VariableLookup.h

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/// \file
/// \brief Lookup variable names, from it's parent scope.
///
/// This file declares a variable-lookup analysis on AST.
/// We do variable lookup for liveness checking, and emit diagnostics
/// like "unused with", or "undefined variable".
/// The implementation aims to be consistent with C++ nix (NixOS/nix).
 
#pragma once
 
#include "nixf/Basic/Diagnostic.h"
#include "nixf/Basic/Nodes/Attrs.h"
#include "nixf/Basic/Nodes/Basic.h"
#include "nixf/Basic/Nodes/Expr.h"
#include "nixf/Basic/Nodes/Lambda.h"
#include "nixf/Basic/Nodes/Simple.h"
 
#include <map>
#include <memory>
#include <string>
#include <vector>
 
namespace nixf {
 
/// \brief Represents a definition
class Definition {
public:
  /// \brief "Source" information so we can know where the def comes from.
  enum DefinitionSource {
    /// \brief From with <expr>;
    DS_With,
 
    /// \brief From let ... in ...
    DS_Let,
 
    /// \brief From ambda arg e.g.  a: a + 1
    DS_LambdaArg,
 
    /// \brief From lambda (noarg) formal, e.g. { a }: a + 1
    DS_LambdaNoArg_Formal,
 
    /// \brief From lambda (with `@arg`) `arg`,
    /// e.g. `a` in `{ foo }@a: foo + 1`
    DS_LambdaWithArg_Arg,
 
    /// \brief From lambda (with `@arg`) formal,
    /// e.g. `foo` in `{ foo }@a: foo + 1`
    DS_LambdaWithArg_Formal,
 
    /// \brief From recursive attribute set.  e.g. rec { }
    DS_Rec,
 
    /// \brief Builtin names.
    DS_Builtin,
  };
 
private:
  std::vector<const ExprVar *> Uses;
  const Node *Syntax;
  DefinitionSource Source;
 
public:
  Definition(const Node *Syntax, DefinitionSource Source)
      : Syntax(Syntax), Source(Source) {}
  Definition(std::vector<const ExprVar *> Uses, const Node *Syntax,
             DefinitionSource Source)
      : Uses(std::move(Uses)), Syntax(Syntax), Source(Source) {}
 
  [[nodiscard]] const Node *syntax() const { return Syntax; }
 
  [[nodiscard]] const std::vector<const ExprVar *> &uses() const {
    return Uses;
  }
 
  [[nodiscard]] DefinitionSource source() const { return Source; }
 
  void usedBy(const ExprVar &User) { Uses.emplace_back(&User); }
 
  [[nodiscard]] bool isBuiltin() const { return Source == DS_Builtin; }
};
 
/// \brief A set of variable definitions, which may inherit parent environment.
class EnvNode {
public:
  using DefMap = std::map<std::string, std::shared_ptr<Definition>>;
 
private:
  const std::shared_ptr<EnvNode> Parent; // Points to the parent node.
 
  DefMap Defs; // Definitions.
 
  const Node *Syntax;
 
public:
  EnvNode(std::shared_ptr<EnvNode> Parent, DefMap Defs, const Node *Syntax)
      : Parent(std::move(Parent)), Defs(std::move(Defs)), Syntax(Syntax) {}
 
  [[nodiscard]] EnvNode *parent() const { return Parent.get(); }
 
  /// \brief Where this node comes from.
  [[nodiscard]] const Node *syntax() const { return Syntax; }
 
  [[nodiscard]] bool isWith() const {
    return Syntax && Syntax->kind() == Node::NK_ExprWith;
  }
 
  [[nodiscard]] const DefMap &defs() const { return Defs; }
 
  [[nodiscard]] bool isLive() const;
};
 
class VariableLookupAnalysis {
public:
  enum class LookupResultKind {
    Undefined,
    FromWith,
    Defined,
    NoSuchVar,
  };
 
  struct LookupResult {
    LookupResultKind Kind;
    std::shared_ptr<const Definition> Def;
  };
 
  using ToDefMap = std::map<const Node *, std::shared_ptr<Definition>>;
  using EnvMap = std::map<const Node *, std::shared_ptr<EnvNode>>;
 
private:
  std::vector<Diagnostic> &Diags;
 
  std::map<const Node *, std::shared_ptr<Definition>>
      WithDefs; // record with ... ; users.
 
  ToDefMap ToDef;
 
  // Record the environment so that we can know which names are available after
  // name lookup, for later references like code completions.
  EnvMap Envs;
 
  void lookupVar(const ExprVar &Var, const std::shared_ptr<EnvNode> &Env);
 
  void checkBuiltins(const ExprSelect &Sel);
 
  std::shared_ptr<EnvNode> dfsAttrs(const SemaAttrs &SA,
                                    const std::shared_ptr<EnvNode> &Env,
                                    const Node *Syntax,
                                    Definition::DefinitionSource Source);
 
  void emitEnvLivenessWarning(const std::shared_ptr<EnvNode> &NewEnv);
 
  void dfsDynamicAttrs(const std::vector<Attribute> &DynamicAttrs,
                       const std::shared_ptr<EnvNode> &Env);
 
  // "dfs" is an abbreviation of "Deep-First-Search".
  void dfs(const ExprLambda &Lambda, const std::shared_ptr<EnvNode> &Env);
  void dfs(const ExprAttrs &Attrs, const std::shared_ptr<EnvNode> &Env);
  void dfs(const ExprLet &Let, const std::shared_ptr<EnvNode> &Env);
  void dfs(const ExprWith &With, const std::shared_ptr<EnvNode> &Env);
 
  void dfs(const Node &Root, const std::shared_ptr<EnvNode> &Env);
 
  void trivialDispatch(const Node &Root, const std::shared_ptr<EnvNode> &Env);
 
  std::map<const ExprVar *, LookupResult> Results;
 
  /// \brief For variables resolved from `with` scopes, track all enclosing
  /// `with` expressions that could potentially provide the binding.
  /// This is needed to detect indirect nested `with` scenarios where
  /// converting an outer `with` to `let/inherit` could change semantics.
  std::map<const ExprVar *, std::vector<const ExprWith *>> VarWithScopes;
 
public:
  VariableLookupAnalysis(std::vector<Diagnostic> &Diags);
 
  /// \brief Perform variable lookup analysis (def-use) on AST.
  /// \note This method should be invoked after any other method called.
  /// \note The result remains immutable thus it can be shared among threads.
  void runOnAST(const Node &Root);
 
  /// \brief Query the which name/with binds to specific varaible.
  [[nodiscard]] LookupResult query(const ExprVar &Var) const {
    if (!Results.contains(&Var))
      return {.Kind = LookupResultKind::NoSuchVar};
    return Results.at(&Var);
  }
 
  /// \brief Get definition record for some name.
  ///
  /// For some cases, we need to get "definition" record to find all references
  /// to this definition, on AST.
  ///
  /// Thus we need to store AST -> Definition
  /// There are many pointers on AST, the convention is:
  ///
  ///   1. attrname "key" syntax is recorded.
  //        For static attrs, they are Node::NK_AttrName.
  ///   2. "with" keyword is recorded.
  ///   3. Lambda arguments, record its identifier.
  [[nodiscard]] const Definition *toDef(const Node &N) const {
    if (ToDef.contains(&N))
      return ToDef.at(&N).get();
    return nullptr;
  }
 
  const EnvNode *env(const Node *N) const;
 
  /// \brief Get all `with` expressions that could provide the binding for a
  /// variable.
  ///
  /// For variables resolved from `with` scopes, multiple enclosing `with`
  /// expressions may potentially provide the binding. This method returns
  /// all such `with` expressions, ordered from innermost to outermost.
  ///
  /// This is useful for detecting indirect nested `with` scenarios where
  /// converting an outer `with` to `let/inherit` could change semantics.
  ///
  /// \param Var The variable to query.
  /// \return A vector of `ExprWith` pointers representing all `with` scopes
  ///         that could provide the variable's binding. Returns an empty
  ///         vector if the variable is not resolved from a `with` scope.
  [[nodiscard]] std::vector<const ExprWith *>
  getWithScopes(const ExprVar &Var) const {
    auto It = VarWithScopes.find(&Var);
    if (It != VarWithScopes.end())
      return It->second;
    return {};
  }
};
 
} // namespace nixf