BEP-006: compiler-errors

Summary

Uniform user-facing experience for compiler errors.

Enumerate all errors
Give them unique 4-digit codes
Specify how each is rendered as a diagnostic message
Specify how each is turned into guidance

Motivation

We want to give users and coding agents a uniform experience when they encounter compiler errors.

Proposed Design

A top-level compiler_error module contains every error category.
```
pub enum CompilerError<Ty> {
  ParseError(ParseError)
  TypeError(TypeError<Ty>)
}
```
(TypeError is parameterized because it's defined here, it needs to contain types, types are defined in another crate that depends on errors, and using types directly would lead to a circular crate dependency).
compiler_error has submodules errors pertinent to specific phases. For example type errors are defined in compiler_error::type_error::TypeError.
```
pub enum TypeError<T> {
  TypeMismatch { expected: T, found: T, span: Span },
  UnknownType { name: String, span: Span }.
  ...
}
```
compiler_error::error_format contains error_report_and_code(), which does a giant match over every error variant and specifies how that variant becomes an ariadne report builder and an error code. render_error() takes these two pieces of data and combines them into an ariadne Report.

This design is implemented and merged already, see PR 27651.

All the errors

The following is a list of every possible error. It is derived from both the new compiler (which follows the new scheme), and the legacy compiler (which has a more exhaustive set of errors due to its longer time in development).

Phase: compiler phase (Lexing, Parsing, NameResolution, Typing, Codegen) Error: very brief description in the form of an Enum Variant

Code	Phase	Error	Spans	Notes
E0001	Typing	TypeMismatch	(1) error location, (2) type constraint origin	Expected X, found Y
E0002	Typing	UnknownType	type reference span	Type name not found in scope
E0003	Typing	UnknownVariable	variable reference span	Variable name not found in scope
E0004	Typing	InvalidOperator	operator span	Binary or unary op invalid for types
E0005	Typing	ArgumentCountMismatch	(1) call site, (2) function definition	Wrong number of args to function
E0006	Typing	NotCallable	call site span	Calling a non-function type
E0007	Typing	NoSuchField	(1) field access span, (2) type definition	Field doesn't exist on type
E0008	Typing	NotIndexable	index access span	Type doesn't support indexing
E0009	Parsing	UnexpectedEof	EOF location span	Unexpected end of file
E0010	Parsing	UnexpectedToken	token span	Expected X, found Y
E0011	NameResolution	DuplicateName	(1) second definition, (2) first definition	Same name defined twice
E0012	Parsing	LiteralParserError	literal span	Invalid literal value
E0013	Validation	ArgumentNotFound	argument usage span	Required argument missing
E0014	Validation	AttributeArgumentNotFound	attribute span	Attribute missing required arg
E0015	Validation	GeneratorArgumentNotFound	generator block span	Generator missing required arg
E0016	Validation	AttributeValidationError	attribute span	Attribute parsing failed
E0017	Validation	DuplicateAttribute	attribute span	Attribute defined multiple times
E0018	Typing	IncompatibleNativeType	native type annotation span	Native type incompatible
E0019	Typing	InvalidNativeTypeArgument	native type argument span	Invalid arg for native type
E0020	Typing	InvalidNativeTypePrefix	native type prefix span	Wrong prefix for native type
E0021	Validation	NativeTypesNotSupported	native type span	Connector doesn't support native types
E0022	Typing	ReservedScalarType	type name span	Using reserved type name
E0023	NameResolution	DuplicateEnumDatabaseName	enum span	Duplicate DB name for enum
E0024	NameResolution	DuplicateModelDatabaseName	(1) new model span, (2) existing model span	Duplicate DB name for model
E0025	NameResolution	DuplicateViewDatabaseName	(1) new view span, (2) existing view span	Duplicate DB name for view
E0026	NameResolution	DuplicateTest	(1) test span, (2+) other test spans	Test name already defined
E0027	NameResolution	DuplicateTopLevel	(1) new definition, (2+) existing definitions	Top-level name collision
E0028	NameResolution	DuplicateConfigKey	config key span	Key already defined in config
E0029	Validation	DuplicateArgument	argument span	Argument specified twice
E0030	Validation	UnusedArgument	argument span	No such argument exists
E0031	Validation	DuplicateDefaultArgument	argument span	Default arg already specified
E0032	NameResolution	DuplicateFunction	function span	Function already defined
E0033	Parsing	InvalidFunctionSyntax	function span	Malformed function definition
E0034	NameResolution	DuplicateEnumValue	(1) new value span, (2) existing value span	Enum value already defined
E0035	NameResolution	DuplicateCompositeTypeField	(1) new field span, (2) existing field span	Field already on composite type
E0036	NameResolution	DuplicateField	(1) new field span, (2) existing field span	Field already on model/class
E0037	Validation	ScalarListFieldsNotSupported	field span	Connector doesn't support scalar lists
E0038	Validation	ModelValidationError	model span	Generic model validation failure
E0039	Validation	NameError	name span	Invalid identifier name
E0040	Validation	EnumValidationError	enum span	Generic enum validation failure
E0041	Validation	CompositeTypeFieldValidation	field span	Composite type field validation
E0042	Validation	FieldValidationError	field span	Generic field validation failure
E0043	Validation	SourceValidationError	datasource span	Datasource validation failure
E0044	Validation	DynamicTypeNotAllowed	@dynamic attribute span	@dynamic not allowed in type_builder
E0045	Validation	ValidationError	relevant span	Generic validation error
E0046	Parsing	LegacyParserError	token span	Legacy parser catch-all
E0047	Typing	OptionalArgumentCountMismatch	native type span	Wrong optional arg count
E0048	Parsing	ParserError	token span	Expected one of: X, Y, Z
E0049	Typing	FunctionalEvaluationError	expression span	Error evaluating expression
E0050	NameResolution	NotFoundError	reference span	Generic not-found with suggestions
E0051	Typing	TypeNotUsedInPrompt	type reference span	Type not in function output
E0052	NameResolution	ClientNotFound	client reference span	Client name not found
E0053	NameResolution	TypeNotFound	type reference span	Type name not found
E0054	Validation	AttributeNotKnown	attribute span	Unknown attribute name
E0055	Validation	PropertyNotKnown	property span	Unknown property in block
E0056	Validation	ArgumentNotKnown	argument span	Unknown argument name
E0057	Typing	ValueParserError	value span	Expected type X, found Y
E0058	Typing	TypeMismatchLegacy	value span	Legacy type mismatch error
E0059	Validation	MissingRequiredProperty	block span	Required property missing
E0060	Validation	ConfigPropertyMissingValue	property span	Property needs a value
E0061	Typing	TypeNotAllowedAsMapKey	map key span	Invalid map key type
E0062	Typing	NonExhaustiveMatch	match expression span	Match does not cover all cases
E0063	Typing	UnreachableMatchArm	match arm span	Match arm masked by previous patterns

Appendix: Runtime Errors (VM)

These errors occur during execution, not compilation. They are out of scope for this BEP.

Code	Category	Error	Notes
R0001	Runtime	StackOverflow	Call stack exceeded
R0002	Runtime	AssertionError	User assertion failed
R0003	Runtime	NoSuchKeyInMap	Map key not found
R0004	Runtime	DivisionByZero	Division by zero
R0005	Internal	InvalidArgumentCount	Wrong arg count (VM bug)
R0006	Internal	UnexpectedEmptyStack	Stack underflow (VM bug)
R0007	Internal	NotEnoughItemsOnStack	Stack depth error (VM bug)
R0008	Internal	InvalidObjectRef	Bad object reference (VM bug)
R0009	Internal	TypeError	VM type error (VM bug)
R0010	Internal	CannotApplyBinOp	Invalid binary op (VM bug)
R0011	Internal	CannotApplyCmpOp	Invalid comparison (VM bug)
R0012	Internal	CannotApplyUnaryOp	Invalid unary op (VM bug)
R0013	Runtime	ArrayIndexOutOfBounds	Array index too large
R0014	Runtime	ArrayIndexIsNegative	Negative array index
R0015	Internal	NegativeInstructionPtr	Bad instruction ptr (VM bug)