Built-time macros strawman

a.js

import { deprecated } from './macros/deprecated.js';

// Using Rust-like syntax only to make it clear these
// are macros and not "decorators"

#[deprecated]
function dontUseMeAnymore() {
  eval('You wrote a bad song, Petey!');
}

/*

Built-time tooling (e.g. a Webpack plugin) would
transform this into:

function dontUseMeAnymore() {
  console.warn('This function is deprecated');
  eval('You wrote a bad song Petey!');
}

*/

deprecated.js

export function deprecated(ast) {
  // In practice, macros would use a library to give 
  // AST mutation and construction better DX
  ast.body.statements.unshift({
    type: 'ExpressionStatement',
    expression: {
      type: 'CallExpression',
      callee: {
        type: 'MemberExpression',
        object: {
          type: 'Identifier',
          value: 'console',
        },
        property: {
          type: 'Identifier',
          value: 'warn',
        }
      },
      arguments: [{
        type: 'StringLiteral',
        value: 'This function is deprecated',
      }],
    },
  });
  return ast;
}