//! This module implements compiler logic related to attribute sets //! (construction, access operators, ...). use super::*; impl Compiler<'_> { pub(super) fn compile_attr(&mut self, slot: LocalIdx, node: ast::Attr) { match node { ast::Attr::Dynamic(dynamic) => { self.compile(slot, dynamic.expr().unwrap()); self.emit_force(&dynamic.expr().unwrap()); } ast::Attr::Str(s) => { self.compile_str(slot, s.clone()); self.emit_force(&s); } ast::Attr::Ident(ident) => self.emit_literal_ident(&ident), } } /// Compiles inherited values in an attribute set. Inherited /// values are *always* inherited from the outer scope, even if /// there is a matching name within a recursive attribute set. fn compile_inherit_attrs( &mut self, slot: LocalIdx, inherits: AstChildren<ast::Inherit>, ) -> usize { // Count the number of inherited values, so that the outer // constructor can emit the correct number of pairs when // constructing attribute sets. let mut count = 0; for inherit in inherits { match inherit.from() { Some(from) => { for ident in inherit.idents() { count += 1; // First emit the identifier itself (this // becomes the new key). self.emit_literal_ident(&ident); let ident_span = self.span_for(&ident); self.scope_mut().declare_phantom(ident_span, true); // Then emit the node that we're inheriting // from. // // TODO: Likely significant optimisation // potential in having a multi-select // instruction followed by a merge, rather // than pushing/popping the same attrs // potentially a lot of times. let val_idx = self.scope_mut().declare_phantom(ident_span, false); self.compile(val_idx, from.expr().unwrap()); self.emit_force(&from.expr().unwrap()); self.emit_literal_ident(&ident); self.push_op(OpCode::OpAttrsSelect, &ident); self.scope_mut().mark_initialised(val_idx); } } None => { for ident in inherit.idents() { let ident_span = self.span_for(&ident); count += 1; // Emit the key to use for OpAttrs self.emit_literal_ident(&ident); self.scope_mut().declare_phantom(ident_span, true); // Emit the value. self.compile_ident(slot, ident); self.scope_mut().declare_phantom(ident_span, true); } } } } count } /// Compile the statically known entries of an attribute set. Which /// keys are which is not known from the iterator, so discovered /// dynamic keys are returned from here. fn compile_static_attr_entries( &mut self, count: &mut usize, entries: AstChildren<ast::AttrpathValue>, ) -> Vec<ast::AttrpathValue> { let mut dynamic_attrs: Vec<ast::AttrpathValue> = vec![]; 'entries: for kv in entries { // Attempt to turn the attrpath into a list of static // strings, but abort this process if any dynamic // fragments are encountered. let static_attrpath: Option<Vec<String>> = kv .attrpath() .unwrap() .attrs() .map(|a| self.expr_static_attr_str(&a)) .collect(); let fragments = match static_attrpath { Some(fragments) => fragments, None => { dynamic_attrs.push(kv); continue 'entries; } }; // At this point we can increase the counter because we // know that this particular attribute is static and can // thus be processed here. *count += 1; let key_count = fragments.len(); for fragment in fragments.into_iter() { self.emit_constant(Value::String(fragment.into()), &kv.attrpath().unwrap()); } // We're done with the key if there was only one fragment, // otherwise we need to emit an instruction to construct // the attribute path. if key_count > 1 { self.push_op( OpCode::OpAttrPath(Count(key_count)), &kv.attrpath().unwrap(), ); } // The value is just compiled as normal so that its // resulting value is on the stack when the attribute set // is constructed at runtime. let value_span = self.span_for(&kv.value().unwrap()); let value_slot = self.scope_mut().declare_phantom(value_span, false); self.compile(value_slot, kv.value().unwrap()); self.scope_mut().mark_initialised(value_slot); } dynamic_attrs } /// Compile the dynamic entries of an attribute set, where keys /// are only known at runtime. fn compile_dynamic_attr_entries( &mut self, count: &mut usize, entries: Vec<ast::AttrpathValue>, ) { for entry in entries.into_iter() { *count += 1; let mut key_count = 0; let key_span = self.span_for(&entry.attrpath().unwrap()); let key_idx = self.scope_mut().declare_phantom(key_span, false); for fragment in entry.attrpath().unwrap().attrs() { // Key fragments can contain dynamic expressions, // which makes accounting for their stack slots very // tricky. // // In order to ensure the locals are correctly cleaned // up, we essentially treat any key fragment after the // first one (which has a locals index that will // become that of the final key itself) as being part // of a separate scope, which results in the somewhat // annoying setup logic below. let fragment_slot = match key_count { 0 => key_idx, 1 => { self.scope_mut().begin_scope(); self.scope_mut().declare_phantom(key_span, false) } _ => self.scope_mut().declare_phantom(key_span, false), }; key_count += 1; self.compile_attr(fragment_slot, fragment); self.scope_mut().mark_initialised(fragment_slot); } // We're done with the key if there was only one fragment, // otherwise we need to emit an instruction to construct // the attribute path. if key_count > 1 { self.push_op( OpCode::OpAttrPath(Count(key_count)), &entry.attrpath().unwrap(), ); // Close the temporary scope that was set up for the // key fragments. self.scope_mut().end_scope(); } // The value is just compiled as normal so that its // resulting value is on the stack when the attribute set // is constructed at runtime. let value_span = self.span_for(&entry.value().unwrap()); let value_slot = self.scope_mut().declare_phantom(value_span, false); self.compile(value_slot, entry.value().unwrap()); self.scope_mut().mark_initialised(value_slot); } } /// Compile attribute set literals into equivalent bytecode. /// /// This is complicated by a number of features specific to Nix /// attribute sets, most importantly: /// /// 1. Keys can be dynamically constructed through interpolation. /// 2. Keys can refer to nested attribute sets. /// 3. Attribute sets can (optionally) be recursive. pub(super) fn compile_attr_set(&mut self, slot: LocalIdx, node: ast::AttrSet) { // Open a scope to track the positions of the temporaries used // by the `OpAttrs` instruction. self.scope_mut().begin_scope(); if node.rec_token().is_some() { self.compile_recursive_scope(slot, true, &node); self.push_op(OpCode::OpAttrs(Count(node.entries().count())), &node); } else { let mut count = self.compile_inherit_attrs(slot, node.inherits()); let dynamic_entries = self.compile_static_attr_entries(&mut count, node.attrpath_values()); self.compile_dynamic_attr_entries(&mut count, dynamic_entries); self.push_op(OpCode::OpAttrs(Count(count)), &node); } // Remove the temporary scope, but do not emit any additional // cleanup (OpAttrs consumes all of these locals). self.scope_mut().end_scope(); } pub(super) fn compile_has_attr(&mut self, slot: LocalIdx, node: ast::HasAttr) { // Put the attribute set on the stack. self.compile(slot, node.expr().unwrap()); self.emit_force(&node); // Push all path fragments with an operation for fetching the // next nested element, for all fragments except the last one. for (count, fragment) in node.attrpath().unwrap().attrs().enumerate() { if count > 0 { self.push_op(OpCode::OpAttrsTrySelect, &fragment); self.emit_force(&fragment); } self.compile_attr(slot, fragment); } // After the last fragment, emit the actual instruction that // leaves a boolean on the stack. self.push_op(OpCode::OpHasAttr, &node); } pub(super) fn compile_select(&mut self, slot: LocalIdx, node: ast::Select) { let set = node.expr().unwrap(); let path = node.attrpath().unwrap(); if node.or_token().is_some() { self.compile_select_or(slot, set, path, node.default_expr().unwrap()); return; } // Push the set onto the stack self.compile(slot, set.clone()); // Compile each key fragment and emit access instructions. // // TODO: multi-select instruction to avoid re-pushing attrs on // nested selects. for fragment in path.attrs() { // Force the current set value. self.emit_force(&fragment); self.compile_attr(slot, fragment.clone()); self.push_op(OpCode::OpAttrsSelect, &fragment); } } /// Compile an `or` expression into a chunk of conditional jumps. /// /// If at any point during attribute set traversal a key is /// missing, the `OpAttrOrNotFound` instruction will leave a /// special sentinel value on the stack. /// /// After each access, a conditional jump evaluates the top of the /// stack and short-circuits to the default value if it sees the /// sentinel. /// /// Code like `{ a.b = 1; }.a.c or 42` yields this bytecode and /// runtime stack: /// /// ```notrust /// Bytecode Runtime stack /// ┌────────────────────────────┐ ┌─────────────────────────┐ /// │ ... │ │ ... │ /// │ 5 OP_ATTRS(1) │ → │ 5 [ { a.b = 1; } ] │ /// │ 6 OP_CONSTANT("a") │ → │ 6 [ { a.b = 1; } "a" ] │ /// │ 7 OP_ATTR_OR_NOT_FOUND │ → │ 7 [ { b = 1; } ] │ /// │ 8 JUMP_IF_NOT_FOUND(13) │ → │ 8 [ { b = 1; } ] │ /// │ 9 OP_CONSTANT("C") │ → │ 9 [ { b = 1; } "c" ] │ /// │ 10 OP_ATTR_OR_NOT_FOUND │ → │ 10 [ NOT_FOUND ] │ /// │ 11 JUMP_IF_NOT_FOUND(13) │ → │ 11 [ ] │ /// │ 12 JUMP(14) │ │ .. jumped over │ /// │ 13 CONSTANT(42) │ → │ 12 [ 42 ] │ /// │ 14 ... │ │ .. .... │ /// └────────────────────────────┘ └─────────────────────────┘ /// ``` fn compile_select_or( &mut self, slot: LocalIdx, set: ast::Expr, path: ast::Attrpath, default: ast::Expr, ) { self.compile(slot, set.clone()); let mut jumps = vec![]; for fragment in path.attrs() { self.emit_force(&fragment); self.compile_attr(slot, fragment.clone()); self.push_op(OpCode::OpAttrsTrySelect, &fragment); jumps.push(self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &fragment)); } let final_jump = self.push_op(OpCode::OpJump(JumpOffset(0)), &path); for jump in jumps { self.patch_jump(jump); } // Compile the default value expression and patch the final // jump to point *beyond* it. self.compile(slot, default); self.patch_jump(final_jump); } }