.pkg, global.file),
@@ -5675,7 +5708,7 @@ infer_all :: proc(checker: ^Checker) {
 		// Backward demands: a global pushes its own (declared or already-resolved) type
 		// onto open numeric slots reachable through names and numeric arithmetic.
 		for global, index in checker.ast_module.globals {
-			if global.external {
+			if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
 				continue
 			}
 			demand := checker.global_types[index]
@@ -5689,7 +5722,7 @@ infer_all :: proc(checker: ^Checker) {
 		// concrete-typed ones) for its side effect of specializing called functions and
 		// recording demands from call arguments in their initializers.
 		for global, index in checker.ast_module.globals {
-			if global.external {
+			if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
 				continue
 			}
 			declared := resolve_inferred_array(
@@ -5765,7 +5798,8 @@ infer_all :: proc(checker: ^Checker) {
 				// No authoritative demand can still arrive. Assign final defaults, then
 				// continue the same fixpoint so dependent globals/specs observe them.
 				for global, index in checker.ast_module.globals {
-					if global.external || is_runtime_type(checker, checker.global_types[index]) {
+					if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC ||
+					   is_runtime_type(checker, checker.global_types[index]) {
 						continue
 					}
 					if checker.global_open_const[index] {
@@ -5807,7 +5841,7 @@ prune_specs :: proc(checker: ^Checker) {
 		mark_spec_demanded(checker, find_spec(checker, checker.io_provider_template, nil), &stack)
 	}
 	for global in checker.ast_module.globals {
-		if global.external {
+		if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
 			continue
 		}
 		_ = infer_expr(checker, global.expr, nil, global.pkg, global.file, &stack)
@@ -5859,6 +5893,23 @@ invalid_hir_expr :: proc(
 	)
 }
 
+invalid_expr_diagnostic :: proc(checker: ^Checker, id: hir.Expr_Id) -> (source.Diagnostic_Id, bool) {
+	if id == hir.INVALID_EXPR || int(id) >= len(checker.module.exprs) {
+		return source.INVALID_DIAGNOSTIC, false
+	}
+	expr := checker.module.exprs[id]
+	return expr.diagnostic, expr.kind == .Invalid && expr.diagnostic != source.INVALID_DIAGNOSTIC
+}
+
+propagate_invalid_expr :: proc(checker: ^Checker, span: source.Span, values: ..hir.Expr_Id) -> (hir.Expr_Id, bool) {
+	for value in values {
+		if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
+			return invalid_hir_expr(checker, span, diagnostic), true
+		}
+	}
+	return hir.INVALID_EXPR, false
+}
+
 add_unique_global :: proc(values: ^[dynamic]hir.Global_Id, value: hir.Global_Id) {
 	for existing in values {
 		if existing == value {
@@ -5935,6 +5986,9 @@ coerce_expr :: proc(
 	if expr_id == hir.INVALID_EXPR {
 		return expr_id
 	}
+	if _, invalid := invalid_expr_diagnostic(checker, expr_id); invalid {
+		return expr_id
+	}
 	actual := checker.module.exprs[expr_id].type
 	if types.equal(actual, expected) {
 		return expr_id
@@ -6918,6 +6972,9 @@ build_compound_expr :: proc(
 			})
 		}
 		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, value); propagated {
+			return invalid
+		}
 		if !hir_is_location(checker, value) {
 			id := source.add(checker.diagnostics, expr.span, "'&' requires an addressable location")
 			return invalid_hir_expr(checker, expr.span, id)
@@ -6936,6 +6993,9 @@ build_compound_expr :: proc(
 		})
 	case .Deref:
 		pointer := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, pointer); propagated {
+			return invalid
+		}
 		pointer_type := checker.module.exprs[pointer].type
 		if !types.is_pointer(pointer_type, store) {
 			id := source.add(checker.diagnostics, expr.span, "postfix '^' requires a pointer")
@@ -6949,6 +7009,9 @@ build_compound_expr :: proc(
 		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 		index := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.USIZE, pkg, file)
 		index = coerce_expr(checker, index, types.USIZE, expr.span)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, container, index); propagated {
+			return invalid
+		}
 		container_type := checker.module.exprs[container].type
 		item, ok := types.container(container_type, store)
 		if !ok {
@@ -6970,6 +7033,9 @@ build_compound_expr :: proc(
 			})
 		}
 		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, container); propagated {
+			return invalid
+		}
 		container_type := checker.module.exprs[container].type
 		item, ok := types.container(container_type, store)
 		if !ok {
@@ -6987,6 +7053,10 @@ build_compound_expr :: proc(
 			if bound != ast.INVALID_EXPR {
 				bounds[index] = build_nested_expr(checker, bound, locals, global_reads, calls, types.USIZE, pkg, file)
 				bounds[index] = coerce_expr(checker, bounds[index], types.USIZE, checker.ast_module.exprs[bound].span)
+				if invalid, propagated := propagate_invalid_expr(checker, expr.span, bounds[index]); propagated {
+					delete(bounds, checker.allocator)
+					return invalid
+				}
 			}
 		}
 		preserve_sentinel := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
@@ -7000,11 +7070,17 @@ build_compound_expr :: proc(
 			return enum_member_hir(checker, enum_type, expr.name, expr.span)
 		}
 		base := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, base); propagated {
+			return invalid
+		}
 		base_type := checker.module.exprs[base].type
 		result, _ := build_field_from_value(checker, expr, base, base_type)
 		return result
 	case .Unwrap:
 		optional := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, optional); propagated {
+			return invalid
+		}
 		optional_type := checker.module.exprs[optional].type
 		if !types.is_optional(optional_type, store) {
 			id := source.add(checker.diagnostics, expr.span, "postfix '?' requires an optional")
@@ -7016,6 +7092,9 @@ build_compound_expr :: proc(
 		})
 	case .Orelse:
 		optional := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, optional); propagated {
+			return invalid
+		}
 		optional_type := checker.module.exprs[optional].type
 		if !types.is_optional(optional_type, store) {
 			id := source.add(checker.diagnostics, expr.span, "'orelse' requires an optional left operand")
@@ -7035,6 +7114,9 @@ build_compound_expr :: proc(
 		}
 		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
 		channel := build_nested_expr(checker, expr.left, locals, global_reads, calls, left_expected, pkg, file)
+		if diagnostic, invalid := invalid_expr_diagnostic(checker, channel); invalid {
+			return invalid_hir_expr(checker, expr.span, diagnostic)
+		}
 		channel_type := checker.module.exprs[channel].type
 		success := types.fallible_success(channel_type, store)
 		if !types.is_valid(success) {
@@ -7076,6 +7158,9 @@ build_compound_expr :: proc(
 	case .Catch:
 		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
 		channel := build_nested_expr(checker, expr.left, locals, global_reads, calls, left_expected, pkg, file)
+		if diagnostic, invalid := invalid_expr_diagnostic(checker, channel); invalid {
+			return invalid_hir_expr(checker, expr.span, diagnostic)
+		}
 		channel_type := checker.module.exprs[channel].type
 		success := types.fallible_success(channel_type, store)
 		if !types.is_valid(success) {
@@ -7145,6 +7230,9 @@ build_compound_expr :: proc(
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
 		}
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
+			return invalid
+		}
 		child := expected_child
 		if !types.is_valid(child) {
 			child = types.widest(checker.module.exprs[left].type, checker.module.exprs[right].type)
@@ -7174,6 +7262,9 @@ build_compound_expr :: proc(
 		})
 	case .Not:
 		operand := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.BOOL, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, operand); propagated {
+			return invalid
+		}
 		operand_type := checker.module.exprs[operand].type
 		if checker.module.exprs[operand].kind != .Invalid && !types.is_bool(operand_type) {
 			id := source.add(checker.diagnostics, expr.span, "'!' requires a bool operand")
@@ -7186,6 +7277,9 @@ build_compound_expr :: proc(
 	case .And, .Or:
 		left := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.BOOL, pkg, file)
 		right := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.BOOL, pkg, file)
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
+			return invalid
+		}
 		left_type := checker.module.exprs[left].type
 		right_type := checker.module.exprs[right].type
 		left_ok := checker.module.exprs[left].kind == .Invalid || types.is_bool(left_type)
@@ -7226,11 +7320,11 @@ build_compound_expr :: proc(
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
 		}
+		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
+			return invalid
+		}
 		left_type := checker.module.exprs[left].type
 		right_type := checker.module.exprs[right].type
-		if checker.module.exprs[left].kind == .Invalid || checker.module.exprs[right].kind == .Invalid {
-			return invalid_hir_expr(checker, expr.span, expr.diagnostic, types.BOOL)
-		}
 		operand_type := types.INVALID
 		if types.is_enum(left_type, store) || types.is_enum(right_type, store) {
 			if !types.equal(left_type, right_type) || (expr.kind != .Eq && expr.kind != .Ne) {
@@ -7420,6 +7514,9 @@ build_binary_arith :: proc(
 	left, right: hir.Expr_Id,
 	span: source.Span,
 ) -> hir.Expr_Id {
+	if invalid, propagated := propagate_invalid_expr(checker, span, left, right); propagated {
+		return invalid
+	}
 	// Pointer arithmetic is only defined for `+` (many-pointer + usize).
 	if op == .Add &&
 	   types.is_many_pointer(checker.module.exprs[left].type, &checker.module.types) &&
@@ -7960,6 +8057,9 @@ build_expr :: proc(
 							checker.diagnostics, expr.span, message,
 							symbol_text(checker, expr.name), len(function.params), len(expr.args),
 						)
+						if !function.imported {
+							source.add_secondary_label(checker.diagnostics, id, function.span, "function declared here")
+						}
 					} else if len(mapping_failure) > 0 {
 						id = source.addf(
 							checker.diagnostics, expr.span,
@@ -8009,6 +8109,11 @@ build_expr :: proc(
 		}
 		if frame.stage == 5 {
 			operand := last
+			if invalid, propagated := propagate_invalid_expr(checker, expr.span, operand); propagated {
+				last = invalid
+				_ = pop(&stack)
+				continue
+			}
 			operand_type := checker.module.exprs[operand].type
 			if !types.is_signed(operand_type, checker.target) && !types.is_float(operand_type, checker.target) {
 				id := source.add(checker.diagnostics, expr.span, "negation requires a signed integer or float")
@@ -8071,6 +8176,17 @@ build_expr :: proc(
 					continue
 				}
 			}
+			if invalid, propagated := propagate_invalid_expr(checker, expr.span, ..stack[frame_index].built_args); propagated {
+				delete(stack[frame_index].arg_types, checker.allocator)
+				stack[frame_index].arg_types = nil
+				delete(stack[frame_index].built_args, checker.allocator)
+				stack[frame_index].built_args = nil
+				delete(stack[frame_index].mapping, checker.allocator)
+				stack[frame_index].mapping = nil
+				last = invalid
+				_ = pop(&stack)
+				continue
+			}
 			function := checker.ast_module.functions[frame.template]
 			comptime_values: []Comptime_Value
 			comptime_ok := false
@@ -8236,6 +8352,11 @@ build_expr :: proc(
 		}
 		if frame.stage == 6 {
 			callee := last
+			if invalid, propagated := propagate_invalid_expr(checker, expr.span, callee); propagated {
+				last = invalid
+				_ = pop(&stack)
+				continue
+			}
 			_, function_item, function_type, ok := types.function_pointer(checker.module.exprs[callee].type, &checker.module.types)
 			if !ok {
 				id := source.add(checker.diagnostics, expr.span, "call target is not a function pointer")
@@ -8279,6 +8400,17 @@ build_expr :: proc(
 					continue
 				}
 			}
+			invalid, propagated := propagate_invalid_expr(checker, expr.span, frame.left)
+			if !propagated {
+				invalid, propagated = propagate_invalid_expr(checker, expr.span, ..stack[frame_index].built_args)
+			}
+			if propagated {
+				delete(stack[frame_index].built_args, checker.allocator)
+				stack[frame_index].built_args = nil
+				last = invalid
+				_ = pop(&stack)
+				continue
+			}
 			callee_type := checker.module.exprs[frame.left].type
 			_, function_item, function_type, ok := types.function_pointer(callee_type, &checker.module.types)
 			if !ok {
@@ -8897,6 +9029,22 @@ build_block :: proc(
 	duplicate_start := scope_start if duplicate_scope_start < 0 else duplicate_scope_start
 	for statement_id in statements {
 		statement := checker.ast_module.statements[statement_id]
+		if statement.diagnostic != source.INVALID_DIAGNOSTIC {
+			if statement.kind == .Declaration && symbol.is_valid(statement.name) {
+				if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); !found {
+					_ = append_build_local(
+						ctx, statement.name, types.I64, !statement.immutable, statement.span,
+					)
+				}
+			}
+			append(&body, hir.stmt_id(len(checker.module.statements)))
+			append(&checker.module.statements, hir.Stmt{
+				kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
+				local = hir.INVALID_LOCAL, diagnostic = statement.diagnostic,
+			})
+			ctx.problematic^ = true
+			continue
+		}
 		switch statement.kind {
 		case .Declaration:
 			// A value block (`x :: { ... yield v }` / `x T = { ... }`): the parser
@@ -9070,6 +9218,15 @@ build_block :: proc(
 					checker, statement.target, ctx.locals^[:], ctx.global_reads, ctx.calls,
 					types.INVALID, ctx.pkg, ctx.file,
 				)
+				if diagnostic, invalid := invalid_expr_diagnostic(checker, target_expr); invalid {
+					append(&body, hir.stmt_id(len(checker.module.statements)))
+					append(&checker.module.statements, hir.Stmt{
+						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
+						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
+					})
+					ctx.problematic^ = true
+					continue
+				}
 				target_type := checker.module.exprs[target_expr].type
 				if !hir_location_writable(checker, target_expr, ctx.locals^[:]) {
 					id := source.add(checker.diagnostics, statement.span, "assignment target is not writable")
@@ -9092,6 +9249,15 @@ build_block :: proc(
 						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
 						rhs_expected, ctx.pkg, ctx.file,
 					)
+					if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
+						append(&body, hir.stmt_id(len(checker.module.statements)))
+						append(&checker.module.statements, hir.Stmt{
+							kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
+							target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
+						})
+						ctx.problematic^ = true
+						continue
+					}
 					if types.is_many_pointer(target_type, &checker.module.types) {
 						assignment_op = .Pointer_Add
 						if statement.assignment_op != .Add {
@@ -9208,6 +9374,15 @@ build_block :: proc(
 					continue
 				}
 				target_expr := build_global_reference(checker, global, statement.span, ctx.global_reads)
+				if diagnostic, invalid := invalid_expr_diagnostic(checker, target_expr); invalid {
+					append(&body, hir.stmt_id(len(checker.module.statements)))
+					append(&checker.module.statements, hir.Stmt{
+						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
+						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
+					})
+					ctx.problematic^ = true
+					continue
+				}
 				target_type := checker.module.exprs[target_expr].type
 				if !hir_location_writable(checker, target_expr, ctx.locals^[:]) {
 					id := source.add(checker.diagnostics, statement.span, "assignment target is not writable")
@@ -9425,7 +9600,14 @@ build_block :: proc(
 			})
 		case .Expression:
 			value := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
-			if !types.is_void(checker.module.exprs[value].type) {
+			if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
+				append(&body, hir.stmt_id(len(checker.module.statements)))
+				append(&checker.module.statements, hir.Stmt{
+					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
+					local = hir.INVALID_LOCAL, diagnostic = diagnostic,
+				})
+				ctx.problematic^ = true
+			} else if !types.is_void(checker.module.exprs[value].type) {
 				id := source.add(checker.diagnostics, statement.span, "non-void expression result must be consumed or assigned to '_'")
 				append(&body, hir.stmt_id(len(checker.module.statements)))
 				append(&checker.module.statements, hir.Stmt{
@@ -11504,15 +11686,10 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 	checker.current_comptime_values = spec.comptime_values
 	defer checker.current_comptime_values = previous_comptime
 	signature_diagnostic := source.INVALID_DIAGNOSTIC
-	if !types.is_void(spec.result) && !is_runtime_type(checker, spec.result) {
+	unresolved_result := !types.is_void(spec.result) && !is_runtime_type(checker, spec.result)
+	if unresolved_result {
 		checker.specs[id].result = types.I64
 		spec.result = types.I64
-		signature_diagnostic = source.addf(
-			checker.diagnostics,
-			function.span,
-			"could not resolve a concrete result type for '%s'",
-			symbol_text(checker, function.name),
-		)
 	}
 	for arg in spec.args {
 		if !is_runtime_type(checker, arg) {
@@ -11582,6 +11759,16 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 	               checker.template_diagnostics[spec.template] != source.INVALID_DIAGNOSTIC
 	native_main := function.pkg == 0 && function.name == checker.main_symbol && checker.entry_point == .Plain
 	if !function.has_body {
+		if unresolved_result && signature_diagnostic == source.INVALID_DIAGNOSTIC &&
+		   checker.template_diagnostics[spec.template] == source.INVALID_DIAGNOSTIC {
+			signature_diagnostic = source.addf(
+				checker.diagnostics,
+				function.span,
+				"could not resolve a concrete result type for '%s'",
+				symbol_text(checker, function.name),
+			)
+			problematic = true
+		}
 		assert(spec.hir_id == hir.function_id(len(checker.module.functions)))
 		append(
 			&checker.module.functions,
@@ -11665,6 +11852,22 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 		append(&body, block_stmt)
 	}
 	delete(block, checker.allocator)
+	if unresolved_result && signature_diagnostic == source.INVALID_DIAGNOSTIC &&
+	   checker.template_diagnostics[spec.template] == source.INVALID_DIAGNOSTIC && !problematic {
+		signature_diagnostic = source.addf(
+			checker.diagnostics,
+			function.span,
+			"could not resolve a concrete result type for '%s'",
+			symbol_text(checker, function.name),
+		)
+		append(&body, hir.stmt_id(len(checker.module.statements)))
+		append(&checker.module.statements, hir.Stmt{
+			kind=.Trap, span=function.span, expr=hir.INVALID_EXPR,
+			local=hir.INVALID_LOCAL, diagnostic=signature_diagnostic,
+		})
+		problematic = true
+		returns = true
+	}
 
 	fallible_void := types.kind(spec.result, &checker.module.types) == .Fallible &&
 	                 types.is_void(types.fallible_success(spec.result, &checker.module.types))
@@ -11773,6 +11976,28 @@ static_integer_value :: proc(module: ^hir.Module, expr_id: hir.Expr_Id) -> (i64,
 
 build_globals :: proc(checker: ^Checker) {
 	for global, global_index in checker.ast_module.globals {
+		if global.diagnostic != source.INVALID_DIAGNOSTIC {
+			global_type := checker.global_types[global_index]
+			if !is_runtime_type(checker, global_type) {
+				global_type = types.I64
+			}
+			expr := hir.INVALID_EXPR
+			if !global.external {
+				expr = invalid_hir_expr(checker, global.span, global.diagnostic, global_type)
+			}
+			append(&checker.module.globals, hir.Global{
+				name=global.name,
+				link_name=strings.clone(global.link_name, checker.allocator),
+				type=global_type,
+				expr=expr,
+				external=global.external,
+				writable=global.writable || !global.immutable,
+				direct_problem=true,
+				problematic=true,
+				diagnostic=global.diagnostic,
+			})
+			continue
+		}
 		if global.external {
 			global_type := checker.global_types[global_index]
 			writable := global.writable
@@ -11841,6 +12066,12 @@ build_globals :: proc(checker: ^Checker) {
 			global_type = checker.module.exprs[expr].type
 		}
 		diagnostic := source.INVALID_DIAGNOSTIC
+		if root, invalid := invalid_expr_diagnostic(checker, expr); invalid {
+			diagnostic = root
+			if !is_runtime_type(checker, global_type) {
+				global_type = types.I64
+			}
+		}
 		if !global.immutable && is_undefined_expr(checker, global.expr) {
 			diagnostic = source.add(
 				checker.diagnostics,
@@ -12188,6 +12419,7 @@ check :: proc(
 	checker.record_field_defaults = make([]types.Type, len(checker.module.types.fields), allocator)
 	checker.record_field_conflicts = make([]types.Type, len(checker.module.types.fields), allocator)
 	checker.record_field_conflict_spans = make([]source.Span, len(checker.module.types.fields), allocator)
+	checker.poisoned_packages = make([]bool, max(len(ast_module.packages), 1), allocator)
 	init_record_field_inference(&checker)
 	checker.external_global_canonical = make([]ast.Global_Id, len(ast_module.globals), allocator)
 	checker.external_global_diagnostics = make([]source.Diagnostic_Id, len(ast_module.globals), allocator)
@@ -12199,8 +12431,17 @@ check :: proc(
 	}
 	checker.constants = make([]Constant, len(ast_module.exprs), allocator)
 	checker.template_diagnostics = make([]source.Diagnostic_Id, len(ast_module.functions), allocator)
-	for &diagnostic in checker.template_diagnostics {
-		diagnostic = source.INVALID_DIAGNOSTIC
+	for &diagnostic, index in checker.template_diagnostics {
+		diagnostic = ast_module.functions[index].diagnostic
+		pkg := ast_module.functions[index].pkg
+		if diagnostic != source.INVALID_DIAGNOSTIC && int(pkg) < len(checker.poisoned_packages) {
+			checker.poisoned_packages[pkg] = true
+		}
+	}
+	for global in ast_module.globals {
+		if global.diagnostic != source.INVALID_DIAGNOSTIC && int(global.pkg) < len(checker.poisoned_packages) {
+			checker.poisoned_packages[global.pkg] = true
+		}
 	}
 	defer {
 		for spec in checker.specs {
@@ -12220,6 +12461,7 @@ check :: proc(
 		delete(checker.record_field_defaults, allocator)
 		delete(checker.record_field_conflicts, allocator)
 		delete(checker.record_field_conflict_spans, allocator)
+		delete(checker.poisoned_packages, allocator)
 		delete(checker.external_global_canonical, allocator)
 		delete(checker.external_global_diagnostics, allocator)
 		delete(checker.constants, allocator)

compiler/loader/loader.odin

@@ -1486,7 +1486,7 @@ validate_declaration_aliases :: proc(state: ^State) {
 
 		import_id := find_type_import(state.module, alias.file, alias.qualifier)
 		if import_id == ast.INVALID_IMPORT {
-			alias.diagnostic = source.addf(state.diagnostics, alias.span, "unknown package alias '%s'", symbol.resolve(state.symbols, alias.qualifier))
+			alias.diagnostic = source.addf(state.diagnostics, alias.span, "unknown symbol '%s'", symbol.resolve(state.symbols, alias.qualifier))
 			alias.valid = false
 			continue
 		}
@@ -1613,6 +1613,81 @@ canonical_type :: proc(
 	return resolved
 }
 
+diagnose_qualified_type_uses :: proc(state: ^State) {
+	module := state.module
+	for &type_use in module.type_uses {
+		item, ok := types.node(&module.type_store, type_use.type)
+		if !ok || item.qualifier == 0 || item.kind != .Named {
+			continue
+		}
+		qualifier := symbol.Id(item.qualifier)
+		name := symbol.Id(item.name)
+		import_id := find_type_import(module, type_use.file, qualifier)
+		if import_id == ast.INVALID_IMPORT {
+			type_use.diagnostic = source.addf(
+				state.diagnostics,
+				type_use.span,
+				"unknown symbol '%s'",
+				symbol.resolve(state.symbols, qualifier),
+			)
+			source.set_primary_label(state.diagnostics, type_use.diagnostic, "unknown symbol")
+			continue
+		}
+		import_item := module.imports[import_id]
+		if !import_item.valid || import_item.target == ast.INVALID_PACKAGE ||
+		   int(import_item.target) >= len(module.packages) || !module.packages[import_item.target].available {
+			type_use.diagnostic = source.addf(
+				state.diagnostics,
+				type_use.span,
+				"unavailable imported package '%s'",
+				symbol.resolve(state.symbols, qualifier),
+			)
+			continue
+		}
+		if !types.is_valid(types.find_named(&module.type_store, u32(import_item.target), u32(name))) {
+			type_use.diagnostic = source.addf(
+				state.diagnostics,
+				type_use.span,
+				"package '%s' has no member '%s'",
+				symbol.resolve(state.symbols, qualifier),
+				symbol.resolve(state.symbols, name),
+			)
+		}
+	}
+}
+
+type_resolution_diagnostic :: proc(module: ^ast.Module, value: types.Type, file: ast.File_Id, depth := 0) -> source.Diagnostic_Id {
+	if depth > 64 {
+		return source.INVALID_DIAGNOSTIC
+	}
+	for type_use in module.type_uses {
+		if type_use.file == file && type_use.type == value && type_use.diagnostic != source.INVALID_DIAGNOSTIC {
+			return type_use.diagnostic
+		}
+	}
+	item, ok := types.node(&module.type_store, value)
+	if !ok {
+		return source.INVALID_DIAGNOSTIC
+	}
+	if diagnostic := type_resolution_diagnostic(module, item.child, file, depth+1);
+	   diagnostic != source.INVALID_DIAGNOSTIC {
+		return diagnostic
+	}
+	if diagnostic := type_resolution_diagnostic(module, item.extra, file, depth+1);
+	   diagnostic != source.INVALID_DIAGNOSTIC {
+		return diagnostic
+	}
+	if item.kind == .Function {
+		for param in types.params_for(&module.type_store, value) {
+			if diagnostic := type_resolution_diagnostic(module, param.type, file, depth+1);
+			   diagnostic != source.INVALID_DIAGNOSTIC {
+				return diagnostic
+			}
+		}
+	}
+	return source.INVALID_DIAGNOSTIC
+}
+
 canonicalize_types :: proc(module: ^ast.Module, allocator: mem.Allocator) {
 	original_count := len(module.type_store.nodes)
 	mapping := make([]types.Type, original_count, allocator)
@@ -1620,6 +1695,19 @@ canonicalize_types :: proc(module: ^ast.Module, allocator: mem.Allocator) {
 	defer delete(mapping, allocator)
 	defer delete(visiting, allocator)
 	for &function in module.functions {
+		for param in function.params {
+			if diagnostic := type_resolution_diagnostic(module, param.type, function.file);
+			   diagnostic != source.INVALID_DIAGNOSTIC && function.diagnostic == source.INVALID_DIAGNOSTIC {
+				function.diagnostic = diagnostic
+			}
+		}
+		signature_results := [2]types.Type{function.result, function.error}
+		for value in signature_results {
+			if diagnostic := type_resolution_diagnostic(module, value, function.file);
+			   diagnostic != source.INVALID_DIAGNOSTIC && function.diagnostic == source.INVALID_DIAGNOSTIC {
+				function.diagnostic = diagnostic
+			}
+		}
 		for &param in function.params {
 			param.type = canonical_type(module, param.type, mapping, visiting)
 		}
@@ -1627,9 +1715,17 @@ canonicalize_types :: proc(module: ^ast.Module, allocator: mem.Allocator) {
 		function.error = canonical_type(module, function.error, mapping, visiting)
 	}
 	for &global in module.globals {
+		if diagnostic := type_resolution_diagnostic(module, global.type, global.file);
+		   diagnostic != source.INVALID_DIAGNOSTIC && global.diagnostic == source.INVALID_DIAGNOSTIC {
+			global.diagnostic = diagnostic
+		}
 		global.type = canonical_type(module, global.type, mapping, visiting)
 	}
 	for &statement in module.statements {
+		if diagnostic := type_resolution_diagnostic(module, statement.type, ast.File_Id(statement.span.file));
+		   diagnostic != source.INVALID_DIAGNOSTIC && statement.diagnostic == source.INVALID_DIAGNOSTIC {
+			statement.diagnostic = diagnostic
+		}
 		statement.type = canonical_type(module, statement.type, mapping, visiting)
 	}
 	for &field in module.type_fields {
@@ -1687,6 +1783,7 @@ load :: proc(
 	}
 	validate_imports(&state)
 	validate_declaration_aliases(&state)
+	diagnose_qualified_type_uses(&state)
 	canonicalize_types(&module, allocator)
 	return module, !state.root_failed
 }

compiler/parser/parser.odin

@@ -428,6 +428,13 @@ parse_type_atom :: proc(parser: ^Parser) -> ast.Type_Syntax {
 				count_expr=u32(call),
 			})
 		}
+		append(&parser.module.type_uses, ast.Type_Use{
+			type=named,
+			span=first.span,
+			pkg=parser.pkg,
+			file=parser.file,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 		return named
 	}
 	source.add(parser.diagnostics, tok.span, "expected a type")

compiler/source/source.odin

@@ -2,6 +2,7 @@ package source
 
 import "core:fmt"
 import "core:mem"
+import "core:strings"
 
 Source_Id :: distinct u32
 Diagnostic_Id :: distinct u32
@@ -58,6 +59,20 @@ Diagnostic :: struct {
 	severity: Severity,
 }
 
+Annotation_Kind :: enum u8 {
+	Primary,
+	Secondary,
+	Note,
+	Help,
+}
+
+Annotation :: struct {
+	owner:   Diagnostic_Id,
+	span:    Span,
+	message: string,
+	kind:    Annotation_Kind,
+}
+
 Severity :: enum u8 {
 	Error,
 	Warning,
@@ -67,6 +82,7 @@ Diagnostics :: struct {
 	source:    ^Source,
 	store:     ^Store,
 	items:     [dynamic]Diagnostic,
+	annotations: [dynamic]Annotation,
 	index:     map[Diagnostic_Key]Diagnostic_Id,
 	allocator: mem.Allocator,
 }
@@ -130,6 +146,7 @@ init_diagnostics :: proc(source_file: ^Source, allocator := context.allocator) -
 	result.source = source_file
 	result.allocator = allocator
 	result.items.allocator = allocator
+	result.annotations.allocator = allocator
 	result.index.allocator = allocator
 	return result
 }
@@ -139,6 +156,7 @@ init_store_diagnostics :: proc(store: ^Store, allocator := context.allocator) ->
 	result.store = store
 	result.allocator = allocator
 	result.items.allocator = allocator
+	result.annotations.allocator = allocator
 	result.index.allocator = allocator
 	return result
 }
@@ -148,7 +166,11 @@ destroy_diagnostics :: proc(diagnostics: ^Diagnostics) {
 	for diagnostic in diagnostics.items {
 		delete(diagnostic.message, diagnostics.allocator)
 	}
+	for annotation in diagnostics.annotations {
+		delete(annotation.message, diagnostics.allocator)
+	}
 	delete(diagnostics.items)
+	delete(diagnostics.annotations)
 }
 
 add_with_severity :: proc(diagnostics: ^Diagnostics, span: Span, message: string, severity: Severity) -> Diagnostic_Id {
@@ -192,6 +214,50 @@ addf_warning :: proc(diagnostics: ^Diagnostics, span: Span, format: string, args
 	return addf_with_severity(diagnostics, span, .Warning, format, ..args)
 }
 
+add_annotation :: proc(
+	diagnostics: ^Diagnostics,
+	owner: Diagnostic_Id,
+	kind: Annotation_Kind,
+	span: Span,
+	message: string,
+) {
+	if _, ok := diagnostic_index(owner, len(diagnostics.items)); !ok {
+		return
+	}
+	for annotation in diagnostics.annotations {
+		if annotation.owner == owner && annotation.kind == kind && annotation.span == span &&
+		   annotation.message == message {
+			return
+		}
+	}
+	append(&diagnostics.annotations, Annotation{
+		owner=owner,
+		kind=kind,
+		span=span,
+		message=fmt.aprintf("%s", message, allocator=diagnostics.allocator),
+	})
+}
+
+set_primary_label :: proc(diagnostics: ^Diagnostics, owner: Diagnostic_Id, message: string) {
+	index, ok := diagnostic_index(owner, len(diagnostics.items))
+	if !ok {
+		return
+	}
+	add_annotation(diagnostics, owner, .Primary, diagnostics.items[index].span, message)
+}
+
+add_secondary_label :: proc(diagnostics: ^Diagnostics, owner: Diagnostic_Id, span: Span, message: string) {
+	add_annotation(diagnostics, owner, .Secondary, span, message)
+}
+
+add_note :: proc(diagnostics: ^Diagnostics, owner: Diagnostic_Id, message: string) {
+	add_annotation(diagnostics, owner, .Note, {}, message)
+}
+
+add_help :: proc(diagnostics: ^Diagnostics, owner: Diagnostic_Id, message: string) {
+	add_annotation(diagnostics, owner, .Help, {}, message)
+}
+
 line_and_column :: proc(source_file: ^Source, offset: Offset) -> (line, column: int) {
 	if len(source_file.line_starts) > 0 {
 		limit := min(int(offset), len(source_file.text))
@@ -232,6 +298,140 @@ source_for_span :: proc(diagnostics: ^Diagnostics, span: Span) -> ^Source {
 	return diagnostics.source
 }
 
+annotation_for :: proc(diagnostics: ^Diagnostics, owner: Diagnostic_Id, kind: Annotation_Kind) -> (Annotation, bool) {
+	for annotation in diagnostics.annotations {
+		if annotation.owner == owner && annotation.kind == kind {
+			return annotation, true
+		}
+	}
+	return {}, false
+}
+
+line_bounds :: proc(source_file: ^Source, line: int) -> (start, end: int, ok: bool) {
+	if line <= 0 {
+		return 0, 0, false
+	}
+	if len(source_file.line_starts) == 0 {
+		current := 1
+		start = 0
+		for value, index in transmute([]byte)source_file.text {
+			if current == line && value == '\n' {
+				end = index
+				if end > start && source_file.text[end-1] == '\r' {
+					end -= 1
+				}
+				return start, end, true
+			}
+			if value == '\n' {
+				current += 1
+				start = index+1
+			}
+		}
+		if current == line {
+			return start, len(source_file.text), true
+		}
+		return 0, 0, false
+	}
+	if line > len(source_file.line_starts) {
+		return 0, 0, false
+	}
+	start = int(source_file.line_starts[line-1])
+	end = len(source_file.text)
+	if line < len(source_file.line_starts) {
+		end = int(source_file.line_starts[line])-1
+	}
+	if end > start && source_file.text[end-1] == '\r' {
+		end -= 1
+	}
+	return start, end, true
+}
+
+write_spaces :: proc(builder: ^strings.Builder, count: int) {
+	for _ in 0..<max(count, 0) {
+		strings.write_byte(builder, ' ')
+	}
+}
+
+write_expanded :: proc(builder: ^strings.Builder, text: string, start_column := 0) -> int {
+	column := start_column
+	for value in transmute([]byte)text {
+		if value == '\t' {
+			width := 4-column%4
+			write_spaces(builder, width)
+			column += width
+		} else {
+			strings.write_byte(builder, value)
+			column += 1
+		}
+	}
+	return column
+}
+
+display_width :: proc(text: string, start_column := 0) -> int {
+	column := start_column
+	for value in transmute([]byte)text {
+		column += 4-column%4 if value == '\t' else 1
+	}
+	return column-start_column
+}
+
+decimal_width :: proc(value: int) -> int {
+	width := 1
+	for remaining := value; remaining >= 10; remaining /= 10 {
+		width += 1
+	}
+	return width
+}
+
+write_excerpt :: proc(
+	builder: ^strings.Builder,
+	diagnostics: ^Diagnostics,
+	span: Span,
+	label: string,
+	primary: bool,
+) -> bool {
+	if span == (Span{}) {
+		return false
+	}
+	source_file := source_for_span(diagnostics, span)
+	if source_file == nil {
+		return false
+	}
+	line, column := line_and_column(source_file, span.start)
+	line_start, line_end, ok := line_bounds(source_file, line)
+	if !ok {
+		return false
+	}
+	prefix := " -->" if primary else " :::"
+	fmt.sbprintf(builder, "%s %s:%d:%d\n", prefix, source_file.path, line, column)
+	gutter := decimal_width(line)
+	write_spaces(builder, gutter+1)
+	strings.write_string(builder, "|\n")
+	fmt.sbprintf(builder, "%*d | ", gutter, line)
+	_ = write_expanded(builder, source_file.text[line_start:line_end])
+	strings.write_byte(builder, '\n')
+	write_spaces(builder, gutter+1)
+	strings.write_string(builder, "| ")
+	start := clamp(int(span.start), line_start, line_end)
+	indent := display_width(source_file.text[line_start:start])
+	width := 1
+	if start < line_end {
+		end := clamp(int(span.end), start+1, line_end)
+		width = max(display_width(source_file.text[start:end], indent), 1)
+	}
+	write_spaces(builder, indent)
+	marker := u8('^') if primary else u8('-')
+	for _ in 0..<width {
+		strings.write_byte(builder, marker)
+	}
+	if len(label) > 0 {
+		strings.write_byte(builder, ' ')
+		strings.write_string(builder, label)
+	}
+	strings.write_byte(builder, '\n')
+	return true
+}
+
 format :: proc(diagnostics: ^Diagnostics, id: Diagnostic_Id, allocator := context.allocator) -> string {
 	index, ok := diagnostic_index(id, len(diagnostics.items))
 	if !ok {
@@ -246,19 +446,42 @@ format :: proc(diagnostics: ^Diagnostics, id: Diagnostic_Id, allocator := contex
 	diagnostic := diagnostics.items[index]
 	source_file := source_for_span(diagnostics, diagnostic.span)
 	severity := "warning" if diagnostic.severity == .Warning else "error"
+	if diagnostic.span == (Span{}) {
+		path := source_file.path if source_file != nil else "<unknown>"
+		return fmt.aprintf("%s: %s: %s", path, severity, diagnostic.message, allocator=allocator)
+	}
 	if source_file == nil {
 		return fmt.aprintf("<unknown>: %s: %s", severity, diagnostic.message, allocator=allocator)
 	}
-	line, column := line_and_column(source_file, diagnostic.span.start)
-	return fmt.aprintf(
-		"%s:%d:%d: %s: %s",
-		source_file.path,
-		line,
-		column,
-		severity,
-		diagnostic.message,
-		allocator=allocator,
-	)
+	builder := strings.builder_make(allocator)
+	fmt.sbprintf(&builder, "%s: %s\n", severity, diagnostic.message)
+	primary_label := ""
+	if annotation, found := annotation_for(diagnostics, id, .Primary); found {
+		primary_label = annotation.message
+	}
+	_ = write_excerpt(&builder, diagnostics, diagnostic.span, primary_label, true)
+	for annotation in diagnostics.annotations {
+		if annotation.owner == id && annotation.kind == .Secondary {
+			_ = write_excerpt(&builder, diagnostics, annotation.span, annotation.message, false)
+		}
+	}
+	for annotation in diagnostics.annotations {
+		if annotation.owner != id {
+			continue
+		}
+		#partial switch annotation.kind {
+		case .Note:
+			fmt.sbprintf(&builder, "note: %s\n", annotation.message)
+		case .Help:
+			fmt.sbprintf(&builder, "help: %s\n", annotation.message)
+		case:
+		}
+	}
+	result := strings.to_string(builder)
+	if len(result) > 0 && result[len(result)-1] == '\n' {
+		return result[:len(result)-1]
+	}
+	return result
 }
 
 print_all :: proc(diagnostics: ^Diagnostics) {