Enum EvalError 

#[non_exhaustive]
pub enum EvalError {
    UndefinedVar(String),
    TypeError(String),
    AttrNotFound(String),
    TypeMismatch {
        expected: &'static str,
        got: &'static str,
    },
    AssertionFailed(String),
    DivisionByZero,
    InfiniteRecursion(String),
    IoError {
        context: String,
        message: String,
    },
    Throw(String),
    Abort(String),
    NotImplemented(String),
    ParseError(String),
    RecursionLimit(String),
}

Evaluation errors produced by the Nix evaluator.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

UndefinedVar(String)

A variable was referenced but not bound in scope.

TypeError(String)

A type mismatch or coercion failure.

AttrNotFound(String)

An attribute was selected from a set that does not contain it.

TypeMismatch

A type mismatch with structured expected/got information.

Fields

expected: &'static str

got: &'static str

AssertionFailed(String)

An assert expression’s condition evaluated to false.

DivisionByZero

Integer division by zero.

InfiniteRecursion(String)

Infinite recursion detected (thunk blackhole or eval depth).

IoError

An I/O error from the host filesystem.

Fields

context: String

message: String

Throw(String)

Explicit throw from Nix code — CATCHABLE by builtins.tryEval.

Abort(String)

An abort from Nix code — UNCATCHABLE (CppNix’s abort/builtins.abort is a hard error tryEval does NOT catch, unlike throw/assert). Verified: nix eval '(builtins.tryEval (abort "x")).success' errors.

NotImplemented(String)

A language feature that is not yet implemented.

ParseError(String)

A syntax error in the input expression.

RecursionLimit(String)

Maximum recursion depth exceeded.

Implementations

impl EvalError

pub fn type_error(msg: impl Into<String>) -> Self

Convenience constructor for a TypeError variant.

pub fn type_mismatch(expected: &'static str, got: &'static str) -> Self

Convenience constructor for a TypeMismatch variant.

pub fn builtin_type(builtin: &str, expected: &str, got: &str) -> Self

Create a type error for a builtin argument type mismatch.

pub fn op_type(op: &str, lhs: &str, rhs: &str) -> Self

Create a type error for a binary operator type mismatch.

CARRIES THE EVAL FILE (added 2026-07-20). Every arithmetic/comparison raise site routes through here, and none of them appended eval_file_ctx() — unlike the ~12 sibling raise sites in eval.rs that do — so an operator type error named no file at all. Nor could the frame stack help: NixTraceGuard::drop pops every frame during unwind, so by the time the error surfaces attach_trace has nothing left to attach.

The cost of that was concrete. “cannot add string and null” was the sole symptom of the ident-cache aliasing bug that stopped sui evaluating nixpkgs, and it pointed nowhere: four parallel investigations each spent most of their budget just locating it, and the only tool that worked was SUI_TRACE_EVAL=1 dumping 521k lines to be read backwards. One format! argument here would have named make-derivation.nix immediately.

Fixing it in op_type rather than at the Add arm means every operator — add, sub, mul, div, comparison, update — gains the context at once, instead of the next one to bite us needing its own patch.

pub fn is_throw(&self) -> bool

Whether this error was caused by throw or abort.

pub fn is_infinite_recursion(&self) -> bool

Whether this error is an infinite recursion.

Trait Implementations

impl Clone for EvalError

fn clone(&self) -> EvalError

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for EvalError

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for EvalError

fn fmt(&self, __formatter: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for EvalError

impl Error for EvalError

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

impl PartialEq for EvalError

fn eq(&self, other: &EvalError) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for EvalError