Move variable resolution to middleend

- Introduce AST-based visitor and resolver in middleend
- Update guard field and parser to use AstNode<Expr>
- Remove backend variable_resolution and adjust exports
- Expose middleend in the library

src/middleend/variable_resolution.rs

@@ -0,0 +1,99 @@
+use std::collections::HashMap;
+
+use crate::{
+    backend::environment::EnvironmentStack,
+    common::{
+        ast::{AstNode, Expr, Stmt},
+        lox_result::{LoxError, LoxResult},
+    },
+    frontend::source_registry::SourceSlice,
+    middleend::visit_ast::{walk_expr, walk_stmt, Visitor},
+};
+
+struct Resolver {
+    scopes: EnvironmentStack<bool>,
+    locals: HashMap<SourceSlice, usize>,
+}
+
+impl Resolver {
+    pub fn new() -> Self {
+        Resolver {
+            scopes: EnvironmentStack::new(),
+            locals: HashMap::new(),
+        }
+    }
+
+    fn declare(&mut self, name: &String) {
+        if self.scopes.is_empty() {
+            return;
+        }
+        let _ = self.scopes.set(name.clone(), false);
+    }
+
+    fn define(&mut self, name: &String) {
+        if self.scopes.is_empty() {
+            return;
+        }
+        let _ = self.scopes.set(name.clone(), true);
+    }
+
+    fn resolve_local(&mut self, name: &String) {
+        let depth = self.scopes.depth();
+        for i in (0..depth).rev() {
+            if self.scopes.scope_contains(i, name) {
+                self.locals.insert(, i);
+            }
+        }
+    }
+}
+
+impl Visitor for Resolver {
+    fn visit_stmt(&mut self, stmt: &AstNode<Stmt>) -> LoxResult<()> {
+        match &stmt.node {
+            Stmt::VarDeclaration { name, .. } => {
+                self.declare(name);
+                // `walk_stmt` resolves the initializer (if present).
+                walk_stmt(self, stmt)?;
+                self.define(name);
+                Ok(())
+            }
+            Stmt::VarAssigment { name, .. } => {
+                match self.scopes.get(name) {
+                    Ok(true) => (),
+                    Ok(false) => {
+                        return Err(LoxError::RuntimeError {
+                            source_slice: SourceSlice::default(),
+                            message: "Cant read local variable in it own lintilizer".to_string(),
+                        })
+                    }
+                    Err(err) => return Err(err),
+                }
+                // `walk_stmt` resolves the assigned value.
+                walk_stmt(self, stmt)
+            }
+            Stmt::Block { .. } => {
+                self.scopes.push_new_scope();
+                walk_stmt(self, stmt)?;
+                self.scopes.pop_scope();
+                Ok(())
+            }
+            // Expression, Print, Return, If, While, For: default traversal.
+            _ => walk_stmt(self, stmt),
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &AstNode<Expr>) -> LoxResult<()> {
+        match &expr.node {
+            Expr::Identifier { name, .. } => {
+                if !self.scopes.is_empty() && self.scopes.get(name).is_ok() {
+                    return Err(LoxError::ParseError {
+                        source_slice: SourceSlice::default(),
+                        message: "Cant read local varialbe in it own initializer".to_string(),
+                    });
+                }
+                Ok(())
+            }
+            _ => walk_expr(self, expr),
+        }
+    }
+}