Move Declaration And Initialization Together

Problem

A variable that exists before it has a meaningful value creates a temporary mystery. The reader must track an empty slot, scan forward for assignment, and check whether the value can be observed before it becomes valid. This is one instance of a broader live-context problem: the longer a fact is in scope, the longer a reader has to consider whether it can affect later code.

Preferred Move

Declare a value where it becomes known. Prefer immutable locals and narrow scopes so a name starts life with meaning. Preserve evaluation order, but move each declaration as close as practical to the expression that gives it a valid value and the code that uses it.

Tradeoff

Some language constructs, cleanup paths, or ownership constraints require broader scope. Do not duplicate expensive work or contort control flow only to narrow a variable. If moving the declaration requires manually tracking too many ordering constraints, take a smaller step or add a test first; this cleanup should not quietly change execution order.

Agent Instruction

Move local declarations to the point where their value becomes meaningful. Avoid placeholder values
that readers must remember across unrelated work.

Examples

Bad: the name exists before it has a valid value.

let mut slug = None;

if let Some(input) = request.slug() {
    slug = Some(Slug::parse(input)?);
}

let pattern = load_pattern(slug.ok_or(Error::MissingSlug)?)?;

Good: the value is declared when it becomes available.

let input = request.slug().ok_or(Error::MissingSlug)?;
let slug = Slug::parse(input)?;

let pattern = load_pattern(slug)?;

References

Source	Use	Note
Tidy First, Ch. 7	`adapts`	Keep declaration and initialization close, preserving order.

Summary