//! This module implements the scope-tracking logic of the Tvix
//! compiler.
//!
//! Scoping in Nix is fairly complicated, there are features like
//! mutually recursive bindings, `with`, upvalue capturing, builtin
//! poisoning and so on that introduce a fair bit of complexity.
//!
//! Tvix attempts to do as much of the heavy lifting of this at
//! compile time, and leave the runtime to mostly deal with known
//! stack indices. To do this, the compiler simulates where locals
//! will be at runtime using the data structures implemented here.
use std::{
collections::{hash_map, HashMap},
ops::Index,
};
use smol_str::SmolStr;
use crate::opcode::{StackIdx, UpvalueIdx};
#[derive(Debug)]
enum LocalName {
/// Normally declared local with a statically known name.
Ident(String),
/// Phantom stack value (e.g. attribute set used for `with`) that
/// must be accounted for to calculate correct stack offsets.
Phantom,
}
/// Represents a single local already known to the compiler.
#[derive(Debug)]
pub struct Local {
// Identifier of this local. This is always a statically known
// value (Nix does not allow dynamic identifier names in locals),
// or a "phantom" value not accessible by users.
name: LocalName,
// Source span at which this local was declared.
pub span: codemap::Span,
// Scope depth of this local.
pub depth: usize,
// Is this local initialised?
pub initialised: bool,
// Is this local known to have been used at all?
pub used: bool,
// Does this local need to be finalised after the enclosing scope
// is completely constructed?
pub needs_finaliser: bool,
}
impl Local {
/// Does this local live above the other given depth?
pub fn above(&self, theirs: usize) -> bool {
self.depth > theirs
}
/// Does the name of this local match the given string?
pub fn has_name(&self, other: &str) -> bool {
match &self.name {
LocalName::Ident(name) => name == other,
// Phantoms are *never* accessible by a name.
LocalName::Phantom => false,
}
}
/// Is this local intentionally ignored? (i.e. name starts with `_`)
pub fn is_ignored(&self) -> bool {
match &self.name {
LocalName::Ident(name) => name.starts_with('_'),
LocalName::Phantom => false,
}
}
}
/// Represents the current position of a local as resolved in a scope.
pub enum LocalPosition {
/// Local is not known in this scope.
Unknown,
/// Local is known at the given local index.
Known(LocalIdx),
/// Local is known, but is being accessed recursively within its
/// own initialisation. Depending on context, this is either an
/// error or forcing a closure/thunk.
Recursive(LocalIdx),
}
/// Represents the different ways in which upvalues can be captured in
/// closures or thunks.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum UpvalueKind {
/// This upvalue captures a local from the stack.
Local(LocalIdx),
/// This upvalue captures an enclosing upvalue.
Upvalue(UpvalueIdx),
/// This upvalue captures a dynamically resolved value (i.e.
/// `with`).
///
/// It stores the identifier with which to perform a dynamic
/// lookup, as well as the optional upvalue index in the enclosing
/// function (if any).
Dynamic {
name: SmolStr,
up: Option<UpvalueIdx>,
},
}
#[derive(Clone, Debug)]
pub struct Upvalue {
pub kind: UpvalueKind,
pub node: rnix::ast::Ident,
}
/// Represents the index of a local in the scope's local array, which
/// is subtly different from its `StackIdx` (which excludes
/// uninitialised values in between).
#[repr(transparent)]
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
pub struct LocalIdx(usize);
/// Represents a scope known during compilation, which can be resolved
/// directly to stack indices.
#[derive(Debug, Default)]
pub struct Scope {
pub locals: Vec<Local>,
pub upvalues: Vec<Upvalue>,
// How many scopes "deep" are these locals?
pub scope_depth: usize,
// Current size of the `with`-stack at runtime.
with_stack_size: usize,
// Users are allowed to override globally defined symbols like
// `true`, `false` or `null` in scopes. We call this "scope
// poisoning", as it requires runtime resolution of those tokens.
//
// To support this efficiently, the depth at which a poisoning
// occured is tracked here.
poisoned_tokens: HashMap<&'static str, usize>,
}
impl Index<LocalIdx> for Scope {
type Output = Local;
fn index(&self, index: LocalIdx) -> &Self::Output {
&self.locals[index.0]
}
}
impl Scope {
/// Mark a globally defined token as poisoned.
pub fn poison(&mut self, name: &'static str, depth: usize) {
match self.poisoned_tokens.entry(name) {
hash_map::Entry::Occupied(_) => {
/* do nothing, as the token is already poisoned at a
* lower scope depth */
}
hash_map::Entry::Vacant(entry) => {
entry.insert(depth);
}
}
}
/// Inherit scope details from a parent scope (required for
/// correctly nesting scopes in lambdas and thunks when special
/// scope features like poisoning are present).
pub fn inherit(&self) -> Self {
Self {
poisoned_tokens: self.poisoned_tokens.clone(),
scope_depth: self.scope_depth,
..Default::default()
}
}
/// Check whether a given token is poisoned.
pub fn is_poisoned(&self, name: &str) -> bool {
self.poisoned_tokens.contains_key(name)
}
/// "Unpoison" tokens that were poisoned at a given depth. Used
/// when scopes are closed.
pub fn unpoison(&mut self, depth: usize) {
self.poisoned_tokens
.retain(|_, poisoned_at| *poisoned_at != depth);
}
/// Increase the `with`-stack size of this scope.
pub fn push_with(&mut self) {
self.with_stack_size += 1;
}
/// Decrease the `with`-stack size of this scope.
pub fn pop_with(&mut self) {
self.with_stack_size -= 1;
}
/// Does this scope currently require dynamic runtime resolution
/// of identifiers that could not be found?
pub fn has_with(&self) -> bool {
self.with_stack_size > 0
}
/// Resolve the stack index of a statically known local.
pub fn resolve_local(&mut self, name: &str) -> LocalPosition {
for (idx, local) in self.locals.iter_mut().enumerate().rev() {
if local.has_name(name) {
local.used = true;
// This local is still being initialised, meaning that
// we know its final runtime stack position, but it is
// not yet on the stack.
if !local.initialised {
return LocalPosition::Recursive(LocalIdx(idx));
}
return LocalPosition::Known(LocalIdx(idx));
}
}
LocalPosition::Unknown
}
/// Declare a local variable that occupies a stack slot and should
/// be accounted for, but is not directly accessible by users
/// (e.g. attribute sets used for `with`).
pub fn declare_phantom(&mut self, span: codemap::Span) -> LocalIdx {
let idx = self.locals.len();
self.locals.push(Local {
name: LocalName::Phantom,
span,
depth: self.scope_depth,
initialised: false,
needs_finaliser: false,
used: true,
});
LocalIdx(idx)
}
/// Declare an uninitialised local variable.
pub fn declare_local(&mut self, name: String, span: codemap::Span) -> LocalIdx {
let idx = self.locals.len();
self.locals.push(Local {
name: LocalName::Ident(name),
span,
depth: self.scope_depth,
initialised: false,
needs_finaliser: false,
used: false,
});
LocalIdx(idx)
}
/// Mark local as initialised after compiling its expression.
pub fn mark_initialised(&mut self, idx: LocalIdx) {
self.locals[idx.0].initialised = true;
}
/// Mark local as needing a finaliser.
pub fn mark_needs_finaliser(&mut self, idx: LocalIdx) {
self.locals[idx.0].needs_finaliser = true;
}
/// Compute the runtime stack index for a given local by
/// accounting for uninitialised variables at scopes below this
/// one.
pub fn stack_index(&self, idx: LocalIdx) -> StackIdx {
let uninitialised_count = self.locals[..(idx.0)]
.iter()
.filter(|l| !l.initialised && self[idx].above(l.depth))
.count();
StackIdx(idx.0 - uninitialised_count)
}
}
