Immediate Argument Evaluation in defer in Golang

[novahe]

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• Yes, I've searched similar issues on GitHub and didn't find any.

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Your feature request related to a problem? Please describe

I'm not sure if golangci-lint can catch this, thanks in advance.

in Go, arguments to deferred function calls are evaluated immediately
when the defer statement is executed, not when the deferred function
runs. This caused SinceInSeconds(startTime) to be computed at the
start of request handling instead of at the end.

package main

import (
	"fmt"
	"time"
)

func main() {
	start := time.Now()

	// --- 1. ❌ INCORRECT WAY: Immediate Argument Evaluation ---
	// The function time.Since(start) is an ARGUMENT to fmt.Println.
	// Go's defer rule states that arguments are evaluated IMMEDIATELY
	// when the defer statement is executed.
	// Result: Reports a time duration close to zero.
	defer fmt.Println("❌ Incorrect Value (Immediate Evaluation):", time.Since(start)) 

	// --- 2. ✅ CORRECT WAY: Using a Closure/Anonymous Function ---
	// The deferred function is now the anonymous function itself (func() {}).
	// The code inside the closure (time.Since(start)) is NOT evaluated
	// until the function exits and the deferred call is actually made.
	// Result: Reports the true duration of the main function's execution.
	defer func() {
		fmt.Println("✅ Correct Value (Delayed Evaluation):", time.Since(start))
	}()

	fmt.Println("Main function execution STARTING...")

	// Simulate a time-consuming operation (e.g., an HTTP request or database query)
	time.Sleep(100 * time.Millisecond) 

	fmt.Println("Main function execution FINISHED.")
	// When the function exits, deferred statements are executed (LIFO).
}

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[alexandear]

The govet (defers analyzer) caught this:

$ golangci-lint run --enable-only govet
issue6268/main.go:16:67: defers: call to time.Since is not deferred (govet)
        defer fmt.Println("❌ Incorrect Value (Immediate Evaluation):", time.Since(start))
                                                                         ^
1 issues:
* govet: 1