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Implement AST multi-error recovery #383

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Implement AST multi-error recovery #383

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fdfae8b
feat: add Error to TypeInner
LesterEvSe Jul 23, 2026
ceb772c
feat: add Error arm to ast.rs file
LesterEvSe Jul 23, 2026
872a7a3
refactor: split parse_from_str and parse_from_content by error policy
LesterEvSe Jul 24, 2026
0d80370
feat: add multi-errors to ast.rs file
LesterEvSe Jul 28, 2026
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30 changes: 28 additions & 2 deletions src/types.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,9 @@ pub enum TypeInner<A> {
/// Nominal enum type, represented as a balanced sum of its variants'
/// payload types
Enum(EnumInfo),
/// Type of a recovered subtree. Compatible with every type; never re-reported.

@apoelstra apoelstra Aug 17, 2026

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In fdfae8b:

What is a "recovered subtree" what does it mean to "re-report" a type?

/// A diagnostic was already emitted for its span.
Never,
}

/// One variant of a nominal enum type: its name and payload types.
Expand Down Expand Up @@ -201,6 +204,7 @@ impl<A> TypeInner<A> {
}
},
TypeInner::Enum(info) => write!(f, "{}", info.name()),
TypeInner::Never => write!(f, "<never>"),
}
}
}
Expand Down Expand Up @@ -459,6 +463,14 @@ impl ResolvedType {
pub fn as_inner(&self) -> &TypeInner<Arc<Self>> {
&self.0
}

pub const fn never() -> Self {
Self(TypeInner::Never)
}

pub fn is_never(&self) -> bool {
matches!(self.0, TypeInner::Never)
}
}

/// Nominal enum types.
Expand Down Expand Up @@ -571,7 +583,9 @@ impl TypeDeconstructible for ResolvedType {
impl TreeLike for &ResolvedType {
fn as_node(&self) -> Tree<Self> {
match &self.0 {
TypeInner::Boolean | TypeInner::UInt(..) | TypeInner::Enum(..) => Tree::Nullary,
TypeInner::Boolean | TypeInner::UInt(..) | TypeInner::Enum(..) | TypeInner::Never => {
Tree::Nullary
}
TypeInner::Option(l) | TypeInner::Array(l, _) | TypeInner::List(l, _) => Tree::Unary(l),
TypeInner::Either(l, r) => Tree::Binary(l, r),
TypeInner::Tuple(elements) => Tree::Nary(elements.iter().map(Arc::as_ref).collect()),
Expand Down Expand Up @@ -729,6 +743,10 @@ impl AliasedType {
Self(AliasedInner::Builtin(builtin))
}

pub const fn error() -> Self {
Self(AliasedInner::Inner(TypeInner::Never))
}

/// Resolve all aliases in the type based on the given map of `aliases` to types.
pub fn resolve<F, E>(&self, mut get_alias: F) -> Result<ResolvedType, E>
where
Expand Down Expand Up @@ -775,6 +793,7 @@ impl AliasedType {
TypeInner::Enum(info) => {
output.push(ResolvedType::enumeration(info.clone()));
}
TypeInner::Never => output.push(ResolvedType::never()),
},
}
}
Expand Down Expand Up @@ -809,6 +828,7 @@ impl_require_feature!(TypeInner<Arc<AliasedType>> {
Array(element, _),
List(element, _),
Enum(_),
Never,
});

impl TypeConstructible for AliasedType {
Expand Down Expand Up @@ -901,7 +921,10 @@ impl TreeLike for &AliasedType {
match &self.0 {
AliasedInner::Alias(_) | AliasedInner::Builtin(_) => Tree::Nullary,
AliasedInner::Inner(inner) => match inner {
TypeInner::Boolean | TypeInner::UInt(..) | TypeInner::Enum(..) => Tree::Nullary,
TypeInner::Boolean
| TypeInner::UInt(..)
| TypeInner::Enum(..)
| TypeInner::Never => Tree::Nullary,
TypeInner::Option(l) | TypeInner::Array(l, _) | TypeInner::List(l, _) => {
Tree::Unary(l)
}
Expand Down Expand Up @@ -1205,6 +1228,9 @@ impl From<&ResolvedType> for StructuralType {
TypeInner::Enum(info) => {
output.push(StructuralType::balanced_sum(info.structural_variants()));
}
TypeInner::Never => unreachable!(
"poisoned type reached codegen; compilation must be gated on zero errors"
),
}
}
debug_assert_eq!(output.len(), 1);
Expand Down
4 changes: 3 additions & 1 deletion src/value.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -864,6 +864,7 @@ impl Value {
}
}
}
TypeInner::Never => unreachable!("poisoned type in value reconstruction; values exist only for error-free programs"),
}
}
debug_assert_eq!(output.len(), 1);
Expand Down Expand Up @@ -937,6 +938,7 @@ impl crate::ArbitraryOfType for Value {
.collect::<arbitrary::Result<Vec<Self>>>()?;
Ok(Self::list(elements, ty.as_ref().clone(), *bound))
}
TypeInner::Never => unreachable!("cannot generate a value of a poisoned type"),

@apoelstra apoelstra Aug 17, 2026

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In fdfae8b:

This one is not unreachable. We should return an Arbitrary error here indicating that there is no value of the never type.

@LesterEvSe LesterEvSe Sep 1, 2026

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Agreed with @KyrylR that this PR is too large to review as one unit. I'm splitting it into a sequence:

(Rough split, not yet validated)

  1. Add the uninhabited Never type to TypeInner (+ is_never,
    ResolvedType::compatible, and the Arbitrary impl returning an error
    instead of unreachable!). Standalone and useful on its own.
  2. Error variants in the parse tree + parser recovery (Implement error states in parser #205).
  3. Diagnostics sink in Scope; containers collect instead of aborting (Error recovery in analysis #207).
  4. Split parse_from_str / parse_from_content by error policy.

@apoelstra, I plan to address your comment about the Arbitrary branch in (1). I'll also rewrite the doc comment "recovered subtree" and "re-report" are not defined anywhere. Sorry for the churn.

Keeping this open as a draft for the overall design; I'll close it once (4)
is merged.

}
}
}
Expand Down Expand Up @@ -1257,7 +1259,7 @@ impl TreeLike for Destructor<'_> {
Self::WrongType => return Tree::Nullary,
};
match ty.as_inner() {
TypeInner::Boolean | TypeInner::UInt(..) => Tree::Nullary,
TypeInner::Boolean | TypeInner::UInt(..) | TypeInner::Never => Tree::Nullary,
TypeInner::Enum(info) => match destruct::as_enum_leaf(value, info.variants().len()) {
Some((index, leaf)) => {
Tree::Unary(Self::new(leaf, info.variants()[index].payload_type()))
Expand Down