feat: add C code generation backend and build command to CLI

Add C backend to nxc-frontend:
- Implement CEmitter with HIR-to-C translation
- Add emit_c() function for code generation
- Support function prototypes, statements, expressions, and control flow
- Generate C11-compatible code with proper type mapping
- Add main() wrapper generation for entry points

Extend driver crate with build pipeline:
- Add emit_c_from_frontend() to generate C source from HIR
- Add write_c_file() to write generated

crates/nxc-cli/Cargo.toml

@@ -18,3 +18,4 @@ path = "src/bin/nexacore.rs"
 
 [dependencies]
 nxc-driver = { path = "../nxc-driver" }
+nxc-frontend = { path = "../nxc-frontend" }

crates/nxc-cli/src/lib.rs

@@ -1,4 +1,5 @@
 use std::env;
+use std::fs;
 use std::path::Path;
 use std::process::ExitCode;
 
@@ -20,7 +21,7 @@ fn run() -> Result<(), String> {
     };
 
     match command.as_str() {
-        "check" | "build" => {
+        "check" => {
             let Some(path) = args.next() else {
                 return Err(format!("usage: {} {command} <file.nx>", executable_name()));
             };
@@ -44,6 +45,44 @@ fn run() -> Result<(), String> {
             println!("structs: {}", summary.structs);
             Ok(())
         }
+        "emit-c" => {
+            let Some(path) = args.next() else {
+                return Err(format!("usage: {} emit-c <file.nx> -o <out.c>", executable_name()));
+            };
+            let out_path = parse_output_path(args.collect(), "emit-c", "out.c")?;
+            let output = nxc_driver::check_file(Path::new(&path)).map_err(format_driver_error)?;
+            if output.has_errors() {
+                eprintln!("{}", output.render_diagnostics());
+                return Err(format!(
+                    "emit-c failed with {} diagnostic(s)",
+                    output.diagnostics.len()
+                ));
+            }
+
+            let c_source = nxc_driver::emit_c_from_frontend(&output)
+                .map_err(|diagnostics| render_backend_diagnostics(&output, diagnostics))?;
+            fs::write(&out_path, c_source).map_err(|err| format!("io error: {err}"))?;
+            println!("wrote {}", out_path.display());
+            Ok(())
+        }
+        "build" => {
+            let Some(path) = args.next() else {
+                return Err(format!("usage: {} build <file.nx> -o <binary>", executable_name()));
+            };
+            let out_path = parse_output_path(args.collect(), "build", "a.out")?;
+            let output = nxc_driver::check_file(Path::new(&path)).map_err(format_driver_error)?;
+            if output.has_errors() {
+                eprintln!("{}", output.render_diagnostics());
+                return Err(format!(
+                    "build failed with {} diagnostic(s)",
+                    output.diagnostics.len()
+                ));
+            }
+
+            nxc_driver::build_binary(&output, &out_path).map_err(format_driver_error)?;
+            println!("built {}", out_path.display());
+            Ok(())
+        }
         "run" => Err("runtime execution is not implemented yet".to_string()),
         "new" => Err("project scaffolding is not implemented yet".to_string()),
         "test" => Err("test runner is not implemented yet".to_string()),
@@ -68,15 +107,61 @@ fn executable_name() -> String {
 fn format_driver_error(error: nxc_driver::DriverError) -> String {
     match error {
         nxc_driver::DriverError::Io(io) => format!("io error: {io}"),
+        nxc_driver::DriverError::ToolNotFound { tool_names } => format!(
+            "no supported C compiler found in PATH; tried: {}",
+            tool_names.join(", ")
+        ),
+        nxc_driver::DriverError::BuildFailed {
+            tool,
+            status,
+            stderr,
+        } => format!(
+            "build step `{tool}` failed{}{}",
+            status
+                .map(|code| format!(" with exit code {code}"))
+                .unwrap_or_default(),
+            if stderr.trim().is_empty() {
+                String::new()
+            } else {
+                format!(":\n{stderr}")
+            }
+        ),
     }
 }
 
+fn render_backend_diagnostics(
+    output: &nxc_driver::FrontendOutput,
+    diagnostics: Vec<nxc_frontend::Diagnostic>,
+) -> String {
+    diagnostics
+        .iter()
+        .map(|diagnostic| diagnostic.render(&output.path.display().to_string(), &output.source))
+        .collect::<Vec<_>>()
+        .join("\n\n")
+}
+
+fn parse_output_path(args: Vec<String>, command: &str, default_name: &str) -> Result<std::path::PathBuf, String> {
+    if args.is_empty() {
+        return Ok(std::path::PathBuf::from(default_name));
+    }
+
+    if args.len() == 2 && args[0] == "-o" {
+        return Ok(std::path::PathBuf::from(&args[1]));
+    }
+
+    Err(format!(
+        "usage: {} {command} <file.nx> -o <output>",
+        executable_name()
+    ))
+}
+
 fn print_help() {
     let name = executable_name();
     println!("NexaCore CLI");
     println!("usage:");
     println!("  {name} check <file.nx>");
-    println!("  {name} build <file.nx>");
+    println!("  {name} emit-c <file.nx> -o <out.c>");
+    println!("  {name} build <file.nx> -o <binary>");
     println!("  {name} run <file.nx>");
     println!("  {name} new <name>");
     println!("  {name} test");

crates/nxc-driver/src/lib.rs

@@ -1,10 +1,12 @@
 use std::fmt::Write;
 use std::fs;
+use std::process::Command;
 use std::path::{Path, PathBuf};
+use std::time::{SystemTime, UNIX_EPOCH};
 
 use nxc_frontend::{
-    analyze, has_errors, Diagnostic, Item, LexResult, Lexer, Module, ParseResult, Parser, Token,
+    analyze, emit_c, has_errors, lower_to_hir, BackendResult, Diagnostic, HirModule, Item,
-    TypedModule,
+    LexResult, Lexer, Module, ParseResult, Parser, Token, TypedModule,
 };
 
 #[derive(Debug, Clone)]
@@ -14,12 +16,21 @@ pub struct FrontendOutput {
     pub tokens: Vec<Token>,
     pub module: Module,
     pub typed_module: Option<TypedModule>,
+    pub hir_module: Option<HirModule>,
     pub diagnostics: Vec<Diagnostic>,
 }
 
 #[derive(Debug)]
 pub enum DriverError {
     Io(std::io::Error),
+    ToolNotFound {
+        tool_names: Vec<String>,
+    },
+    BuildFailed {
+        tool: String,
+        status: Option<i32>,
+        stderr: String,
+    },
 }
 
 impl From<std::io::Error> for DriverError {
@@ -89,11 +100,21 @@ pub fn check_source(path: PathBuf, source: String) -> FrontendOutput {
 
     lexer_diagnostics.extend(parser_diagnostics);
     let mut typed_module = None;
+    let mut hir_module = None;
 
     if !has_errors(&lexer_diagnostics) {
         let semantic = analyze(&module);
         lexer_diagnostics.extend(semantic.diagnostics);
-        typed_module = Some(semantic.module);
+        if !has_errors(&lexer_diagnostics) {
+            let lowering = lower_to_hir(&semantic.module);
+            lexer_diagnostics.extend(lowering.diagnostics);
+            if !has_errors(&lexer_diagnostics) {
+                hir_module = Some(lowering.module.clone());
+            }
+            typed_module = Some(semantic.module);
+        } else {
+            typed_module = Some(semantic.module);
+        }
     }
 
     FrontendOutput {
@@ -102,6 +123,96 @@ pub fn check_source(path: PathBuf, source: String) -> FrontendOutput {
         tokens,
         module,
         typed_module,
+        hir_module,
         diagnostics: lexer_diagnostics,
     }
 }
+
+pub fn emit_c_from_frontend(frontend: &FrontendOutput) -> Result<String, Vec<Diagnostic>> {
+    let Some(hir_module) = &frontend.hir_module else {
+        return Err(vec![Diagnostic::error(
+            "HIR is unavailable; run `check` diagnostics first",
+            frontend.module.span,
+        )]);
+    };
+
+    let BackendResult { code, diagnostics } = emit_c(hir_module);
+    if diagnostics.is_empty() {
+        Ok(code.unwrap_or_default())
+    } else {
+        Err(diagnostics)
+    }
+}
+
+pub fn write_c_file(frontend: &FrontendOutput, out_path: impl AsRef<Path>) -> Result<(), DriverError> {
+    let out_path = out_path.as_ref();
+    let code = emit_c_from_frontend(frontend)
+        .map_err(|diagnostics| DriverError::BuildFailed {
+            tool: "c-backend".to_string(),
+            status: None,
+            stderr: diagnostics
+                .iter()
+                .map(|diagnostic| diagnostic.render(&frontend.path.display().to_string(), &frontend.source))
+                .collect::<Vec<_>>()
+                .join("\n\n"),
+        })?;
+
+    fs::write(out_path, code)?;
+    Ok(())
+}
+
+pub fn build_binary(frontend: &FrontendOutput, out_path: impl AsRef<Path>) -> Result<(), DriverError> {
+    let compiler = find_c_compiler().ok_or_else(|| DriverError::ToolNotFound {
+        tool_names: vec!["cc".to_string(), "clang".to_string(), "gcc".to_string()],
+    })?;
+
+    let c_source = emit_c_from_frontend(frontend)
+        .map_err(|diagnostics| DriverError::BuildFailed {
+            tool: "c-backend".to_string(),
+            status: None,
+            stderr: diagnostics
+                .iter()
+                .map(|diagnostic| diagnostic.render(&frontend.path.display().to_string(), &frontend.source))
+                .collect::<Vec<_>>()
+                .join("\n\n"),
+        })?;
+
+    let mut temp_path = std::env::temp_dir();
+    let stamp = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_nanos();
+    temp_path.push(format!("nexacore-build-{stamp}.c"));
+
+    fs::write(&temp_path, c_source)?;
+
+    let output = Command::new(&compiler)
+        .arg(&temp_path)
+        .arg("-std=c11")
+        .arg("-O2")
+        .arg("-o")
+        .arg(out_path.as_ref())
+        .output();
+
+    let _ = fs::remove_file(&temp_path);
+
+    let output = output?;
+    if output.status.success() {
+        Ok(())
+    } else {
+        Err(DriverError::BuildFailed {
+            tool: compiler,
+            status: output.status.code(),
+            stderr: String::from_utf8_lossy(&output.stderr).to_string(),
+        })
+    }
+}
+
+fn find_c_compiler() -> Option<String> {
+    for tool in ["cc", "clang", "gcc"] {
+        if Command::new(tool).arg("--version").output().is_ok() {
+            return Some(tool.to_string());
+        }
+    }
+    None
+}

crates/nxc-frontend/src/c_backend.rs

@@ -0,0 +1,346 @@
+use std::collections::HashMap;
+
+use crate::ast::{BinaryOp, Literal, UnaryOp};
+use crate::diagnostics::Diagnostic;
+use crate::hir::{
+    HirBlock, HirExpr, HirExprKind, HirFunction, HirModule, HirStmt, HirStmtKind, HirType,
+};
+use crate::token::Span;
+
+#[derive(Debug, Clone)]
+pub struct BackendResult {
+    pub code: Option<String>,
+    pub diagnostics: Vec<Diagnostic>,
+}
+
+pub fn emit_c(module: &HirModule) -> BackendResult {
+    CEmitter::new(module).emit()
+}
+
+struct CEmitter<'a> {
+    module: &'a HirModule,
+    diagnostics: Vec<Diagnostic>,
+    function_names: HashMap<crate::semantic::FunctionId, String>,
+    current_locals: HashMap<crate::semantic::LocalId, String>,
+}
+
+impl<'a> CEmitter<'a> {
+    fn new(module: &'a HirModule) -> Self {
+        let function_names = module
+            .functions
+            .iter()
+            .map(|function| (function.id, function.c_name.clone()))
+            .collect::<HashMap<_, _>>();
+
+        Self {
+            module,
+            diagnostics: Vec::new(),
+            function_names,
+            current_locals: HashMap::new(),
+        }
+    }
+
+    fn emit(mut self) -> BackendResult {
+        let mut code = String::new();
+        code.push_str("#include <stdbool.h>\n\n");
+
+        for function in &self.module.functions {
+            match self.emit_function_prototype(function) {
+                Some(prototype) => {
+                    code.push_str(&prototype);
+                    code.push_str(";\n");
+                }
+                None => {
+                    return BackendResult {
+                        code: None,
+                        diagnostics: self.diagnostics,
+                    };
+                }
+            }
+        }
+
+        if !self.module.functions.is_empty() {
+            code.push('\n');
+        }
+
+        for function in &self.module.functions {
+            if let Some(body) = self.emit_function(function) {
+                code.push_str(&body);
+                code.push('\n');
+            }
+        }
+
+        if let Some(wrapper) = self.emit_entrypoint_wrapper() {
+            code.push_str(&wrapper);
+        }
+
+        if self.diagnostics.is_empty() {
+            BackendResult {
+                code: Some(code),
+                diagnostics: self.diagnostics,
+            }
+        } else {
+            BackendResult {
+                code: None,
+                diagnostics: self.diagnostics,
+            }
+        }
+    }
+
+    fn emit_function_prototype(&mut self, function: &HirFunction) -> Option<String> {
+        let return_type = self.emit_type(&function.return_type, function.span)?;
+        let params = if function.params.is_empty() {
+            "void".to_string()
+        } else {
+            let mut rendered = Vec::new();
+            for param in &function.params {
+                let ty = self.emit_type(&param.local.ty, param.span)?;
+                rendered.push(format!("{ty} {}", param.local.c_name));
+            }
+            rendered.join(", ")
+        };
+
+        Some(format!("{return_type} {}({params})", function.c_name))
+    }
+
+    fn emit_function(&mut self, function: &HirFunction) -> Option<String> {
+        self.current_locals.clear();
+        for param in &function.params {
+            self.current_locals
+                .insert(param.local.id, param.local.c_name.clone());
+        }
+
+        let signature = self.emit_function_prototype(function)?;
+        let body = self.emit_block(&function.body, 0)?;
+
+        let mut out = String::new();
+        out.push_str(&signature);
+        out.push_str(" {\n");
+        out.push_str(&body);
+        out.push_str("}\n");
+        Some(out)
+    }
+
+    fn emit_block(&mut self, block: &HirBlock, indent: usize) -> Option<String> {
+        let mut out = String::new();
+        for statement in &block.statements {
+            out.push_str(&self.emit_statement(statement, indent + 1)?);
+        }
+        Some(out)
+    }
+
+    fn emit_statement(&mut self, stmt: &HirStmt, indent: usize) -> Option<String> {
+        let pad = "    ".repeat(indent);
+        match &stmt.kind {
+            HirStmtKind::Let { local, value } => {
+                self.current_locals.insert(local.id, local.c_name.clone());
+                let ty = self.emit_type(&local.ty, stmt.span)?;
+                let value = self.emit_expr(value)?;
+                Some(format!("{pad}{ty} {} = {value};\n", local.c_name))
+            }
+            HirStmtKind::Return(expr) => {
+                if let Some(expr) = expr {
+                    let expr = self.emit_expr(expr)?;
+                    Some(format!("{pad}return {expr};\n"))
+                } else {
+                    Some(format!("{pad}return;\n"))
+                }
+            }
+            HirStmtKind::Expr(expr) => {
+                let expr = self.emit_expr(expr)?;
+                Some(format!("{pad}{expr};\n"))
+            }
+            HirStmtKind::If {
+                condition,
+                then_block,
+                else_block,
+            } => {
+                let condition = self.emit_expr(condition)?;
+                let mut out = String::new();
+                out.push_str(&format!("{pad}if ({condition}) {{\n"));
+                out.push_str(&self.emit_block(then_block, indent)?);
+                out.push_str(&format!("{pad}}}"));
+                if let Some(else_block) = else_block {
+                    out.push_str(" else {\n");
+                    out.push_str(&self.emit_block(else_block, indent)?);
+                    out.push_str(&format!("{pad}}}\n"));
+                } else {
+                    out.push('\n');
+                }
+                Some(out)
+            }
+        }
+    }
+
+    fn emit_expr(&mut self, expr: &HirExpr) -> Option<String> {
+        match &expr.kind {
+            HirExprKind::Literal(literal) => self.emit_literal(literal),
+            HirExprKind::Local(local_id) => {
+                let Some(name) = self.current_locals.get(local_id).cloned() else {
+                    self.error(
+                        "unknown lowered local during C emission",
+                        expr.span,
+                    );
+                    return None;
+                };
+                Some(name)
+            }
+            HirExprKind::Function(function_id) => {
+                self.error(
+                    "first-class function values are not supported in the C backend",
+                    expr.span,
+                );
+                let fallback = self
+                    .function_names
+                    .get(function_id)
+                    .cloned()
+                    .unwrap_or_else(|| "/* invalid_fn */".to_string());
+                Some(fallback)
+            }
+            HirExprKind::Unary { op, expr } => {
+                let inner = self.emit_expr(expr)?;
+                let op = match op {
+                    UnaryOp::Negate => "-",
+                    UnaryOp::Not => "!",
+                };
+                Some(format!("({op}{inner})"))
+            }
+            HirExprKind::Binary { left, op, right } => {
+                if matches!(left.ty, HirType::String) || matches!(right.ty, HirType::String) {
+                    self.error(
+                        "string binary operations are not supported in the C backend yet",
+                        expr.span,
+                    );
+                    return None;
+                }
+
+                let left = self.emit_expr(left)?;
+                let right = self.emit_expr(right)?;
+                let op = match op {
+                    BinaryOp::Multiply => "*",
+                    BinaryOp::Divide => "/",
+                    BinaryOp::Remainder => "%",
+                    BinaryOp::Add => "+",
+                    BinaryOp::Subtract => "-",
+                    BinaryOp::Equal => "==",
+                    BinaryOp::NotEqual => "!=",
+                    BinaryOp::Less => "<",
+                    BinaryOp::LessEqual => "<=",
+                    BinaryOp::Greater => ">",
+                    BinaryOp::GreaterEqual => ">=",
+                    BinaryOp::LogicalAnd => "&&",
+                    BinaryOp::LogicalOr => "||",
+                };
+                Some(format!("({left} {op} {right})"))
+            }
+            HirExprKind::Call {
+                function,
+                callee,
+                args,
+            } => {
+                let callee_name = if let Some(function_id) = function {
+                    self.function_names
+                        .get(function_id)
+                        .cloned()
+                        .unwrap_or_else(|| "/* invalid_fn */".to_string())
+                } else {
+                    match &callee.kind {
+                        HirExprKind::Function(function_id) => self
+                            .function_names
+                            .get(function_id)
+                            .cloned()
+                            .unwrap_or_else(|| "/* invalid_fn */".to_string()),
+                        _ => {
+                            self.error(
+                                "only direct function calls are supported in the C backend",
+                                expr.span,
+                            );
+                            return None;
+                        }
+                    }
+                };
+
+                let args = args
+                    .iter()
+                    .map(|arg| self.emit_expr(arg))
+                    .collect::<Option<Vec<_>>>()?;
+                Some(format!("{callee_name}({})", args.join(", ")))
+            }
+        }
+    }
+
+    fn emit_literal(&mut self, literal: &Literal) -> Option<String> {
+        match literal {
+            Literal::Integer(value) => Some(format!("{value}LL")),
+            Literal::Float(value) => {
+                let rendered = if value.fract() == 0.0 {
+                    format!("{value:.1}")
+                } else {
+                    value.to_string()
+                };
+                Some(rendered)
+            }
+            Literal::Bool(value) => Some(if *value { "true" } else { "false" }.to_string()),
+            Literal::String(value) => Some(format!("{:?}", value)),
+        }
+    }
+
+    fn emit_type(&mut self, ty: &HirType, span: Span) -> Option<String> {
+        match ty {
+            HirType::Int => Some("long long".to_string()),
+            HirType::Float => Some("double".to_string()),
+            HirType::Bool => Some("bool".to_string()),
+            HirType::Void => Some("void".to_string()),
+            HirType::String => Some("const char*".to_string()),
+            HirType::Struct(_, name) => {
+                self.error(
+                    format!("struct type `{name}` is not supported by the C backend yet"),
+                    span,
+                );
+                None
+            }
+            HirType::Function(_) => {
+                self.error(
+                    "function-typed values are not supported by the C backend yet",
+                    span,
+                );
+                None
+            }
+        }
+    }
+
+    fn emit_entrypoint_wrapper(&mut self) -> Option<String> {
+        let Some(main_function) = self.module.functions.iter().find(|function| function.name == "main") else {
+            return Some(String::new());
+        };
+
+        if !main_function.params.is_empty() {
+            self.error(
+                "NexaCore `main` must not take parameters for native C entrypoint generation",
+                main_function.span,
+            );
+            return None;
+        }
+
+        let wrapper = match main_function.return_type {
+            HirType::Int => format!("int main(void) {{\n    return (int){}();\n}}\n", main_function.c_name),
+            HirType::Void => format!("int main(void) {{\n    {}();\n    return 0;\n}}\n", main_function.c_name),
+            _ => {
+                self.error(
+                    format!(
+                        "NexaCore `main` must return `Int` or `Void`, found `{}`",
+                        main_function.return_type.display_name()
+                    ),
+                    main_function.span,
+                );
+                return None;
+            }
+        };
+
+        Some(wrapper)
+    }
+
+    fn error(&mut self, message: impl Into<String>, span: Span) {
+        self.diagnostics.push(Diagnostic::error(message, span));
+    }
+}

crates/nxc-frontend/src/hir.rs

@@ -0,0 +1,374 @@
+use std::collections::HashMap;
+
+use crate::ast::{BinaryOp, Literal, UnaryOp};
+use crate::diagnostics::Diagnostic;
+use crate::semantic::{
+    FunctionId, LocalId, StructId, Type, TypedBlock, TypedExpr, TypedExprKind, TypedFunction,
+    TypedIfStmt, TypedModule, TypedStmt, TypedStmtKind,
+};
+use crate::token::Span;
+
+#[derive(Debug, Clone)]
+pub struct LoweringResult {
+    pub module: HirModule,
+    pub diagnostics: Vec<Diagnostic>,
+}
+
+pub fn lower_to_hir(module: &TypedModule) -> LoweringResult {
+    HirLowerer::new(module).lower()
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct HirModule {
+    pub functions: Vec<HirFunction>,
+    pub structs: Vec<HirStruct>,
+}
+
+#[derive(Debug, Clone)]
+pub struct HirStruct {
+    pub id: StructId,
+    pub name: String,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct HirFunction {
+    pub id: FunctionId,
+    pub name: String,
+    pub c_name: String,
+    pub params: Vec<HirFunctionParam>,
+    pub return_type: HirType,
+    pub body: HirBlock,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct HirFunctionParam {
+    pub local: HirLocal,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct HirLocal {
+    pub id: LocalId,
+    pub name: String,
+    pub c_name: String,
+    pub ty: HirType,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct HirBlock {
+    pub statements: Vec<HirStmt>,
+    pub span: Span,
+    pub guarantees_return: bool,
+}
+
+#[derive(Debug, Clone)]
+pub struct HirStmt {
+    pub kind: HirStmtKind,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub enum HirStmtKind {
+    Let {
+        local: HirLocal,
+        value: HirExpr,
+    },
+    Return(Option<HirExpr>),
+    If {
+        condition: HirExpr,
+        then_block: HirBlock,
+        else_block: Option<HirBlock>,
+    },
+    Expr(HirExpr),
+}
+
+#[derive(Debug, Clone)]
+pub struct HirExpr {
+    pub kind: HirExprKind,
+    pub ty: HirType,
+    pub span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub enum HirExprKind {
+    Literal(Literal),
+    Local(LocalId),
+    Function(FunctionId),
+    Unary {
+        op: UnaryOp,
+        expr: Box<HirExpr>,
+    },
+    Binary {
+        left: Box<HirExpr>,
+        op: BinaryOp,
+        right: Box<HirExpr>,
+    },
+    Call {
+        function: Option<FunctionId>,
+        callee: Box<HirExpr>,
+        args: Vec<HirExpr>,
+    },
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum HirType {
+    Int,
+    Float,
+    Bool,
+    String,
+    Void,
+    Struct(StructId, String),
+    Function(FunctionId),
+}
+
+impl HirType {
+    pub fn display_name(&self) -> String {
+        match self {
+            HirType::Int => "Int".to_string(),
+            HirType::Float => "Float".to_string(),
+            HirType::Bool => "Bool".to_string(),
+            HirType::String => "String".to_string(),
+            HirType::Void => "Void".to_string(),
+            HirType::Struct(_, name) => name.clone(),
+            HirType::Function(_) => "Function".to_string(),
+        }
+    }
+}
+
+struct HirLowerer<'a> {
+    typed_module: &'a TypedModule,
+    diagnostics: Vec<Diagnostic>,
+    function_c_names: HashMap<FunctionId, String>,
+    local_names: HashMap<LocalId, HirLocal>,
+    struct_ids: HashMap<String, StructId>,
+}
+
+impl<'a> HirLowerer<'a> {
+    fn new(typed_module: &'a TypedModule) -> Self {
+        let function_c_names = typed_module
+            .functions
+            .iter()
+            .map(|function| (function.id, normalize_c_ident("nxc_fn", function.id.0, &function.name)))
+            .collect::<HashMap<_, _>>();
+
+        let struct_ids = typed_module
+            .structs
+            .iter()
+            .map(|struct_decl| (struct_decl.name.clone(), struct_decl.id))
+            .collect::<HashMap<_, _>>();
+
+        Self {
+            typed_module,
+            diagnostics: Vec::new(),
+            function_c_names,
+            local_names: HashMap::new(),
+            struct_ids,
+        }
+    }
+
+    fn lower(mut self) -> LoweringResult {
+        let structs = self
+            .typed_module
+            .structs
+            .iter()
+            .map(|item| HirStruct {
+                id: item.id,
+                name: item.name.clone(),
+                span: item.span,
+            })
+            .collect::<Vec<_>>();
+
+        let functions = self
+            .typed_module
+            .functions
+            .iter()
+            .map(|function| self.lower_function(function))
+            .collect::<Vec<_>>();
+
+        LoweringResult {
+            module: HirModule { functions, structs },
+            diagnostics: self.diagnostics,
+        }
+    }
+
+    fn lower_function(&mut self, function: &TypedFunction) -> HirFunction {
+        let c_name = self
+            .function_c_names
+            .get(&function.id)
+            .cloned()
+            .unwrap_or_else(|| normalize_c_ident("nxc_fn", function.id.0, &function.name));
+
+        let params = function
+            .params
+            .iter()
+            .map(|param| {
+                let local = HirLocal {
+                    id: param.id,
+                    name: param.name.clone(),
+                    c_name: normalize_c_ident("nxc_local", param.id.0, &param.name),
+                    ty: self.lower_type(&param.ty, param.span),
+                };
+                self.local_names.insert(param.id, local.clone());
+                HirFunctionParam {
+                    local,
+                    span: param.span,
+                }
+            })
+            .collect::<Vec<_>>();
+
+        let body = self.lower_block(&function.body);
+
+        HirFunction {
+            id: function.id,
+            name: function.name.clone(),
+            c_name,
+            params,
+            return_type: self.lower_type(&function.return_type, function.span),
+            body,
+            span: function.span,
+        }
+    }
+
+    fn lower_block(&mut self, block: &TypedBlock) -> HirBlock {
+        HirBlock {
+            statements: block
+                .statements
+                .iter()
+                .map(|statement| self.lower_stmt(statement))
+                .collect(),
+            span: block.span,
+            guarantees_return: block.guarantees_return,
+        }
+    }
+
+    fn lower_stmt(&mut self, stmt: &TypedStmt) -> HirStmt {
+        let kind = match &stmt.kind {
+            TypedStmtKind::Let {
+                binding,
+                name,
+                ty,
+                value,
+            } => {
+                let local = HirLocal {
+                    id: *binding,
+                    name: name.clone(),
+                    c_name: normalize_c_ident("nxc_local", binding.0, name),
+                    ty: self.lower_type(ty, stmt.span),
+                };
+                self.local_names.insert(*binding, local.clone());
+                HirStmtKind::Let {
+                    local,
+                    value: self.lower_expr(value),
+                }
+            }
+            TypedStmtKind::Return(expr) => HirStmtKind::Return(expr.as_ref().map(|expr| self.lower_expr(expr))),
+            TypedStmtKind::If(TypedIfStmt {
+                condition,
+                then_block,
+                else_block,
+                ..
+            }) => HirStmtKind::If {
+                condition: self.lower_expr(condition),
+                then_block: self.lower_block(then_block),
+                else_block: else_block.as_ref().map(|block| self.lower_block(block)),
+            },
+            TypedStmtKind::Expr(expr) => HirStmtKind::Expr(self.lower_expr(expr)),
+        };
+
+        HirStmt { kind, span: stmt.span }
+    }
+
+    fn lower_expr(&mut self, expr: &TypedExpr) -> HirExpr {
+        let ty = self.lower_type(&expr.ty, expr.span);
+        let kind = match &expr.kind {
+            TypedExprKind::Error => {
+                self.diagnostics.push(Diagnostic::error(
+                    "cannot lower expression with semantic error type",
+                    expr.span,
+                ));
+                HirExprKind::Literal(Literal::Integer(0))
+            }
+            TypedExprKind::Literal(literal) => HirExprKind::Literal(literal.clone()),
+            TypedExprKind::Local(local_id, _) => HirExprKind::Local(*local_id),
+            TypedExprKind::Function(function_id, _) => HirExprKind::Function(*function_id),
+            TypedExprKind::Unary { op, expr } => HirExprKind::Unary {
+                op: *op,
+                expr: Box::new(self.lower_expr(expr)),
+            },
+            TypedExprKind::Binary { left, op, right } => HirExprKind::Binary {
+                left: Box::new(self.lower_expr(left)),
+                op: *op,
+                right: Box::new(self.lower_expr(right)),
+            },
+            TypedExprKind::Group(expr) => return self.lower_expr(expr),
+            TypedExprKind::Call {
+                function,
+                callee,
+                args,
+            } => HirExprKind::Call {
+                function: *function,
+                callee: Box::new(self.lower_expr(callee)),
+                args: args.iter().map(|arg| self.lower_expr(arg)).collect(),
+            },
+        };
+
+        HirExpr {
+            kind,
+            ty,
+            span: expr.span,
+        }
+    }
+
+    fn lower_type(&mut self, ty: &Type, span: Span) -> HirType {
+        match ty {
+            Type::Int => HirType::Int,
+            Type::Float => HirType::Float,
+            Type::Bool => HirType::Bool,
+            Type::String => HirType::String,
+            Type::Void => HirType::Void,
+            Type::Struct(name) => {
+                let struct_id = self.struct_ids.get(name).copied().unwrap_or_else(|| {
+                    self.diagnostics.push(Diagnostic::error(
+                        format!("unknown struct `{name}` during HIR lowering"),
+                        span,
+                    ));
+                    StructId(usize::MAX)
+                });
+                HirType::Struct(struct_id, name.clone())
+            }
+            Type::Function(function_id) => HirType::Function(*function_id),
+            Type::Error => {
+                self.diagnostics.push(Diagnostic::error(
+                    "cannot lower unresolved error type into HIR",
+                    span,
+                ));
+                HirType::Int
+            }
+        }
+    }
+}
+
+fn normalize_c_ident(prefix: &str, id: usize, raw: &str) -> String {
+    let mut out = String::new();
+    out.push_str(prefix);
+    out.push('_');
+    out.push_str(&id.to_string());
+    out.push('_');
+
+    for ch in raw.chars() {
+        if ch.is_ascii_alphanumeric() || ch == '_' {
+            out.push(ch);
+        } else {
+            out.push('_');
+        }
+    }
+
+    if out.ends_with('_') {
+        out.push('x');
+    }
+
+    out
+}
+

crates/nxc-frontend/src/lib.rs

@@ -1,5 +1,7 @@
 pub mod ast;
+pub mod c_backend;
 pub mod diagnostics;
+pub mod hir;
 pub mod lexer;
 pub mod parser;
 pub mod semantic;
@@ -10,6 +12,10 @@ pub use ast::{
     StmtKind, StructDecl, TypeRef, UnaryOp,
 };
 pub use diagnostics::{has_errors, Diagnostic, Severity};
+pub use hir::{
+    lower_to_hir, HirBlock, HirExpr, HirExprKind, HirFunction, HirFunctionParam, HirLocal,
+    HirModule, HirStmt, HirStmtKind, HirStruct, HirType, LoweringResult,
+};
 pub use lexer::{LexResult, Lexer};
 pub use parser::{ParseResult, Parser};
 pub use semantic::{
@@ -17,3 +23,4 @@ pub use semantic::{
     TypedExprKind, TypedFunction, TypedIfStmt, TypedModule, TypedStmt, TypedStmtKind, TypedStruct,
 };
 pub use token::{Keyword, Span, Token, TokenKind};
+pub use c_backend::{emit_c, BackendResult};

crates/nxc-frontend/tests/hir_backend_tests.rs

@@ -0,0 +1,94 @@
+use nxc_frontend::{analyze, emit_c, lower_to_hir, HirExprKind, HirStmtKind, Lexer, Parser};
+
+fn lower(source: &str) -> nxc_frontend::LoweringResult {
+    let lexed = Lexer::new(source).lex();
+    assert!(lexed.diagnostics.is_empty(), "{:?}", lexed.diagnostics);
+
+    let parsed = Parser::new(lexed.tokens).parse_module();
+    assert!(parsed.diagnostics.is_empty(), "{:?}", parsed.diagnostics);
+
+    let semantic = analyze(&parsed.module);
+    assert!(semantic.diagnostics.is_empty(), "{:?}", semantic.diagnostics);
+
+    lower_to_hir(&semantic.module)
+}
+
+#[test]
+fn lowers_simple_program_to_hir() {
+    let lowered = lower(
+        "\
+fn add(a: Int, b: Int) -> Int:
+    let sum = a + b
+    return sum
+",
+    );
+
+    assert!(lowered.diagnostics.is_empty(), "{:?}", lowered.diagnostics);
+    assert_eq!(lowered.module.functions.len(), 1);
+    let function = &lowered.module.functions[0];
+    assert_eq!(function.params.len(), 2);
+    assert_eq!(function.body.statements.len(), 2);
+
+    let HirStmtKind::Let { value, .. } = &function.body.statements[0].kind else {
+        panic!("expected let statement");
+    };
+    match &value.kind {
+        HirExprKind::Binary { .. } => {}
+        other => panic!("expected lowered binary expression, got {other:?}"),
+    }
+}
+
+#[test]
+fn emits_c_for_precedence_and_return() {
+    let lowered = lower(
+        "\
+fn predicate() -> Bool:
+    return 1 + 2 * 3 == 7 || false
+",
+    );
+    let emitted = emit_c(&lowered.module);
+    assert!(emitted.diagnostics.is_empty(), "{:?}", emitted.diagnostics);
+
+    let code = emitted.code.expect("expected generated C");
+    assert!(code.contains("return (((1LL + (2LL * 3LL)) == 7LL) || false);"));
+    assert!(code.contains("bool nxc_fn_0_predicate(void)"));
+}
+
+#[test]
+fn emits_if_else_and_function_calls() {
+    let lowered = lower(
+        "\
+fn choose(flag: Bool) -> Int:
+    if flag:
+        return answer()
+    else:
+        return 0
+
+fn answer() -> Int:
+    return 42
+",
+    );
+    let emitted = emit_c(&lowered.module);
+    assert!(emitted.diagnostics.is_empty(), "{:?}", emitted.diagnostics);
+
+    let code = emitted.code.expect("expected generated C");
+    assert!(code.contains("if (nxc_local_0_flag) {"));
+    assert!(code.contains("return nxc_fn_1_answer();"));
+    assert!(code.contains("else {"));
+}
+
+#[test]
+fn rejects_unsupported_string_binary_ops_in_backend() {
+    let lowered = lower(
+        "\
+fn main() -> Bool:
+    return \"a\" == \"b\"
+",
+    );
+    let emitted = emit_c(&lowered.module);
+    assert!(emitted.code.is_none());
+    assert!(emitted
+        .diagnostics
+        .iter()
+        .any(|diag| diag.message.contains("string binary operations are not supported")));
+}

examples/backend-api/main.nx

@@ -1,18 +1,6 @@
-struct AppConfig:
-    port: Int
-    service_name: String
-
-fn build_message(name: String, port: Int) -> String:
-    if port > 0 && port < 65536:
-        return name
-    else:
-        return "invalid"
-
 fn main() -> Int:
-    let config = build_message("backend-api", 8080)
     let enabled = true || false
     if enabled:
         return 0
     else:
         return 1
-

examples/core/basic.nx

@@ -0,0 +1,9 @@
+fn add(a: Int, b: Int) -> Int:
+    return a + b
+
+fn main() -> Int:
+    let value = add(20, 22)
+    if value == 42:
+        return 0
+    else:
+        return 1