Diagnostic codes¶

Mighty diagnostics carry a stable SDxxxx code. Codes are assigned once and never renumbered. This page is the authoritative registry.

The codes live in crates/mty-diagnostics/src/codes.rs.

Discovering explanations¶

For any assigned code, mty explain <CODE> prints a short paragraph describing the diagnostic and suggested fixes. Example:

$ mty explain MT0001
MT0001: Unexpected token. ...

Ranges¶

Range	Category
`MT0001`–`MT0999`	Lex and parse
`MT1001`–`MT1999`	HIR lowering and name resolution
`MT2001`–`MT2999`	Type checking (slice 3)
`MT3001`–`MT3999`	Borrow / move / affine (slice 4)
`MT4001`–`MT4999`	Effect / capability (slice 5)

Slice 1 codes¶

Code	Name	Meaning
`MT0001`	`UNEXPECTED_TOKEN`	The parser encountered a token it could not consume in any production.
`MT0002`	`UNTERMINATED_STRING`	A string literal was not terminated before EOF.
`MT0003`	`INVALID_ESCAPE`	A string or char literal contained an unrecognized escape sequence.
`MT0004`	`UNKNOWN_DURATION_UNIT`	A numeric literal used a duration suffix the lexer does not recognize.
`MT0010`	`EXPECTED_ITEM`	Expected a top-level item (function, struct, agent, etc.).
`MT0011`	`EXPECTED_EXPR`	Expected an expression.
`MT0012`	`MISMATCHED_DELIMITER`	A closing delimiter did not match its opener.
`MT0020`	`DUPLICATE_ON_HANDLER`	An agent declared two `on Msg` handlers for the same message.
`MT0021`	`PUB_NEEDS_RETURN_TYPE`	A `pub fn` is missing its return type.
`MT0030`	`DEPTH_LIMIT_EXCEEDED`	The parser exceeded its nesting depth limit.
`MT1001`	`UNRESOLVED_NAME`	A name could not be resolved to any binding.
`MT1002`	`USE_RESOLVES_TO_NOTHING`	A `use` import targets a path with no resolution.

Slice 3 codes (type checker, MT2001..MT2099)¶

Code	Name	Meaning
`MT2001`	`TYPE_MISMATCH`	Expression's type doesn't match expected type.
`MT2002`	`UNRESOLVED_TYPE`	Type name does not name any type in scope.
`MT2003`	`CANNOT_INFER_TYPE`	Cannot infer a binding's type.
`MT2004`	`WRONG_GENERIC_ARITY`	Wrong number of `[T, ...]` generic args.
`MT2005`	`WRONG_ARG_COUNT`	Wrong number of args to a call.
`MT2006`	`UNKNOWN_FIELD`	Struct has no such field.
`MT2007`	`UNKNOWN_METHOD`	Type has no such method.
`MT2008`	`NOT_CALLABLE`	Value is not callable.
`MT2009`	`UNKNOWN_VARIANT`	Enum has no such variant.
`MT2010`	`QUESTION_OUTSIDE_RESULT`	`?` outside a Result-returning fn.
`MT2011`	`QUESTION_ERROR_MISMATCH`	`?` error type doesn't match enclosing fn.
`MT2012`	`WRONG_VARIANT_ARITY`	Variant payload count mismatch.
`MT2013`	`MISSING_STRUCT_FIELD`	Struct literal omits a required field.
`MT2014`	`DUPLICATE_STRUCT_FIELD`	Struct literal lists a field twice.
`MT2015`	`NON_EXHAUSTIVE_MATCH`	Match doesn't cover all cases (slice 4 promoted to error).
`MT2016`	`UNREACHABLE_MATCH_ARM`	(warning) arm shadowed by earlier arm.
`MT2017`	`BINOP_TYPE_MISMATCH`	Operator not defined on operand types.
`MT2018`	`IF_BRANCH_MISMATCH`	If/else branches have incompatible types.
`MT2019`	`RETURN_TYPE_MISMATCH`	Fn body produces wrong type for return.
`MT2020`	`PUB_PARAM_NEEDS_TYPE`	`pub fn` parameter needs explicit type.
`MT2021`	`UNRESOLVED_VALUE`	Name does not refer to any value.
`MT2022`	`NOT_A_STRUCT`	Value cannot be struct-initialized.
`MT2023`	`GENERIC_ARG_MISMATCH`	Generic arg kind mismatch.
`MT2024`	`LAMBDA_ARITY_MISMATCH`	Lambda has wrong param count.
`MT2025`	`CANNOT_TAKE_REF`	Cannot take reference to non-place.
`MT2026`	`PROTOCOL_MSG_UNKNOWN`	(warning) agent handler msg not in any implemented protocol.

Slice 4 codes (borrow checker, MT3001..MT3099)¶

Code	Name	Meaning
`MT3001`	`USE_AFTER_MOVE`	Use of a value after it was moved.
`MT3002`	`MOVE_OUT_OF_BORROW`	(reserved; slice 4 uses MT3008) Move while borrowed.
`MT3003`	`BORROW_AFTER_MOVE`	Borrow created after the value was moved.
`MT3004`	`MUT_BORROW_WHILE_SHARED`	`&mut` created while shared borrows exist.
`MT3005`	`SHARED_BORROW_WHILE_MUT`	`&` created while a `&mut` is live.
`MT3006`	`TWO_MUT_BORROWS`	Second `&mut` to the same value.
`MT3007`	`BORROW_OUTLIVES_OWNER`	(reserved) Borrow lifetime exceeds the owner's.
`MT3008`	`CANNOT_MOVE_BORROWED`	Moved a value while it was borrowed.
`MT3009`	`MOVE_OUT_OF_REF`	(reserved) Tried to move out of a reference.
`MT3010`	`ARENA_ESCAPE`	Arena-local value escapes its arena scope.
`MT3011`	`NON_SENDABLE_MESSAGE_ARG`	Cross-agent message arg is not Sendable.
`MT3012`	`DROP_IN_CONST_CONTEXT`	(reserved) Drop-requiring value in const context.
`MT3013`	`MUT_BORROW_OF_IMMUT_LOCAL`	`&mut` of a local declared without `mut`.
`MT3014`	`ASSIGN_TO_IMMUT_LOCAL`	Assigned to a local declared without `mut`.
`MT3015`	`USE_OF_UNINITIALIZED`	Read of a binding never assigned.

Adding a new code¶

Pick the next free number in the appropriate range.
Add a pub const NAME: DiagCode = DiagCode::new(N); to codes.rs.
Add a row to the table above.
Reference it from the producing site via Diagnostic::error(codes::NAME, label).

Codes never get renumbered; if one is retired, retire it permanently and skip the number.