[perf] deno source code should explicit return type

[loynoir]

https://typescript-eslint.io/rules/explicit-function-return-type/

Functions in TypeScript often don't need to be given an explicit return type annotation. Leaving off the return type is less > code to read or write and allows the compiler to infer it from the contents of the function.

However, explicit return types do make it visually more clear what type is returned by a function. They can also speed up > TypeScript type checking performance in large codebases with many large functions.

This rule enforces that functions do have an explicit return type annotation.

1 + 1 === 2 at worst case deno test takes 30 seconds, due to type checking is included by default.

denoland/deno#16116

https://deno.land/std@0.158.0/node/assert.ts?source#L240

[kt3k]

We intentionally don't write return types if they are void or Promise<void>. We consider those are unnecessary boilerplate

ref: denoland/deno#8832

Also the performance issue reported in denoland/deno#16116 looks like caused by the network download, not by typing issue. deno test now doesn't type check the remote modules by default. ref: https://deno.com/blog/v1.23#no-type-checking-by-default