esp32.json

{
  "arch": "xtensa",
  "cpu": "generic",
  "crt-objects-fallback": "false",
  "data-layout": "e-m:e-p:32:32-i8:8:32-i16:16:32-i64:64-n32",
  "emit-debug-gdb-scripts": false,
  "is-builtin": false,
  "linker": "xtensa-esp32-elf-gcc",
  "linker-flavor": "gnu-cc",
  "llvm-target": "xtensa-none-elf",
  "max-atomic-width": 32,
  "metadata": {
    "description": "Xtensa ESP32",
    "host_tools": false,
    "std": false,
    "tier": 3
  },
  "panic-strategy": "abort",
  "relocation-model": "static",
  "target-pointer-width": "32"
}

no-deps.rs

#![no_std]
#![no_core]
#![no_main]

#![feature(no_core)]
#![feature(lang_items)]

#[lang = "sized"]
trait Sized {}

#[no_mangle]
fn hello_xtensa() -> i32 {
   1 
}

xtensa upstream compile.md

rustc +esp-master test.rs --target esp32.json --emit llvm-ir