Rust Compiler ICE: Resolving Vars

Nov 17, 2025 by Alex Johnson 34 views

Rust Compiler ICE: 'Should Have Resolved Vars Before Calling'

This article delves into a specific issue encountered within the Rust compiler, marked by an Internal Compiler Error (ICE). Specifically, the error message 'should have resolved vars before calling' indicates a problem in the compiler's internal processes during type checking. We'll explore the context of this error, examine the provided code snippet that triggers it, and discuss the implications for Rust developers.

Understanding the ICE

An ICE in a compiler, such as rustc, is a critical error. It signifies that the compiler has encountered an unexpected state, a bug in its own logic. Unlike errors that are the result of incorrect code written by the programmer, an ICE means the compiler itself has failed to correctly process the given source code. When you encounter an ICE, it's essential to report it because it can indicate a deeper problem within the compiler that needs addressing. The phrase "should have resolved vars before calling" points to a failure in the variable resolution stage, a crucial step during type checking.

Code Analysis and the Trigger

The provided code snippet is a minimal, reproducible example (MRE) designed to trigger the ICE. Let's analyze the code:

#![feature(non_lifetime_binders)]
pub trait T {
    fn t<Tail>(&self, _: F) {}
}

pub fn crash<V>(v: &V)
where
    for<F> F: T + 'static,
{
    v.t(|| {});
}

This code defines a trait T with a method t that takes a generic type F. The crash function then takes a reference to a generic type V and calls the t method on it, passing in a closure. The where clause imposes a constraint: for any F, it must implement the trait T and have a 'static lifetime. The #![feature(non_lifetime_binders)] attribute enables a feature that is still under development, and this can be a contributing factor to the ICE. The core issue likely stems from how the compiler handles the generic type F and its interaction with the closure within the t method call, especially in combination with the experimental non_lifetime_binders feature. The error occurs during the rustc_trait_selection phase, which is responsible for resolving traits and their associated types. The error message indicates that the compiler expected all variables to be resolved before calling a specific function, but that didn't happen.

Compiler Version and Environment

The provided information includes the Rust compiler version and environment details. Specifically, the reported version is rustc 1.93.0-nightly (518b42830 2025-11-16). It is important to provide such information in a bug report, this gives the developers of the compiler the ability to reproduce the error and resolve it accordingly. The LLVM version is also specified: LLVM version: 21.1.5. The nightly tag means that the compiler is not fully stable and contains the latest features, this can also increase the chances of facing ICE errors. The flags used during compilation, especially the -Zcrate-attr=feature(non_lifetime_binders), are also critical, because they provide context for the compilation process.

Error Messages and Troubleshooting

The error messages provide valuable clues about the problem. Aside from the core ICE, there are additional errors and warnings. For example, error[E0412]: cannot find type indicates that the compiler cannot resolve the type F within the t method's signature. This is compounded with a warning regarding non_lifetime_binders as incomplete and potentially unsafe to use. The error related to main not being found is also present, this can be related to the test environment. When you encounter an ICE, the first step is to ensure that you are using the latest stable version of the Rust compiler or the latest nightly. Then, try simplifying your code to see if that resolves the issue, this helps to narrow down the source of the problem. If the problem persists, report a bug to the Rust project, providing a minimal, reproducible example and all the relevant version information. The error messages, specifically the query stack during panic, can be extremely useful for developers to debug the issue.

Implications and Solutions

The root cause of this ICE likely lies within the compiler's handling of generics, traits, and closures, particularly in conjunction with the non_lifetime_binders feature. The fact that the error occurs during trait selection indicates that the compiler struggles to determine the correct types and lifetimes during the process of trait resolution. The crash function calls v.t(|| {}); which passes a closure to the function, that might be a place where the compiler has issues while matching types. Since the error is within the compiler, the primary solution is for the Rust developers to identify and fix the bug. As a workaround, developers encountering this ICE could try:

Simplifying Code: Reducing the complexity of the code, especially around generic types, traits, and closures, can sometimes bypass the issue.
Avoiding the Trigger: Refactoring code to avoid the specific pattern that triggers the ICE.
Waiting for a Fix: Because this is a compiler issue, the ultimate solution lies in waiting for a new version of the compiler that addresses the bug. Regularly updating the compiler is recommended.

Reporting and Contributing

If you encounter this ICE, the best course of action is to report it to the Rust project on GitHub. When reporting, include a minimal, reproducible example (the code snippet provided is an excellent starting point), the compiler version, the operating system, and any other relevant details. It's also important to follow the guidelines for reporting bugs, providing all the necessary information to help the developers understand and fix the problem. Additionally, contributing to the Rust project, even in small ways, can help improve the quality and stability of the compiler. This can be by reviewing code, testing patches, or even helping to write documentation.

Conclusion

The "should have resolved vars before calling" ICE highlights an interesting issue within the Rust compiler, touching on the interaction of traits, generics, and closures. It's a reminder of the complex nature of compiler development and the need for rigorous testing and continuous improvement. The provided MRE and the steps to reproduce the error are invaluable for the developers of Rust. As Rust continues to evolve, developers must stay up-to-date with compiler updates and report any bugs they encounter. It is through these efforts that the Rust language becomes more stable and reliable.

For more information on Rust's internal workings, you can check the official Rust documentation.