BE-71: HashQL: Consolidate AST special-form expansion stages into one

.claude/skills/testing-hashql/references/compiletest-guide.md

@@ -236,7 +236,7 @@ cargo run -p hashql-compiletest suites --json
 ### Suite Categories
 
 - `parse/*` - Parsing tests (e.g., `parse/syntax-dump`)
-- `ast/lowering/*` - AST lowering phases
+- `ast/lower/*` - AST lowering phases
 - `hir/lower/*` - HIR lowering phases
 - `hir/reify` - HIR generation from AST
 - `mir/*` - MIR passes and generation

libs/@local/graph/api/src/rest/hashql/compile.rs

@@ -68,7 +68,7 @@ impl<'heap> Compilation<'heap> {
         let Success {
             value: types,
             advisories,
-        } = hashql_ast::lowering::lower(sym::path::main, &mut ast, &env, &modules)
+        } = hashql_ast::lower::lower(sym::path::main, &mut ast, &env, &modules, &mut *scratch)
             .map_category(|category| {
                 HashQlDiagnosticCategory::Ast(AstDiagnosticCategory::Lowering(category))
             })

libs/@local/hashql/ast/Cargo.toml

@@ -16,10 +16,6 @@ hashql-diagnostics = { workspace = true, public = true }
 # Private workspace dependencies
 
 # Private third-party dependencies
-derive_more    = { workspace = true, features = ["display"] }
-enum-iterator  = { workspace = true }
-foldhash       = { workspace = true }
-hashbrown      = { workspace = true }
 simple-mermaid = { workspace = true }
 tracing        = { workspace = true }
 

libs/@local/hashql/ast/src/error.rs

@@ -3,7 +3,7 @@ use alloc::borrow::Cow;
 use hashql_core::span::SpanId;
 use hashql_diagnostics::{Diagnostic, category::DiagnosticCategory};
 
-use crate::lowering::error::LoweringDiagnosticCategory;
+use crate::lower::error::LoweringDiagnosticCategory;
 
 pub type AstDiagnostic = Diagnostic<AstDiagnosticCategory, SpanId>;
 

libs/@local/hashql/ast/src/format/dump.rs

@@ -27,14 +27,12 @@ use crate::node::{
     expr::{
         AsExpr, CallExpr, ClosureExpr, DictExpr, Expr, ExprKind, FieldExpr, IfExpr, IndexExpr,
         InputExpr, LetExpr, ListExpr, LiteralExpr, NewTypeExpr, StructExpr, TupleExpr, TypeExpr,
-        UseExpr,
         call::{Argument, LabeledArgument},
         closure::{ClosureParam, ClosureSignature},
         dict::DictEntry,
         list::ListElement,
         r#struct::StructEntry,
         tuple::TupleElement,
-        r#use::{Glob, UseBinding, UseKind},
     },
     generic::{GenericArgument, GenericConstraint, GenericParam, Generics},
     id::NodeId,
@@ -352,62 +350,6 @@ impl_syntax_dump!(struct TypeExpr(name); []constraints value body);
 
 impl_syntax_dump!(struct NewTypeExpr(name); []constraints value body);
 
-impl SyntaxDump for UseBinding<'_> {
-    fn syntax_dump(&self, fmt: &mut Formatter, depth: usize) -> fmt::Result {
-        let Self {
-            id,
-            span,
-            name,
-            alias,
-        } = self;
-
-        let mut properties = vec![format!("name: {name}")];
-        if let Some(alias) = alias {
-            properties.push(format!("alias: {alias}"));
-        }
-
-        write_header(
-            fmt,
-            depth,
-            "UseBinding",
-            Some(*id),
-            Some(*span),
-            Some(&properties.join(", ")),
-        )
-    }
-}
-
-impl SyntaxDump for Glob {
-    fn syntax_dump(&self, fmt: &mut Formatter, depth: usize) -> fmt::Result {
-        let Self { id, span } = self;
-
-        write_header(fmt, depth, "Glob", Some(*id), Some(*span), None)
-    }
-}
-
-impl SyntaxDump for UseKind<'_> {
-    fn syntax_dump(&self, fmt: &mut Formatter, depth: usize) -> fmt::Result {
-        match self {
-            UseKind::Named(bindings) => {
-                write_header(fmt, depth, "UseKind", None, None, Some("Named"))?;
-
-                for binding in bindings {
-                    binding.syntax_dump(fmt, depth + 1)?;
-                }
-
-                Ok(())
-            }
-            UseKind::Glob(glob) => {
-                write_header(fmt, depth, "UseKind", None, None, Some("Glob"))?;
-
-                glob.syntax_dump(fmt, depth + 1)
-            }
-        }
-    }
-}
-
-impl_syntax_dump!(struct UseExpr(); path kind body);
-
 impl_syntax_dump!(struct InputExpr(name); r#type ?default);
 
 #[rustfmt::skip]
@@ -478,11 +420,6 @@ impl SyntaxDump for ExprKind<'_> {
 
                 new_type_expr.syntax_dump(fmt, depth + 1)
             }
-            Self::Use(use_expr) => {
-                write_header(fmt, depth, "ExprKind", None, None, Some("Use"))?;
-
-                use_expr.syntax_dump(fmt, depth + 1)
-            }
             Self::Input(input_expr) => {
                 write_header(fmt, depth, "ExprKind", None, None, Some("Input"))?;
 

libs/@local/hashql/ast/src/lib.rs

@@ -1,47 +1,16 @@
-//! # HashQL Abstract Syntax Tree
+//! HashQL abstract syntax tree and lowering pipeline.
 //!
-//! This crate defines the Abstract Syntax Tree (AST) for HashQL, a side-effect free,
-//! purely functional query language designed for bi-temporal graph databases.
+//! This crate defines the AST for HashQL and the [`lower`] pipeline that
+//! transforms it into a form suitable for HIR construction. The AST is
+//! frontend-agnostic: nodes are defined independently of any syntax, with
+//! J-Expr as the current primary parser.
 //!
-//! ## Overview
+//! # Modules
 //!
-//! The HashQL AST provides a structured representation of HashQL programs after parsing.
-//! It captures the hierarchical structure of the code with nodes for expressions, types,
-//! and declarations. This AST is the foundation for subsequent compilation phases including
-//! type checking, optimization, and evaluation.
-//!
-//! ## Key Features
-//!
-//! - **Frontend Agnostic**: The AST is designed to be independent of any particular syntax. While
-//!   the current primary interface is through J-Expr (JSON-based expressions), the AST can support
-//!   multiple frontend syntaxes.
-//!
-//! - **Memory Efficient**: Uses arena allocation through a custom [`heap`] module to minimize
-//!   allocations and improve performance during parsing and transformation.
-//!
-//! - **Source Tracking**: Each node maintains its location in the source code via span identifiers.
-//!
-//! - **Comprehensive Language Model**: Supports the full range of language constructs in HashQL,
-//!   including expressions, types, path references, and special forms.
-//!
-//! ## Core Modules
-//!
-//! - [`node`]: Defines the AST node types that represent language constructs
-//! - [`lowering`]: Defines the lowering process for AST nodes, converting them into a more
-//!   optimized form suitable for conversion into the HIR.
-//!
-//! ## Special Forms
-//!
-//! HashQL implements several language constructs as "special forms" that are initially parsed
-//! as function calls and then transformed into specialized AST nodes. These include:
-//!
-//! - `let` for variable binding
-//! - `if` for conditional expressions
-//! - `fn` for closure definitions
-//! - `use` for module imports
-//! - Field and index access expressions
-//!
-//! [`heap`]: hashql_core::heap
+//! - [`node`]: AST node types (expressions, types, paths, generics).
+//! - [`lower`]: Lowering pipeline (expansion, sanitization, type extraction).
+//! - [`visit`]: Visitor trait for AST traversal.
+//! - [`format`](mod@format): Debug formatting for AST dumps.
 //!
 //! ## Workspace dependencies
 #![cfg_attr(doc, doc = simple_mermaid::mermaid!("../docs/dependency-diagram.mmd"))]
@@ -50,6 +19,7 @@
     // Language Features
     coverage_attribute,
     macro_metavar_expr_concat,
+    default_field_values,
 
     // Library Features
     allocator_api,
@@ -61,6 +31,6 @@ extern crate alloc;
 
 pub mod error;
 pub mod format;
-pub mod lowering;
+pub mod lower;
 pub mod node;
 pub mod visit;

libs/@local/hashql/ast/src/lowering/error.rs → libs/@local/hashql/ast/src/lower/error.rs

@@ -4,19 +4,17 @@ use hashql_core::span::SpanId;
 use hashql_diagnostics::{Diagnostic, category::DiagnosticCategory};
 
 use super::{
-    import_resolver::error::ImportResolverDiagnosticCategory,
-    sanitizer::SanitizerDiagnosticCategory,
-    special_form_expander::error::SpecialFormExpanderDiagnosticCategory,
+    expander::error::ExpanderDiagnosticCategory, sanitizer::SanitizerDiagnosticCategory,
     type_extractor::error::TypeExtractorDiagnosticCategory,
 };
 
 pub type LoweringDiagnostic = Diagnostic<LoweringDiagnosticCategory, SpanId>;
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum LoweringDiagnosticCategory {
-    Expander(SpecialFormExpanderDiagnosticCategory),
+    Expander(ExpanderDiagnosticCategory),
     Sanitizer(SanitizerDiagnosticCategory),
-    Resolver(ImportResolverDiagnosticCategory),
+
     Extractor(TypeExtractorDiagnosticCategory),
 }
 
@@ -31,9 +29,8 @@ impl DiagnosticCategory for LoweringDiagnosticCategory {
 
     fn subcategory(&self) -> Option<&dyn DiagnosticCategory> {
         match self {
-            Self::Expander(special_form) => Some(special_form),
+            Self::Expander(expander) => Some(expander),
             Self::Sanitizer(sanitizer) => Some(sanitizer),
-            Self::Resolver(resolver) => Some(resolver),
             Self::Extractor(extractor) => Some(extractor),
         }
     }

libs/@local/hashql/ast/src/lower/expander/access.rs

@@ -0,0 +1,134 @@
+use core::mem;
+
+use hashql_core::{
+    heap::BumpAllocator,
+    span::SpanId,
+    symbol::{Ident, IdentKind},
+    value::Primitive,
+};
+
+use super::Expander;
+use crate::{
+    lower::expander::error,
+    node::{
+        expr::{CallExpr, Expr, ExprKind, FieldExpr, call::Argument},
+        id::NodeId,
+    },
+};
+
+/// Extract a field identifier from an argument.
+///
+/// Handles both named field access (identifiers like `name`) and indexed field
+/// access (integer literals like `0` for tuple fields). Integer literals are
+/// validated for bounds.
+fn argument_to_field<'heap, S>(
+    expander: &mut Expander<'_, 'heap, S>,
+    argument: &Argument<'heap>,
+) -> Option<Ident<'heap>> {
+    // Integer literal for tuple field access
+    if let ExprKind::Literal(literal) = &argument.value.kind {
+        if let Some(annotation) = &literal.r#type {
+            expander
+                .diagnostics
+                .push(error::field_literal_type_annotation(annotation.span));
+        }
+
+        let Primitive::Integer(integer) = literal.kind else {
+            expander
+                .diagnostics
+                .push(error::invalid_field_literal_type(literal.span));
+            return None;
+        };
+
+        if integer.as_usize().is_none() {
+            expander
+                .diagnostics
+                .push(error::field_index_out_of_bounds(literal.span));
+            return None;
+        }
+
+        return Some(Ident {
+            span: literal.span,
+            value: integer.as_symbol(),
+            kind: IdentKind::Lexical,
+        });
+    }
+
+    // Named field access
+    if let ExprKind::Path(path) = &argument.value.kind
+        && let Some(&ident) = path.as_ident()
+    {
+        return Some(ident);
+    }
+
+    expander
+        .diagnostics
+        .push(error::invalid_access_field(argument));
+
+    None
+}
+
+fn lower_access_impl<'heap, S>(
+    span: SpanId,
+    expander: &mut Expander<'_, 'heap, S>,
+
+    value: &mut Argument<'heap>,
+    field: &Argument<'heap>,
+) -> Expr<'heap>
+where
+    S: BumpAllocator,
+{
+    let Some(field) = argument_to_field(expander, field) else {
+        return Expr::dummy();
+    };
+
+    let mut value = mem::replace(&mut value.value, Expr::dummy());
+    expander.visit(&mut value);
+
+    Expr {
+        id: NodeId::PLACEHOLDER,
+        span,
+        kind: ExprKind::Field(FieldExpr {
+            id: NodeId::PLACEHOLDER,
+            span,
+            value: Box::new_in(value, expander.heap),
+            field,
+        }),
+    }
+}
+
+/// Lowers a `.` call into a [`FieldExpr`].
+///
+/// Form: `(. value field)`. The field must be either a named identifier
+/// or a non-negative integer literal (for positional tuple access).
+///
+/// [`FieldExpr`]: crate::node::expr::FieldExpr
+pub(super) fn lower_access<'heap, S>(
+    expander: &mut Expander<'_, 'heap, S>,
+    CallExpr {
+        id: _,
+        span,
+        function: _,
+        arguments,
+        labeled_arguments,
+    }: &mut CallExpr<'heap>,
+) -> Expr<'heap>
+where
+    S: BumpAllocator,
+{
+    if !labeled_arguments.is_empty() {
+        expander
+            .diagnostics
+            .push(error::labeled_arguments_in_access(labeled_arguments));
+    }
+
+    if let [value, field] = &mut **arguments {
+        lower_access_impl(*span, expander, value, field)
+    } else {
+        expander
+            .diagnostics
+            .push(error::invalid_access_argument_count(*span, arguments));
+
+        Expr::dummy()
+    }
+}