You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backend.rs
    path of file: ./repos/wasmtime/cranelift/jit/src
    
    the code of the file until where you have to start completion: //! Defines `JITModule`.

use crate::{compiled_blob::CompiledBlob, memory::BranchProtection, memory::Memory};
use cranelift_codegen::binemit::Reloc;
use cranelift_codegen::isa::{OwnedTargetIsa, TargetIsa};
use cranelift_codegen::settings::Configurable;
use cranelift_codegen::{ir, settings, FinalizedMachReloc};
use cranelift_control::ControlPlane;
use cranelift_entity::SecondaryMap;
use cranelift_module::{
    DataDescription, DataId, FuncId, Init, Linkage, Module, ModuleDeclarations, ModuleError,
    ModuleReloc, ModuleRelocTarget, ModuleResult,
};
use log::info;
use std::cell::RefCell;
use std::collections::HashMap;
use std::ffi::CString;
use std::io::Write;
use std::ptr;
use std::ptr::NonNull;
use std::sync::atomic::{AtomicPtr, Ordering};
use target_lexicon::PointerWidth;

const WRITABLE_DATA_ALIGNMENT: u64 = 0x8;
const READONLY_DATA_ALIGNMENT: u64 = 0x1;

/// A builder for `JITModule`.
pub struct JITBuilder {
    isa: OwnedTargetIsa,
    symbols: HashMap<String, *const u8>,
    lookup_symbols: Vec<Box<dyn Fn(&str) -> Option<*const u8>>>,
    libcall_names: 
    
    pub fn finalize_definitions(&mut self) -> ModuleResult<()> {
        for func in std::mem::take(&mut self.functions_to_finalize) {
            let decl = self.declarations.get_function_decl(func);
            assert!(decl.linkage.is_definable());
            let func = self.compiled_functions[func]
                .as_ref()
                .expect("function must be compiled before it can be finalized");
            func.perform_relocations(
                |name| self.get_address(name),
                |name| self.get_got_address(name).as_ptr().cast(),
                |name| self.get_plt_address(name),
            );
        }

        for data in std::mem::take(&mut self.data_objects_to_finalize) {
            let decl = self.declarations.get_data_decl(data);
            assert!(decl.linkage.is_definable());
            let data = self.compiled_data_objects[data]
                .as_ref()
                .expect("data object must be compiled before it can be finalized");
            data.perform_relocations(
                |name| self.get_address(name),
                |name| self.get_got_address(name).as_ptr().cast(),
                |name| self.get_plt_address(name),
            );
        }

        // Now that we're done patching, prepare the memory for execution!
        self.memory.readonly.set_readonly()?;
        self.memory.code.set_readable_and_executable()?;

        for update in self.pending_got_updates.drain(..) {
            unsafe { update.entry.as_ref() }.store(update.ptr as *mut _, Ordering::SeqCst);
        }
        Ok(())
    }