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-rw-r--r--tvix/eval/src/compiler/mod.rs559
1 files changed, 121 insertions, 438 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 21f131e4543a..de614be4dad6 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -13,7 +13,7 @@
 //! by the code in this module, `debug_assert!` has been used to catch
 //! mistakes early during development.
 
-mod attrs;
+mod bindings;
 mod scope;
 mod spans;
 
@@ -507,6 +507,126 @@ impl Compiler<'_> {
         self.scope_mut().end_scope();
     }
 
+    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),
+        }
+    }
+
+    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);
+    }
+
+    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);
+    }
+
     fn compile_assert(&mut self, slot: LocalIdx, node: ast::Assert) {
         // Compile the assertion condition to leave its value on the stack.
         self.compile(slot, node.condition().unwrap());
@@ -552,311 +672,6 @@ impl Compiler<'_> {
         self.patch_jump(else_idx); // patch jump *over* else body
     }
 
-    fn compile_recursive_scope<N>(&mut self, slot: LocalIdx, rec_attrs: bool, node: &N) -> usize
-    where
-        N: ToSpan + ast::HasEntry,
-    {
-        let mut count = 0;
-        self.scope_mut().begin_scope();
-
-        // First pass to find all plain inherits (if they are not useless).
-        // Since they always resolve to a higher scope, we can just compile and
-        // declare them immediately. This needs to happen *before* we declare
-        // any other locals in the scope or the stack order gets messed up.
-        // While we are looping through the inherits, already note all inherit
-        // (from) expressions, that may very well resolve recursively and need
-        // to be compiled like normal let in bindings.
-        let mut inherit_froms: Vec<(ast::Expr, String, Span)> = vec![];
-        for inherit in node.inherits() {
-            match inherit.from() {
-                // Within a `let` binding, inheriting from the outer
-                // scope is a no-op *if* the identifier can be
-                // statically resolved.
-                None if !rec_attrs && !self.scope().has_with() => {
-                    self.emit_warning(&inherit, WarningKind::UselessInherit);
-                    continue;
-                }
-
-                None => {
-                    for attr in inherit.attrs() {
-                        let name = match self.expr_static_attr_str(&attr) {
-                            Some(name) => name,
-                            None => {
-                                // TODO(tazjin): error variant for dynamic
-                                // key in *inherit* (or generalise it)
-                                self.emit_error(&attr, ErrorKind::DynamicKeyInLet);
-                                continue;
-                            }
-                        };
-
-                        count += 1;
-
-                        // If the identifier resolves statically in a
-                        // `let`, it has precedence over dynamic
-                        // bindings, and the inherit is useless.
-                        if !rec_attrs
-                            && matches!(
-                                self.scope_mut().resolve_local(&name),
-                                LocalPosition::Known(_)
-                            )
-                        {
-                            self.emit_warning(&attr, WarningKind::UselessInherit);
-                            continue;
-                        }
-
-                        if rec_attrs {
-                            self.emit_constant(Value::String(SmolStr::new(&name).into()), &attr);
-                            let span = self.span_for(&attr);
-                            self.scope_mut().declare_phantom(span, true);
-                        }
-
-                        self.compile_identifier_access(slot, &name, &attr);
-                        let idx = self.declare_local(&attr, &name);
-                        self.scope_mut().mark_initialised(idx);
-                    }
-                }
-
-                Some(from) => {
-                    for attr in inherit.attrs() {
-                        let name = match self.expr_static_attr_str(&attr) {
-                            Some(name) => name,
-                            None => {
-                                // TODO(tazjin): error variant for dynamic
-                                // key in *inherit* (or generalise it)
-                                self.emit_error(&attr, ErrorKind::DynamicKeyInLet);
-                                continue;
-                            }
-                        };
-
-                        count += 1;
-                        inherit_froms.push((from.expr().unwrap(), name, self.span_for(&attr)));
-                    }
-                }
-            }
-        }
-
-        // Data structures to track the bindings observed in the
-        // second pass, and forward the information needed to compile
-        // their value.
-        enum BindingKind {
-            InheritFrom {
-                namespace: ast::Expr,
-                name: SmolStr,
-                span: Span,
-            },
-
-            Plain {
-                expr: ast::Expr,
-            },
-        }
-
-        struct KeySlot {
-            slot: LocalIdx,
-            name: SmolStr,
-        }
-
-        struct TrackedBinding {
-            key_slot: Option<KeySlot>,
-            value_slot: LocalIdx,
-            kind: BindingKind,
-        }
-
-        // Vector to track these observed bindings.
-        let mut bindings: Vec<TrackedBinding> = vec![];
-
-        // Begin second pass to ensure that all remaining identifiers
-        // (that may resolve recursively) are known.
-
-        // Begin with the inherit (from)s since they all become a thunk anyway
-        for (from, name, span) in inherit_froms {
-            let key_slot = if rec_attrs {
-                Some(KeySlot {
-                    slot: self.scope_mut().declare_phantom(span, false),
-                    name: SmolStr::new(&name),
-                })
-            } else {
-                None
-            };
-
-            let value_slot = self.declare_local(&span, &name);
-
-            bindings.push(TrackedBinding {
-                key_slot,
-                value_slot,
-                kind: BindingKind::InheritFrom {
-                    namespace: from,
-                    name: SmolStr::new(&name),
-                    span,
-                },
-            });
-        }
-
-        // Declare all regular bindings
-        for entry in node.attrpath_values() {
-            count += 1;
-
-            let mut path = match self.normalise_ident_path(entry.attrpath().unwrap().attrs()) {
-                Ok(p) => p,
-                Err(err) => {
-                    self.errors.push(err);
-                    continue;
-                }
-            };
-
-            if path.len() != 1 {
-                self.emit_error(
-                    &entry,
-                    ErrorKind::NotImplemented("nested bindings in recursive scope :("),
-                );
-                continue;
-            }
-
-            let key_slot = if rec_attrs {
-                let span = self.span_for(&entry.attrpath().unwrap());
-                Some(KeySlot {
-                    slot: self.scope_mut().declare_phantom(span, false),
-                    name: SmolStr::new(&path[0]),
-                })
-            } else {
-                None
-            };
-
-            let value_slot = self.declare_local(&entry.attrpath().unwrap(), path.pop().unwrap());
-
-            bindings.push(TrackedBinding {
-                key_slot,
-                value_slot,
-                kind: BindingKind::Plain {
-                    expr: entry.value().unwrap(),
-                },
-            });
-        }
-
-        // Third pass to place the values in the correct stack slots.
-        let mut value_indices: Vec<LocalIdx> = vec![];
-        for binding in bindings.into_iter() {
-            value_indices.push(binding.value_slot);
-
-            if let Some(key_slot) = binding.key_slot {
-                let span = self.scope()[key_slot.slot].span;
-                self.emit_constant(Value::String(key_slot.name.into()), &span);
-                self.scope_mut().mark_initialised(key_slot.slot);
-            }
-
-            match binding.kind {
-                // This entry is an inherit (from) expr. The value is
-                // placed on the stack by selecting an attribute.
-                BindingKind::InheritFrom {
-                    namespace,
-                    name,
-                    span,
-                } => {
-                    // Create a thunk wrapping value (which may be one as well) to
-                    // avoid forcing the from expr too early.
-                    self.thunk(binding.value_slot, &namespace, move |c, n, s| {
-                        c.compile(s, n.clone());
-                        c.emit_force(n);
-
-                        c.emit_constant(Value::String(name.into()), &span);
-                        c.push_op(OpCode::OpAttrsSelect, &span);
-                    })
-                }
-
-                // Binding is "just" a plain expression that needs to
-                // be compiled.
-                BindingKind::Plain { expr } => self.compile(binding.value_slot, expr),
-            }
-
-            // Any code after this point will observe the value in the
-            // right stack slot, so mark it as initialised.
-            self.scope_mut().mark_initialised(binding.value_slot);
-        }
-
-        // Fourth pass to emit finaliser instructions if necessary.
-        for idx in value_indices {
-            if self.scope()[idx].needs_finaliser {
-                let stack_idx = self.scope().stack_index(idx);
-                self.push_op(OpCode::OpFinalise(stack_idx), node);
-            }
-        }
-
-        count
-    }
-
-    /// Compile a standard `let ...; in ...` expression.
-    ///
-    /// Unless in a non-standard scope, the encountered values are
-    /// simply pushed on the stack and their indices noted in the
-    /// entries vector.
-    fn compile_let_in(&mut self, slot: LocalIdx, node: ast::LetIn) {
-        self.compile_recursive_scope(slot, false, &node);
-
-        // Deal with the body, then clean up the locals afterwards.
-        self.compile(slot, node.body().unwrap());
-        self.cleanup_scope(&node);
-    }
-
-    /// Resolve and compile access to an identifier in the scope.
-    fn compile_identifier_access<N: ToSpan + Clone>(
-        &mut self,
-        slot: LocalIdx,
-        ident: &str,
-        node: &N,
-    ) {
-        // If the identifier is a global, and it is not poisoned, emit
-        // the global directly.
-        if let Some(global) = self.globals.get(ident) {
-            if !self.scope().is_poisoned(ident) {
-                global.clone()(self, self.span_for(node));
-                return;
-            }
-        }
-
-        match self.scope_mut().resolve_local(ident) {
-            LocalPosition::Unknown => {
-                // Are we possibly dealing with an upvalue?
-                if let Some(idx) = self.resolve_upvalue(self.contexts.len() - 1, ident, node) {
-                    self.push_op(OpCode::OpGetUpvalue(idx), node);
-                    return;
-                }
-
-                // If there is a non-empty `with`-stack (or a parent
-                // context with one), emit a runtime dynamic
-                // resolution instruction.
-                if self.has_dynamic_ancestor() {
-                    self.emit_constant(Value::String(SmolStr::new(ident).into()), node);
-                    self.push_op(OpCode::OpResolveWith, node);
-                    return;
-                }
-
-                // Otherwise, this variable is missing.
-                self.emit_error(node, ErrorKind::UnknownStaticVariable);
-            }
-
-            LocalPosition::Known(idx) => {
-                let stack_idx = self.scope().stack_index(idx);
-                self.push_op(OpCode::OpGetLocal(stack_idx), node);
-            }
-
-            // This identifier is referring to a value from the same
-            // scope which is not yet defined. This identifier access
-            // must be thunked.
-            LocalPosition::Recursive(idx) => self.thunk(slot, node, move |compiler, node, _| {
-                let upvalue_idx = compiler.add_upvalue(
-                    compiler.contexts.len() - 1,
-                    node,
-                    UpvalueKind::Local(idx),
-                );
-                compiler.push_op(OpCode::OpGetUpvalue(upvalue_idx), node);
-            }),
-        };
-    }
-
-    fn compile_ident(&mut self, slot: LocalIdx, node: ast::Ident) {
-        let ident = node.ident_token().unwrap();
-        self.compile_identifier_access(slot, ident.text(), &node);
-    }
-
     /// Compile `with` expressions by emitting instructions that
     /// pop/remove the indices of attribute sets that are implicitly
     /// in scope through `with` on the "with-stack".
@@ -1061,15 +876,6 @@ impl Compiler<'_> {
         self.push_op(OpCode::OpCall, &node);
     }
 
-    fn compile_legacy_let(&mut self, slot: LocalIdx, node: ast::LegacyLet) {
-        self.emit_warning(&node, WarningKind::DeprecatedLegacyLet);
-        self.scope_mut().begin_scope();
-        self.compile_recursive_scope(slot, true, &node);
-        self.push_op(OpCode::OpAttrs(Count(node.entries().count())), &node);
-        self.emit_constant(Value::String(SmolStr::new_inline("body").into()), &node);
-        self.push_op(OpCode::OpAttrsSelect, &node);
-    }
-
     /// Compile an expression into a runtime thunk which should be
     /// lazily evaluated when accessed.
     // TODO: almost the same as Compiler::compile_lambda; unify?
@@ -1246,40 +1052,6 @@ impl Compiler<'_> {
         self.scope_mut().declare_local(name, span)
     }
 
-    fn resolve_upvalue<N: ToSpan>(
-        &mut self,
-        ctx_idx: usize,
-        name: &str,
-        node: &N,
-    ) -> Option<UpvalueIdx> {
-        if ctx_idx == 0 {
-            // There can not be any upvalue at the outermost context.
-            return None;
-        }
-
-        // Determine whether the upvalue is a local in the enclosing context.
-        match self.contexts[ctx_idx - 1].scope.resolve_local(name) {
-            // recursive upvalues are dealt with the same way as
-            // standard known ones, as thunks and closures are
-            // guaranteed to be placed on the stack (i.e. in the right
-            // position) *during* their runtime construction
-            LocalPosition::Known(idx) | LocalPosition::Recursive(idx) => {
-                return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Local(idx)))
-            }
-
-            LocalPosition::Unknown => { /* continue below */ }
-        };
-
-        // If the upvalue comes from even further up, we need to
-        // recurse to make sure that the upvalues are created at each
-        // level.
-        if let Some(idx) = self.resolve_upvalue(ctx_idx - 1, name, node) {
-            return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Upvalue(idx)));
-        }
-
-        None
-    }
-
     /// Determine whether the current lambda context has any ancestors
     /// that use dynamic scope resolution, and mark contexts as
     /// needing to capture their enclosing `with`-stack in their
@@ -1301,31 +1073,6 @@ impl Compiler<'_> {
         ancestor_has_with
     }
 
-    fn add_upvalue<N: ToSpan>(
-        &mut self,
-        ctx_idx: usize,
-        node: &N,
-        kind: UpvalueKind,
-    ) -> UpvalueIdx {
-        // If there is already an upvalue closing over the specified
-        // index, retrieve that instead.
-        for (idx, existing) in self.contexts[ctx_idx].scope.upvalues.iter().enumerate() {
-            if existing.kind == kind {
-                return UpvalueIdx(idx);
-            }
-        }
-
-        let span = self.span_for(node);
-        self.contexts[ctx_idx]
-            .scope
-            .upvalues
-            .push(Upvalue { kind, span });
-
-        let idx = UpvalueIdx(self.contexts[ctx_idx].lambda.upvalue_count);
-        self.contexts[ctx_idx].lambda.upvalue_count += 1;
-        idx
-    }
-
     fn emit_force<N: ToSpan>(&mut self, node: &N) {
         self.push_op(OpCode::OpForce, node);
     }
@@ -1339,70 +1086,6 @@ impl Compiler<'_> {
         let span = self.span_for(node);
         self.errors.push(Error { kind, span })
     }
-
-    /// Convert a non-dynamic string expression to a string if possible.
-    fn expr_static_str(&self, node: &ast::Str) -> Option<String> {
-        let mut parts = node.normalized_parts();
-
-        if parts.len() != 1 {
-            return None;
-        }
-
-        if let Some(ast::InterpolPart::Literal(lit)) = parts.pop() {
-            return Some(lit);
-        }
-
-        None
-    }
-
-    /// Convert the provided `ast::Attr` into a statically known
-    /// string if possible.
-    // TODO(tazjin): these should probably be SmolStr
-    fn expr_static_attr_str(&self, node: &ast::Attr) -> Option<String> {
-        match node {
-            ast::Attr::Ident(ident) => Some(ident.ident_token().unwrap().text().into()),
-            ast::Attr::Str(s) => self.expr_static_str(s),
-
-            // The dynamic node type is just a wrapper. C++ Nix does not
-            // care about the dynamic wrapper when determining whether the
-            // node itself is dynamic, it depends solely on the expression
-            // inside (i.e. `let ${"a"} = 1; in a` is valid).
-            ast::Attr::Dynamic(ref dynamic) => match dynamic.expr().unwrap() {
-                ast::Expr::Str(s) => self.expr_static_str(&s),
-                _ => None,
-            },
-        }
-    }
-
-    /// Construct the error returned when a dynamic attribute is found
-    /// in a `let`-expression.
-    fn dynamic_key_error<N>(&self, node: &N) -> Error
-    where
-        N: ToSpan,
-    {
-        Error {
-            kind: ErrorKind::DynamicKeyInLet,
-            span: self.span_for(node),
-        }
-    }
-
-    /// Convert a single identifier path fragment of a let binding to
-    /// a string if possible, or raise an error about the node being
-    /// dynamic.
-    fn binding_name(&self, node: ast::Attr) -> EvalResult<String> {
-        self.expr_static_attr_str(&node)
-            .ok_or_else(|| self.dynamic_key_error(&node))
-    }
-
-    /// Normalises identifier fragments into a single string vector
-    /// for `let`-expressions; fails if fragments requiring dynamic
-    /// computation are encountered.
-    fn normalise_ident_path<I: Iterator<Item = ast::Attr>>(
-        &self,
-        path: I,
-    ) -> EvalResult<Vec<String>> {
-        path.map(|node| self.binding_name(node)).collect()
-    }
 }
 
 /// Perform tail-call optimisation if the last call within a