From a6f0e87fa8c848ca16e38c6a0dc03518acfd2625 Mon Sep 17 00:00:00 2001
From: Raito Bezarius <raito@lix.systems>
Date: Fri, 21 Nov 2025 17:36:35 +0100
Subject: [PATCH] libexpr/static-analyzer: init

Change-Id: I606718fbe33db1f6c00ca34f45c6553ce468398c
Signed-off-by: Raito Bezarius <raito@lix.systems>
---
 lix/libexpr/nixexpr.cc | 75 ++++++++++++++++++++++++++++++++++++++++++
 lix/libexpr/nixexpr.hh | 28 ++++++++++++++++
 2 files changed, 103 insertions(+)

diff --git a/lix/libexpr/nixexpr.cc b/lix/libexpr/nixexpr.cc
index ec23c0a0c..a0a31b56d 100644
--- a/lix/libexpr/nixexpr.cc
+++ b/lix/libexpr/nixexpr.cc
@@ -321,6 +321,74 @@ JSON printAttrPathToJson(const SymbolTable & symbols, const AttrPath & attrPath)
 /* Computing levels/displacements for variables. */
 
 namespace {
+// This is a one-pass static analyzer for
+// various topics.
+struct StaticAnalyzer : ExprVisitor
+{
+    std::set<Symbol> staticallyUsedVariables;
+    bool usedDynamicVariables = false;
+
+    StaticAnalyzer() {}
+    using ExprVisitor::visit;
+
+    void visit(ExprDebugFrame & e, std::unique_ptr<Expr> & ptr) override
+    {
+        visit(e.inner);
+    }
+    void visit(ExprLiteral & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprVar & e, std::unique_ptr<Expr> & ptr) override
+    {
+        staticallyUsedVariables.insert(e.name);
+    }
+    void visit(ExprInheritFrom & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprSelect & e, std::unique_ptr<Expr> & ptr) override
+    {
+        if (e.isDynamic()) {
+            usedDynamicVariables = true;
+        }
+
+        visit(e.def);
+        visit(e.e);
+    }
+    void visit(ExprOpHasAttr & e, std::unique_ptr<Expr> & ptr) override
+    {
+        if (e.isDynamic()) {
+            usedDynamicVariables = true;
+        }
+
+        visit(e.e);
+    }
+    void visit(ExprSet & e, std::unique_ptr<Expr> & ptr) override
+    {
+        // TODO: oh bro, we need to analyze dynamic attributes for their value expressions.
+    }
+    void visit(ExprList & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprLambda & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprCall & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprLet & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprWith & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprIf & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprAssert & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprOpNot & e, std::unique_ptr<Expr> & ptr) override {}
+#define BINOP(type)                                            \
+    /* NOLINTNEXTLINE(bugprone-macro-parentheses) */           \
+    void visit(type & e, std::unique_ptr<Expr> & ptr) override \
+    {                                                          \
+        visit(e.e1);                                           \
+        visit(e.e2);                                           \
+    }
+    BINOP(ExprOpEq)
+    BINOP(ExprOpNEq)
+    BINOP(ExprOpAnd)
+    BINOP(ExprOpOr)
+    BINOP(ExprOpImpl)
+    BINOP(ExprOpUpdate)
+    BINOP(ExprOpConcatLists)
+#undef BINOP
+    void visit(ExprConcatStrings & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprPos & e, std::unique_ptr<Expr> & ptr) override {}
+    void visit(ExprBlackHole & e, std::unique_ptr<Expr> & ptr) override {}
+};
 struct VarBinder : ExprVisitor
 {
     Evaluator & es;
@@ -590,6 +658,13 @@ void VarBinder::visit(ExprLambda & e, std::unique_ptr<Expr> & ptr)
 {
     withEnv(e.pattern->buildEnv(env.get()), [&] {
         e.pattern->accept(*this);
+        /* TODO: If statically, e.body makes only use of some parameters and not the whole scope.
+         * We shouldn't have to keep around all the environment data which might contain trapped
+         * pointers. Analyze `e.body` and return its statically known set of used variables.
+         * */
+        DirectCallAnalyzer analyzer{es, env};
+        analyzer.visit(e.body);
+        e.shortcut = analyzer.shortcut;
         visit(e.body);
     });
 }
diff --git a/lix/libexpr/nixexpr.hh b/lix/libexpr/nixexpr.hh
index 1fc708a5e..7be86fb7d 100644
--- a/lix/libexpr/nixexpr.hh
+++ b/lix/libexpr/nixexpr.hh
@@ -292,6 +292,17 @@ struct ExprSelect : Expr
     /** The path of attributes being selected. e.g. `bar.baz` in `foo.bar.baz.` */
     AttrPath attrPath;
 
+    bool isDynamic() const
+    {
+        for (auto & name : attrPath) {
+            if (name.expr) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     ExprSelect(const PosIdx & pos, std::unique_ptr<Expr> e, AttrPath attrPath, std::unique_ptr<Expr> def) : Expr(pos), e(std::move(e)), def(std::move(def)), attrPath(std::move(attrPath)) { };
     ExprSelect(const PosIdx & pos, std::unique_ptr<Expr> e, const PosIdx namePos, Symbol name) : Expr(pos), e(std::move(e)) { attrPath.push_back(AttrName(namePos, name)); };
     JSON toJSON(const SymbolTable & symbols) const override;
@@ -303,6 +314,16 @@ struct ExprOpHasAttr : Expr
 {
     std::unique_ptr<Expr> e;
     AttrPath attrPath;
+
+    bool isDynamic() const
+    {
+        for (auto & name : attrPath) {
+            if (name.expr) {
+                return true;
+            }
+        }
+        return false;
+    }
     ExprOpHasAttr(const PosIdx & pos, std::unique_ptr<Expr> e, AttrPath attrPath) : Expr(pos), e(std::move(e)), attrPath(std::move(attrPath)) { };
     JSON toJSON(const SymbolTable & symbols) const override;
     void eval(EvalState & state, Env & env, Value & v) override;
@@ -479,6 +500,13 @@ struct ExprLambda : Expr
     Symbol name;
     std::unique_ptr<Pattern> pattern;
     std::unique_ptr<Expr> body;
+    // This is a shortcut variant which
+    // exhausts the body further lambda constructions
+    // to transform x1: x2: …: xn: b
+    // into { x1, …, xn }: b
+    // This can be used when you know that you are
+    // passing all the arguments at once.
+    std::unique_ptr<ExprLambda> shortcut;
     ExprLambda(PosIdx pos, std::unique_ptr<Pattern> pattern, std::unique_ptr<Expr> body)
         : Expr(pos), pattern(std::move(pattern)), body(std::move(body))
     {