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use crate::ScalarValue;
use facet_core::Def;
/// Check if a scalar value matches a target shape.
///
/// This is a non-generic function to avoid unnecessary monomorphization.
/// It determines whether a parsed scalar value can be deserialized into a given shape.
pub(crate) fn scalar_matches_shape(
scalar: &ScalarValue<'_>,
shape: &'static facet_core::Shape,
) -> bool {
use facet_core::ScalarType;
let Some(scalar_type) = shape.scalar_type() else {
// Not a scalar type - check for Option wrapping null
if matches!(scalar, ScalarValue::Null) {
return matches!(shape.def, Def::Option(_));
}
return false;
};
match scalar {
ScalarValue::Bool(_) => matches!(scalar_type, ScalarType::Bool),
ScalarValue::Char(_) => matches!(scalar_type, ScalarType::Char),
ScalarValue::I64(val) => {
// I64 matches signed types directly
if matches!(
scalar_type,
ScalarType::I8
| ScalarType::I16
| ScalarType::I32
| ScalarType::I64
| ScalarType::I128
| ScalarType::ISize
) {
return true;
}
// I64 can also match unsigned types if the value is non-negative and in range
// This handles TOML's requirement to represent all integers as i64
if *val >= 0 {
let uval = *val as u64;
match scalar_type {
ScalarType::U8 => uval <= u8::MAX as u64,
ScalarType::U16 => uval <= u16::MAX as u64,
ScalarType::U32 => uval <= u32::MAX as u64,
ScalarType::U64 | ScalarType::U128 | ScalarType::USize => true,
_ => false,
}
} else {
false
}
}
ScalarValue::U64(val) => {
// U64 matches unsigned types directly
if matches!(
scalar_type,
ScalarType::U8
| ScalarType::U16
| ScalarType::U32
| ScalarType::U64
| ScalarType::U128
| ScalarType::USize
) {
return true;
}
// U64 can also match signed types if the value fits in the signed range
// This handles JSON's representation of positive integers as u64
if *val <= i64::MAX as u64 {
match scalar_type {
ScalarType::I8 => *val <= i8::MAX as u64,
ScalarType::I16 => *val <= i16::MAX as u64,
ScalarType::I32 => *val <= i32::MAX as u64,
ScalarType::I64 | ScalarType::I128 | ScalarType::ISize => true,
_ => false,
}
} else {
false
}
}
ScalarValue::U128(_) => matches!(scalar_type, ScalarType::U128 | ScalarType::I128),
ScalarValue::I128(_) => matches!(scalar_type, ScalarType::I128 | ScalarType::U128),
ScalarValue::F64(_) => matches!(scalar_type, ScalarType::F32 | ScalarType::F64),
ScalarValue::Str(s) => {
// String scalars match string types directly
if matches!(
scalar_type,
ScalarType::String | ScalarType::Str | ScalarType::CowStr | ScalarType::Char
) {
return true;
}
// For other scalar types, check if the shape has a parse function
// and if so, try parsing the string to see if it would succeed.
// This enables untagged enums to correctly match string values like "4.625"
// to the appropriate variant (f64 vs i64).
// See #1615 for discussion of this double-parse pattern.
const PARSE_PROBE_SIZE: usize = 128;
#[repr(align(64))]
struct ParseProbeStorage([u8; PARSE_PROBE_SIZE]);
#[allow(unsafe_code)]
if shape.vtable.has_parse()
&& shape.layout.sized_layout().is_ok_and(|layout| {
layout.size() <= PARSE_PROBE_SIZE
&& layout.align() <= core::mem::align_of::<ParseProbeStorage>()
})
{
// Attempt to parse - this is a probe, not the actual deserialization
let mut temp = core::mem::MaybeUninit::<ParseProbeStorage>::uninit();
let temp_bytes_ptr = unsafe { core::ptr::addr_of_mut!((*temp.as_mut_ptr()).0) };
let temp_ptr = facet_core::PtrUninit::new(temp_bytes_ptr.cast::<u8>());
// SAFETY: temp buffer is properly aligned and sized for this shape
if let Some(Ok(())) = unsafe { shape.call_parse(s.as_ref(), temp_ptr) } {
// Parse succeeded - drop the temp value
// SAFETY: we just successfully parsed into temp_ptr
unsafe { shape.call_drop_in_place(temp_ptr.assume_init()) };
return true;
}
}
false
}
ScalarValue::Bytes(_) => {
// Bytes don't have a ScalarType - would need to check for Vec<u8> or [u8]
false
}
ScalarValue::Null => {
// Null matches Unit type
matches!(scalar_type, ScalarType::Unit)
}
ScalarValue::Unit => {
// Unit matches Unit type
matches!(scalar_type, ScalarType::Unit)
}
}
}
/// Return how strong a scalar-to-shape match is.
///
/// - `0`: direct type match (preferred)
/// - `1`: coercive/parsed match (fallback)
///
/// `None` means the scalar cannot be deserialized into the shape.
pub(crate) fn scalar_match_quality(
scalar: &ScalarValue<'_>,
shape: &'static facet_core::Shape,
) -> Option<u8> {
if !scalar_matches_shape(scalar, shape) {
return None;
}
if is_exact_scalar_match(scalar, shape) {
Some(0)
} else {
Some(1)
}
}
fn is_exact_scalar_match(scalar: &ScalarValue<'_>, shape: &'static facet_core::Shape) -> bool {
use facet_core::ScalarType;
let scalar_type = shape.scalar_type();
match scalar {
ScalarValue::Bool(_) => matches!(scalar_type, Some(ScalarType::Bool)),
ScalarValue::Char(_) => matches!(scalar_type, Some(ScalarType::Char)),
ScalarValue::I64(_) | ScalarValue::U64(_) | ScalarValue::U128(_) | ScalarValue::I128(_) => {
matches!(
scalar_type,
Some(
ScalarType::U8
| ScalarType::U16
| ScalarType::U32
| ScalarType::U64
| ScalarType::U128
| ScalarType::USize
| ScalarType::I8
| ScalarType::I16
| ScalarType::I32
| ScalarType::I64
| ScalarType::I128
| ScalarType::ISize
)
)
}
ScalarValue::F64(_) => matches!(scalar_type, Some(ScalarType::F32 | ScalarType::F64)),
ScalarValue::Str(_) => {
// The string-shaped scalars are exactly these four. This used to sniff
// `shape.type_identifier` for `"::String"` and `"Cow<str"`, which could
// never match: `type_identifier` is the *bare* name — no module path and
// no generics — so it reads `"String"` and `"Cow"`, never `"Cow<str>"`.
// (`"Cow<str>"` is what `format!("{shape}")` renders; the two are not the
// same thing.) The upshot was that `Cow<'_, str>` fell through to `false`
// and got ranked a *coercive* rather than exact match, losing untagged
// enum variant selection to anything that scored exact.
matches!(
scalar_type,
Some(ScalarType::Str | ScalarType::Char | ScalarType::String | ScalarType::CowStr)
)
}
ScalarValue::Null => {
matches!(scalar_type, Some(ScalarType::Unit))
|| (scalar_type.is_none() && matches!(shape.def, Def::Option(_)))
}
ScalarValue::Unit => matches!(scalar_type, Some(ScalarType::Unit)),
ScalarValue::Bytes(_) => false,
}
}
#[cfg(test)]
mod tests {
extern crate alloc;
use super::*;
use crate::ScalarValue;
use alloc::borrow::Cow;
use facet_core::Facet;
/// Every string-shaped type must rank as an *exact* match for a string
/// scalar — not merely a coercive one.
///
/// `Cow<'_, str>` used to fail this. The check sniffed `type_identifier` for
/// `"Cow<str"`, but `type_identifier` is the *bare* name and reads `"Cow"`;
/// `"Cow<str>"` is only what `Display` renders. So `Cow<'_, str>` scored 1
/// (coercive) instead of 0 (exact) and lost untagged enum variant selection
/// to any sibling that scored 0.
///
/// Asserted as a table so a fifth string type cannot be added without a row.
#[test]
fn string_shaped_types_are_exact_matches_for_str_scalars() {
let rows: &[(&str, &'static facet_core::Shape)] = &[
("String", <alloc::string::String as Facet>::SHAPE),
("Cow<'_, str>", <Cow<'_, str> as Facet>::SHAPE),
("&str", <&str as Facet>::SHAPE),
("char", <char as Facet>::SHAPE),
];
let scalar = ScalarValue::Str(Cow::Borrowed("hello"));
let failures: alloc::vec::Vec<&str> = rows
.iter()
.filter(|(_, shape)| !is_exact_scalar_match(&scalar, shape))
.map(|(label, _)| *label)
.collect();
assert!(
failures.is_empty(),
"these string-shaped types were not treated as exact matches for a \
string scalar, so they lose untagged-enum selection to anything that \
is: {failures:?}"
);
}
/// The guard above is only meaningful if non-string types still fail it.
#[test]
fn non_string_types_are_not_exact_matches_for_str_scalars() {
let scalar = ScalarValue::Str(Cow::Borrowed("hello"));
assert!(!is_exact_scalar_match(&scalar, <u32 as Facet>::SHAPE));
assert!(!is_exact_scalar_match(&scalar, <bool as Facet>::SHAPE));
}
}