//! This module implements the backing representation of runtime
//! values in the Nix language.
use std::cmp::Ordering;
use std::fmt::Display;
use std::num::{NonZeroI32, NonZeroUsize};
use std::path::PathBuf;
use std::rc::Rc;
use lexical_core::format::CXX_LITERAL;
use serde::Deserialize;
#[cfg(feature = "arbitrary")]
mod arbitrary;
mod attrs;
mod builtin;
mod function;
mod json;
mod list;
mod path;
mod string;
mod thunk;
use crate::errors::ErrorKind;
use crate::opcode::StackIdx;
use crate::vm::generators::{self, GenCo};
use crate::AddContext;
pub use attrs::NixAttrs;
pub use builtin::{Builtin, BuiltinResult};
pub(crate) use function::Formals;
pub use function::{Closure, Lambda};
pub use list::NixList;
pub use path::canon_path;
pub use string::NixString;
pub use thunk::Thunk;
pub use self::thunk::{SharedThunkSet, ThunkSet};
use lazy_static::lazy_static;
#[warn(variant_size_differences)]
#[derive(Clone, Debug, Deserialize)]
#[serde(untagged)]
pub enum Value {
Null,
Bool(bool),
Integer(i64),
Float(f64),
String(NixString),
#[serde(skip)]
Path(Box<PathBuf>),
Attrs(Box<NixAttrs>),
List(NixList),
#[serde(skip)]
Closure(Rc<Closure>), // must use Rc<Closure> here in order to get proper pointer equality
#[serde(skip)]
Builtin(Builtin),
// Internal values that, while they technically exist at runtime,
// are never returned to or created directly by users.
#[serde(skip_deserializing)]
Thunk(Thunk),
// See [`compiler::compile_select_or()`] for explanation
#[serde(skip)]
AttrNotFound,
// this can only occur in Chunk::Constants and nowhere else
#[serde(skip)]
Blueprint(Rc<Lambda>),
#[serde(skip)]
DeferredUpvalue(StackIdx),
#[serde(skip)]
UnresolvedPath(Box<PathBuf>),
#[serde(skip)]
Json(serde_json::Value),
}
lazy_static! {
static ref WRITE_FLOAT_OPTIONS: lexical_core::WriteFloatOptions =
lexical_core::WriteFloatOptionsBuilder::new()
.trim_floats(true)
.round_mode(lexical_core::write_float_options::RoundMode::Round)
.positive_exponent_break(Some(NonZeroI32::new(5).unwrap()))
.max_significant_digits(Some(NonZeroUsize::new(6).unwrap()))
.build()
.unwrap();
}
// Helper macros to generate the to_*/as_* macros while accounting for
// thunks.
/// Generate an `as_*` method returning a reference to the expected
/// type, or a type error. This only works for types that implement
/// `Copy`, as returning a reference to an inner thunk value is not
/// possible.
/// Generate an `as_*/to_*` accessor method that returns either the
/// expected type, or a type error.
macro_rules! gen_cast {
( $name:ident, $type:ty, $expected:expr, $variant:pat, $result:expr ) => {
pub fn $name(&self) -> Result<$type, ErrorKind> {
match self {
$variant => Ok($result),
Value::Thunk(thunk) => Self::$name(&thunk.value()),
other => Err(type_error($expected, &other)),
}
}
};
}
/// Generate an `as_*_mut/to_*_mut` accessor method that returns either the
/// expected type, or a type error.
macro_rules! gen_cast_mut {
( $name:ident, $type:ty, $expected:expr, $variant:ident) => {
pub fn $name(&mut self) -> Result<&mut $type, ErrorKind> {
match self {
Value::$variant(x) => Ok(x),
other => Err(type_error($expected, &other)),
}
}
};
}
/// Generate an `is_*` type-checking method.
macro_rules! gen_is {
( $name:ident, $variant:pat ) => {
pub fn $name(&self) -> bool {
match self {
$variant => true,
Value::Thunk(thunk) => Self::$name(&thunk.value()),
_ => false,
}
}
};
}
/// Describes what input types are allowed when coercing a `Value` to a string
#[derive(Clone, Copy, PartialEq, Debug)]
pub enum CoercionKind {
/// Only coerce already "stringly" types like strings and paths, but also
/// coerce sets that have a `__toString` attribute. Equivalent to
/// `!coerceMore` in C++ Nix.
Weak,
/// Coerce all value types included by `Weak`, but also coerce `null`,
/// booleans, integers, floats and lists of coercible types. Equivalent to
/// `coerceMore` in C++ Nix.
Strong,
}
impl<T> From<T> for Value
where
T: Into<NixString>,
{
fn from(t: T) -> Self {
Self::String(t.into())
}
}
/// Constructors
impl Value {
/// Construct a [`Value::Attrs`] from a [`NixAttrs`].
pub fn attrs(attrs: NixAttrs) -> Self {
Self::Attrs(Box::new(attrs))
}
}
/// Controls what kind of by-pointer equality comparison is allowed.
///
/// See `//tvix/docs/value-pointer-equality.md` for details.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum PointerEquality {
/// Pointer equality not allowed at all.
ForbidAll,
/// Pointer equality comparisons only allowed for nested values.
AllowNested,
/// Pointer equality comparisons are allowed in all contexts.
AllowAll,
}
impl Value {
/// Deeply forces a value, traversing e.g. lists and attribute sets and forcing
/// their contents, too.
///
/// This is a generator function.
pub(super) async fn deep_force(
self,
co: GenCo,
thunk_set: SharedThunkSet,
) -> Result<Value, ErrorKind> {
// Get rid of any top-level thunks, and bail out of self-recursive
// thunks.
let value = if let Value::Thunk(ref t) = &self {
if !thunk_set.insert(t) {
return Ok(self);
}
generators::request_force(&co, self).await
} else {
self
};
match &value {
// Short-circuit on already evaluated values, or fail on internal values.
Value::Null
| Value::Bool(_)
| Value::Integer(_)
| Value::Float(_)
| Value::String(_)
| Value::Path(_)
| Value::Closure(_)
| Value::Builtin(_) => return Ok(value),
Value::List(list) => {
for val in list {
generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await;
}
}
Value::Attrs(attrs) => {
for (_, val) in attrs.iter() {
generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await;
}
}
Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),
Value::AttrNotFound
| Value::Blueprint(_)
| Value::DeferredUpvalue(_)
| Value::UnresolvedPath(_)
| Value::Json(_) => panic!(
"Tvix bug: internal value left on stack: {}",
value.type_of()
),
};
Ok(value)
}
/// Coerce a `Value` to a string. See `CoercionKind` for a rundown of what
/// input types are accepted under what circumstances.
pub async fn coerce_to_string(self, co: GenCo, kind: CoercionKind) -> Result<Value, ErrorKind> {
let value = generators::request_force(&co, self).await;
match (value, kind) {
// coercions that are always done
tuple @ (Value::String(_), _) => Ok(tuple.0),
// TODO(sterni): Think about proper encoding handling here. This needs
// general consideration anyways, since one current discrepancy between
// C++ Nix and Tvix is that the former's strings are arbitrary byte
// sequences without NUL bytes, whereas Tvix only allows valid
// Unicode. See also b/189.
(Value::Path(p), _) => {
// TODO(tazjin): there are cases where coerce_to_string does not import
let imported = generators::request_path_import(&co, *p).await;
Ok(imported.to_string_lossy().into_owned().into())
}
// Attribute sets can be converted to strings if they either have an
// `__toString` attribute which holds a function that receives the
// set itself or an `outPath` attribute which should be a string.
// `__toString` is preferred.
(Value::Attrs(attrs), kind) => {
if let Some(s) = attrs.try_to_string(&co, kind).await {
return Ok(Value::String(s));
}
if let Some(out_path) = attrs.select("outPath") {
let s = generators::request_string_coerce(&co, out_path.clone(), kind).await;
return Ok(Value::String(s));
}
Err(ErrorKind::NotCoercibleToString { from: "set", kind })
}
// strong coercions
(Value::Null, CoercionKind::Strong) | (Value::Bool(false), CoercionKind::Strong) => {
Ok("".into())
}
(Value::Bool(true), CoercionKind::Strong) => Ok("1".into()),
(Value::Integer(i), CoercionKind::Strong) => Ok(format!("{i}").into()),
(Value::Float(f), CoercionKind::Strong) => {
// contrary to normal Display, coercing a float to a string will
// result in unconditional 6 decimal places
Ok(format!("{:.6}", f).into())
}
// Lists are coerced by coercing their elements and interspersing spaces
(Value::List(list), CoercionKind::Strong) => {
let mut out = String::new();
for (idx, elem) in list.into_iter().enumerate() {
if idx > 0 {
out.push(' ');
}
let s = generators::request_string_coerce(&co, elem, kind).await;
out.push_str(s.as_str());
}
Ok(Value::String(out.into()))
}
(Value::Thunk(_), _) => panic!("Tvix bug: force returned unforced thunk"),
val @ (Value::Closure(_), _)
| val @ (Value::Builtin(_), _)
| val @ (Value::Null, _)
| val @ (Value::Bool(_), _)
| val @ (Value::Integer(_), _)
| val @ (Value::Float(_), _)
| val @ (Value::List(_), _) => Err(ErrorKind::NotCoercibleToString {
from: val.0.type_of(),
kind,
}),
(Value::AttrNotFound, _)
| (Value::Blueprint(_), _)
| (Value::DeferredUpvalue(_), _)
| (Value::UnresolvedPath(_), _)
| (Value::Json(_), _) => {
panic!("tvix bug: .coerce_to_string() called on internal value")
}
}
}
/// Compare two Nix values for equality, forcing nested parts of the structure
/// as needed.
///
/// This comparison needs to be invoked for nested values (e.g. in lists and
/// attribute sets) as well, which is done by suspending and asking the VM to
/// perform the nested comparison.
///
/// The `top_level` parameter controls whether this invocation is the top-level
/// comparison, or a nested value comparison. See
/// `//tvix/docs/value-pointer-equality.md`
pub(crate) async fn nix_eq(
self,
other: Value,
co: GenCo,
ptr_eq: PointerEquality,
) -> Result<Value, ErrorKind> {
let a = match self {
Value::Thunk(ref thunk) => {
// If both values are thunks, and thunk comparisons are allowed by
// pointer, do that and move on.
if ptr_eq == PointerEquality::AllowAll {
if let Value::Thunk(t1) = &other {
if t1.ptr_eq(thunk) {
return Ok(Value::Bool(true));
}
}
};
generators::request_force(&co, self).await
}
_ => self,
};
let b = match other {
Value::Thunk(_) => generators::request_force(&co, other).await,
_ => other,
};
debug_assert!(!matches!(a, Value::Thunk(_)));
debug_assert!(!matches!(b, Value::Thunk(_)));
let result = match (a, b) {
// Trivial comparisons
(Value::Null, Value::Null) => true,
(Value::Bool(b1), Value::Bool(b2)) => b1 == b2,
(Value::String(s1), Value::String(s2)) => s1 == s2,
(Value::Path(p1), Value::Path(p2)) => p1 == p2,
// Numerical comparisons (they work between float & int)
(Value::Integer(i1), Value::Integer(i2)) => i1 == i2,
(Value::Integer(i), Value::Float(f)) => i as f64 == f,
(Value::Float(f1), Value::Float(f2)) => f1 == f2,
(Value::Float(f), Value::Integer(i)) => i as f64 == f,
// List comparisons
(Value::List(l1), Value::List(l2)) => {
if ptr_eq >= PointerEquality::AllowNested && l1.ptr_eq(&l2) {
return Ok(Value::Bool(true));
}
if l1.len() != l2.len() {
return Ok(Value::Bool(false));
}
for (vi1, vi2) in l1.into_iter().zip(l2.into_iter()) {
if !generators::check_equality(
&co,
vi1,
vi2,
std::cmp::max(ptr_eq, PointerEquality::AllowNested),
)
.await?
{
return Ok(Value::Bool(false));
}
}
true
}
(_, Value::List(_)) | (Value::List(_), _) => false,
// Attribute set comparisons
(Value::Attrs(a1), Value::Attrs(a2)) => {
if ptr_eq >= PointerEquality::AllowNested && a1.ptr_eq(&a2) {
return Ok(Value::Bool(true));
}
// Special-case for derivation comparisons: If both attribute sets
// have `type = derivation`, compare them by `outPath`.
match (a1.select("type"), a2.select("type")) {
(Some(v1), Some(v2)) => {
let s1 = generators::request_force(&co, v1.clone()).await.to_str();
let s2 = generators::request_force(&co, v2.clone()).await.to_str();
if let (Ok(s1), Ok(s2)) = (s1, s2) {
if s1.as_str() == "derivation" && s2.as_str() == "derivation" {
// TODO(tazjin): are the outPaths really required,
// or should it fall through?
let out1 = a1
.select_required("outPath")
.context("comparing derivations")?
.clone();
let out2 = a2
.select_required("outPath")
.context("comparing derivations")?
.clone();
let result = generators::request_force(&co, out1.clone())
.await
.to_str()?
== generators::request_force(&co, out2.clone())
.await
.to_str()?;
return Ok(Value::Bool(result));
}
}
}
_ => {}
};
if a1.len() != a2.len() {
return Ok(Value::Bool(false));
}
let iter1 = a1.into_iter_sorted();
let iter2 = a2.into_iter_sorted();
for ((k1, v1), (k2, v2)) in iter1.zip(iter2) {
if k1 != k2 {
return Ok(Value::Bool(false));
}
if !generators::check_equality(
&co,
v1,
v2,
std::cmp::max(ptr_eq, PointerEquality::AllowNested),
)
.await?
{
return Ok(Value::Bool(false));
}
}
true
}
(Value::Attrs(_), _) | (_, Value::Attrs(_)) => false,
(Value::Closure(c1), Value::Closure(c2)) if ptr_eq >= PointerEquality::AllowNested => {
Rc::ptr_eq(&c1, &c2)
}
// Everything else is either incomparable (e.g. internal types) or
// false.
_ => false,
};
Ok(Value::Bool(result))
}
pub fn type_of(&self) -> &'static str {
match self {
Value::Null => "null",
Value::Bool(_) => "bool",
Value::Integer(_) => "int",
Value::Float(_) => "float",
Value::String(_) => "string",
Value::Path(_) => "path",
Value::Attrs(_) => "set",
Value::List(_) => "list",
Value::Closure(_) | Value::Builtin(_) => "lambda",
// Internal types. Note: These are only elaborated here
// because it makes debugging easier. If a user ever sees
// any of these strings, it's a bug.
Value::Thunk(_) => "internal[thunk]",
Value::AttrNotFound => "internal[attr_not_found]",
Value::Blueprint(_) => "internal[blueprint]",
Value::DeferredUpvalue(_) => "internal[deferred_upvalue]",
Value::UnresolvedPath(_) => "internal[unresolved_path]",
Value::Json(_) => "internal[json]",
}
}
gen_cast!(as_bool, bool, "bool", Value::Bool(b), *b);
gen_cast!(as_int, i64, "int", Value::Integer(x), *x);
gen_cast!(as_float, f64, "float", Value::Float(x), *x);
gen_cast!(to_str, NixString, "string", Value::String(s), s.clone());
gen_cast!(to_attrs, Box<NixAttrs>, "set", Value::Attrs(a), a.clone());
gen_cast!(to_list, NixList, "list", Value::List(l), l.clone());
gen_cast!(
as_closure,
Rc<Closure>,
"lambda",
Value::Closure(c),
c.clone()
);
gen_cast_mut!(as_list_mut, NixList, "list", List);
gen_is!(is_path, Value::Path(_));
gen_is!(is_number, Value::Integer(_) | Value::Float(_));
gen_is!(is_bool, Value::Bool(_));
/// Compare `self` against other using (fallible) Nix ordering semantics.
///
/// Note that as this returns an `Option<Ordering>` it can not directly be
/// used as a generator function in the VM. The exact use depends on the
/// callsite, as the meaning is interpreted in different ways e.g. based on
/// the comparison operator used.
///
/// The function is intended to be used from within other generator
/// functions or `gen!` blocks.
pub async fn nix_cmp_ordering(
self,
other: Self,
co: GenCo,
) -> Result<Option<Ordering>, ErrorKind> {
Self::nix_cmp_ordering_(self, other, co).await
}
async fn nix_cmp_ordering_(
mut myself: Self,
mut other: Self,
co: GenCo,
) -> Result<Option<Ordering>, ErrorKind> {
'outer: loop {
match (myself, other) {
// same types
(Value::Integer(i1), Value::Integer(i2)) => return Ok(i1.partial_cmp(&i2)),
(Value::Float(f1), Value::Float(f2)) => return Ok(f1.partial_cmp(&f2)),
(Value::String(s1), Value::String(s2)) => return Ok(s1.partial_cmp(&s2)),
(Value::List(l1), Value::List(l2)) => {
for i in 0.. {
if i == l2.len() {
return Ok(Some(Ordering::Greater));
} else if i == l1.len() {
return Ok(Some(Ordering::Less));
} else if !generators::check_equality(
&co,
l1[i].clone(),
l2[i].clone(),
PointerEquality::AllowAll,
)
.await?
{
// TODO: do we need to control `top_level` here?
myself = generators::request_force(&co, l1[i].clone()).await;
other = generators::request_force(&co, l2[i].clone()).await;
continue 'outer;
}
}
unreachable!()
}
// different types
(Value::Integer(i1), Value::Float(f2)) => return Ok((i1 as f64).partial_cmp(&f2)),
(Value::Float(f1), Value::Integer(i2)) => return Ok(f1.partial_cmp(&(i2 as f64))),
// unsupported types
(lhs, rhs) => {
return Err(ErrorKind::Incomparable {
lhs: lhs.type_of(),
rhs: rhs.type_of(),
})
}
}
}
}
pub async fn force(self, co: GenCo) -> Result<Value, ErrorKind> {
if let Value::Thunk(thunk) = self {
return thunk.force(co).await;
}
Ok(self)
}
/// Explain a value in a human-readable way, e.g. by presenting
/// the docstrings of functions if present.
pub fn explain(&self) -> String {
match self {
Value::Null => "the 'null' value".into(),
Value::Bool(b) => format!("the boolean value '{}'", b),
Value::Integer(i) => format!("the integer '{}'", i),
Value::Float(f) => format!("the float '{}'", f),
Value::String(s) => format!("the string '{}'", s),
Value::Path(p) => format!("the path '{}'", p.to_string_lossy()),
Value::Attrs(attrs) => format!("a {}-item attribute set", attrs.len()),
Value::List(list) => format!("a {}-item list", list.len()),
Value::Closure(f) => {
if let Some(name) = &f.lambda.name {
format!("the user-defined Nix function '{}'", name)
} else {
"a user-defined Nix function".to_string()
}
}
Value::Builtin(b) => {
let mut out = format!("the builtin function '{}'", b.name());
if let Some(docs) = b.documentation() {
out.push_str("\n\n");
out.push_str(docs);
}
out
}
// TODO: handle suspended thunks with a different explanation instead of panicking
Value::Thunk(t) => t.value().explain(),
Value::AttrNotFound
| Value::Blueprint(_)
| Value::DeferredUpvalue(_)
| Value::UnresolvedPath(_)
| Value::Json(_) => "an internal Tvix evaluator value".into(),
}
}
}
trait TotalDisplay {
fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result;
}
impl Display for Value {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
self.total_fmt(f, &mut Default::default())
}
}
/// Emulates the C++-Nix style formatting of floats, which diverges
/// significantly from Rust's native float formatting.
fn total_fmt_float<F: std::fmt::Write>(num: f64, mut f: F) -> std::fmt::Result {
let mut buf = [b'0'; lexical_core::BUFFER_SIZE];
let mut s = lexical_core::write_with_options::<f64, { CXX_LITERAL }>(
num,
&mut buf,
&WRITE_FLOAT_OPTIONS,
);
// apply some postprocessing on the buffer. If scientific
// notation is used (we see an `e`), and the next character is
// a digit, add the missing `+` sign.)
let mut new_s = Vec::with_capacity(s.len());
if s.contains(&b'e') {
for (i, c) in s.iter().enumerate() {
// encountered `e`
if c == &b'e' {
// next character is a digit (so no negative exponent)
if s.len() > i && s[i + 1].is_ascii_digit() {
// copy everything from the start up to (including) the e
new_s.extend_from_slice(&s[0..=i]);
// add the missing '+'
new_s.push(b'+');
// check for the remaining characters.
// If it's only one, we need to prepend a trailing zero
if s.len() == i + 2 {
new_s.push(b'0');
}
new_s.extend_from_slice(&s[i + 1..]);
break;
}
}
}
// if we modified the scientific notation, flip the reference
if !new_s.is_empty() {
s = &mut new_s
}
}
// else, if this is not scientific notation, and there's a
// decimal point, make sure we really drop trailing zeroes.
// In some cases, lexical_core doesn't.
else if s.contains(&b'.') {
for (i, c) in s.iter().enumerate() {
// at `.``
if c == &b'.' {
// trim zeroes from the right side.
let frac = String::from_utf8_lossy(&s[i + 1..]);
let frac_no_trailing_zeroes = frac.trim_end_matches('0');
if frac.len() != frac_no_trailing_zeroes.len() {
// we managed to strip something, construct new_s
if frac_no_trailing_zeroes.is_empty() {
// if frac_no_trailing_zeroes is empty, the fractional part was all zeroes, so we can drop the decimal point as well
new_s.extend_from_slice(&s[0..=i - 1]);
} else {
// else, assemble the rest of the string
new_s.extend_from_slice(&s[0..=i]);
new_s.extend_from_slice(frac_no_trailing_zeroes.as_bytes());
}
// flip the reference
s = &mut new_s;
break;
}
}
}
}
write!(f, "{}", format!("{}", String::from_utf8_lossy(s)))
}
impl TotalDisplay for Value {
fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
match self {
Value::Null => f.write_str("null"),
Value::Bool(true) => f.write_str("true"),
Value::Bool(false) => f.write_str("false"),
Value::Integer(num) => write!(f, "{}", num),
Value::String(s) => s.fmt(f),
Value::Path(p) => p.display().fmt(f),
Value::Attrs(attrs) => attrs.total_fmt(f, set),
Value::List(list) => list.total_fmt(f, set),
Value::Closure(_) => f.write_str("lambda"), // TODO: print position
Value::Builtin(builtin) => builtin.fmt(f),
// Nix prints floats with a maximum precision of 5 digits
// only. Except when it decides to use scientific notation
// (with a + after the `e`, and zero-padded to 0 digits)
Value::Float(num) => total_fmt_float(*num, f),
// internal types
Value::AttrNotFound => f.write_str("internal[not found]"),
Value::Blueprint(_) => f.write_str("internal[blueprint]"),
Value::DeferredUpvalue(_) => f.write_str("internal[deferred_upvalue]"),
Value::UnresolvedPath(_) => f.write_str("internal[unresolved_path]"),
Value::Json(_) => f.write_str("internal[json]"),
// Delegate thunk display to the type, as it must handle
// the case of already evaluated or cyclic thunks.
Value::Thunk(t) => t.total_fmt(f, set),
}
}
}
impl From<bool> for Value {
fn from(b: bool) -> Self {
Value::Bool(b)
}
}
impl From<i64> for Value {
fn from(i: i64) -> Self {
Self::Integer(i)
}
}
impl From<f64> for Value {
fn from(i: f64) -> Self {
Self::Float(i)
}
}
impl From<PathBuf> for Value {
fn from(path: PathBuf) -> Self {
Self::Path(Box::new(path))
}
}
fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
ErrorKind::TypeError {
expected,
actual: actual.type_of(),
}
}
#[cfg(test)]
mod tests {
mod floats {
use crate::value::total_fmt_float;
#[test]
fn format_float() {
let ff = vec![
(0f64, "0"),
(1.0f64, "1"),
(-0.01, "-0.01"),
(5e+22, "5e+22"),
(1e6, "1e+06"),
(-2E-2, "-0.02"),
(6.626e-34, "6.626e-34"),
(9_224_617.445_991_227, "9.22462e+06"),
];
for (n, expected) in ff.iter() {
let mut buf = String::new();
let res = total_fmt_float(*n, &mut buf);
assert!(res.is_ok());
assert_eq!(
expected, &buf,
"{} should be formatted as {}, but got {}",
n, expected, &buf
);
}
}
}
}
