Confusing panic when using FunctionData without adding to Program first

[sudoytang]

Description

When a user misuses this crate (like me), creates a FunctionData using FunctionData::new() and tries to operate on it directly (without first adding it to a Program), a confusing panic occurs:

thread 'main' panicked at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/koopa-0.0.9/src/ir/builder.rs:445:8:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_fmt
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/panicking.rs:80:14
   2: core::panicking::panic
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/panicking.rs:150:5
   3: core::option::unwrap_failed
             at /rustc/ded5c06cf21d2b93bffd5d884aa6e96934ee4234/library/core/src/option.rs:2174:5
   4: core::option::Option<T>::unwrap
             at .rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:1015:21
   5: <T as koopa::ir::builder::EntityInfoQuerier>::value_type
             at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/koopa-0.0.9/src/ir/builder.rs:445:8
   6: koopa::ir::builder::LocalInstBuilder::ret::{{closure}}
             at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/koopa-0.0.9/src/ir/builder.rs:330:34
   7: core::option::Option<T>::is_none_or
             at .rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:712:24
   8: koopa::ir::builder::LocalInstBuilder::ret
             at .cargo/registry/src/index.crates.io-1949cf8c6b5b557f/koopa-0.0.9/src/ir/builder.rs:330:13

The root cause seems to be that FunctionData::new() creates an object with dfg.globals as an uninitialized Weak reference.

When ret() (or similar methods) tries to query value types via EntityInfoQuerier::value_type(), it calls globals.upgrade().unwrap() which panics.

  fn value_type(&self, value: Value) -> Type {
    self
      .dfg()
      .globals
      .upgrade()
      .unwrap()    // <--- HERE
      .borrow()
      .get(&value)
      .or_else(|| self.dfg().values().get(&value))
      .expect("value does not exist")
      .ty()
      .clone()
  }

(Code of <T as koopa::ir::builder::EntityInfoQuerier>::value_type)

Reproduction

use koopa::ir::builder_traits::*;
use koopa::ir::{FunctionData, Program, Type};

fn main() {
    let mut program = Program::new();

    // User creates FunctionData independently (seems reasonable)
    let mut func = FunctionData::new("@main".into(), vec![], Type::get_i32());

    // Add basic block
    let entry = func.dfg_mut().new_bb().basic_block(Some("%entry".into()));
    func.layout_mut().bbs_mut().push_key_back(entry).unwrap();

    // Try to create ret instruction - PANIC!
    let dfg = func.dfg_mut();
    let zero = dfg.new_value().integer(0);
    let ret = dfg.new_value().ret(Some(zero)); // panics here

    func.layout_mut().bb_mut(entry).insts_mut().push_key_back(ret).unwrap();
    program.new_func(func);
}

Expected Behavior

Either:

1. Compile-time error: The API should prevent this usage pattern, OR
2. Clear runtime error: A meaningful error message like "FunctionData must be added to Program before creating instructions"

Actual Behavior

A confusing panic with no indication of what the user did wrong:

called `Option::unwrap()` on a `None` value

Correct Usage (for reference)

I found that the correct way is to add FunctionData to Program first, then operate through Program:

let main = program.new_func(FunctionData::new("@main".into(), vec![], Type::get_i32()));
let func = program.func_mut(main);  // Get reference through Program
// Now operations work correctly

Suggested Fixes

1. Better error message: Replace .unwrap() with .expect("FunctionData must be added to Program before use")

2. Documentation: Add a warning to FunctionData::new() docs explaining that the returned FunctionData must be added to a Program via new_func() before any operations on it

3. API redesign (optional): Change Program::new_func to accept construction parameters directly instead of a FunctionData:

   // Instead of:
   program.new_func(FunctionData::new("@main".into(), vec![], Type::get_i32()))
   
   // Could be:
   program.new_func("@main".into(), vec![], Type::get_i32())

   This way FunctionData would only be created internally by Program, preventing users from operating on uninitialized instances

Environment

• koopa version: 0.0.9