Rust Const Evaluation Cannot Discard a Value with a Runtime Destructor

A static initializer is evaluated in a restricted compile-time environment. Constructing the final value can be valid while creating and discarding an intermediate value is not, especially when that intermediate has custom Drop behaviour.

The failing fixture builds two Guard values in a tuple and selects .1. The first guard must be discarded during const evaluation, which would invoke its custom destructor. Rust emits E0493.

Drop can run arbitrary runtime code

An implementation of Drop may close handles, update counters, lock state, log, or call other ordinary functions. Const evaluation cannot simply execute arbitrary runtime effects while producing the program image.

The compiler also cannot silently skip the destructor because that would change the type's promised lifecycle semantics.

The official E0493 explanation states that values with custom destructors cannot be dropped during const evaluation.

The discarded temporary causes the failure

Both tuple elements are constructed. Selecting one moves it into the final Runtime, but the other has no destination and must be destroyed. That hidden cleanup is the invalid operation.

The repaired fixture constructs exactly one Guard directly in the static. It becomes part of the final program value instead of being dropped by the initializer.

This is why simplifying a const expression can fix the error even when all named types stay the same.

Const restrictions apply to temporary lifecycles

I inspect not only function calls but also intermediate ownership:

• tuple and array elements later discarded;
• overwritten values;
• pattern matches that drop unused parts;
• blocks with temporary locals;
• method chains that consume and replace owners.

The Reference lists allowed constant expressions and their restrictions. A small reduction often shows which temporary needs cleanup.

Runtime lazy initialization may be the honest boundary

If initialization fundamentally needs allocation, I/O, complex branching, or destructor-bearing temporary work, I move it to runtime. LazyLock, OnceLock, or explicit application startup can create the value once.

This is not a performance failure. Startup initialization is often negligible and yields much clearer error handling. Forcing complex resource construction into const context can make APIs less maintainable.

I reserve compile-time initialization for values whose construction is naturally pure and const-supported.

ManuallyDrop is not a casual escape hatch

ManuallyDrop<T> suppresses automatic destruction and is used in low-level ownership representations. Introducing it solely to make a static initializer compile can leak resources or create unsafe drop invariants.

If I use it, I document exactly who later destroys the value, prove destruction happens once, and audit layout interactions. For ordinary application state, direct construction or runtime initialization is safer.

The final static still has process-lifetime behaviour

A normal immutable static is not dropped at program exit in the way stack locals are. A custom destructor on its field therefore does not imply deterministic shutdown cleanup.

If shutdown must flush or close a resource, I keep it under an application owner whose lifetime and shutdown path are explicit. The E0493 fixture demonstrates compile-time temporary cleanup, not a guarantee that static destructors will run later.

Macro expansion can hide the discarded value

A derive or helper macro may expand a simple-looking initializer into tuples, matches, or replacement operations. When the source does not visibly create a temporary, I inspect expanded code or reproduce the constructor without the macro. I then keep a regression fixture pinned to the supported compiler version. Const evaluation evolves across Rust releases, but a newly accepted expression still deserves a lifecycle review: compilation support does not automatically mean the macro's hidden drop behaviour matches the resource semantics I intended.

My E0493 checklist

• Which type in the const expression implements Drop?
• Which intermediate value would be discarded before becoming the final constant?
• Can I construct the final value directly without that temporary?
• Is a match, tuple projection, or overwrite causing implicit cleanup?
• Does initialization belong at runtime instead?
• Am I considering ManuallyDrop without a complete ownership proof?
• Does the program incorrectly rely on a static destructor at shutdown?
• Can a minimal const fixture isolate the exact unsupported operation?

The core principle is that compile-time evaluation cannot perform arbitrary runtime cleanup. I make destructor-bearing values escape into the final object or move resource lifecycle into explicit runtime ownership.