fix(eslint-plugin): [require-await] Allow concise arrow function bodies

[scottohara]

The @typescript-eslint/require-await rule exists to allow exceptions to the core ESLint require-await rule for async functions that return a promise (where adding an await to the return to suppress the require-await warnings would in turn trigger warnings for no-return-await).

The initial implementation of the @typescript-eslint/require-await rule allowed the following exceptions:

• function expressions that return a promise
• function declarations that return a promise
• arrow function expressions with block bodies that return a promise

(the rule was only checking ReturnStatement nodes)

This PR adds support for:

• arrow function expressions with concise bodies that return a promise

(single-line expressions that don't explicitly include a return statement)

Prior to this PR, the following code would fall through to the core ESLint require-await rule and trigger warnings:

// 1. Async arrow function with promise return
const numberOne = async (): Promise<number> => Promise.resolve(1);

// 2. Async arrow function with async function return
const getAsyncNumber = async function(x: number): Promise<number> {
  return Promise.resolve(x);
}

const numberOne = async (): Promise<number> => getAsyncNumber(1);

// 3. Async arrow function expression with no await
const numberOne = async (): Promise<number> => 1;

After this PR, only the last example should fall through to the core rule and triggers a warning:

// 3. Async arrow function expression with no await
const numberOne = async (): Promise<number> => 1;

[typescript-eslint]

Thanks for the PR, @scottohara!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

---

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[codecov]

Codecov Report

Merging #826 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
+ Coverage   94.07%   94.07%   +<.01%     
==========================================
  Files         113      113              
  Lines        4911     4914       +3     
  Branches     1368     1368              
==========================================
+ Hits         4620     4623       +3     
  Misses        165      165              
  Partials      126      126

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/require-await.ts	92.68% <100%> (+0.57%)	⬆️

[bradzacher]

LGTM, thanks for this