Xtensa
The processor design in the ESP32-S3 and original ESP32, which stock Rust doesn't support yet.
Xtensa is Cadence's configurable processor architecture used in several ESP32 chips; Rust support needs Espressif's forked LLVM and toolchain (installed with espup), unlike the RISC-V ESP32 parts.
Examples
The original ESP32, ESP32-S2 and ESP32-S3 are Xtensa chips, so Rust for them needs Espressif's fork installed with
espup.The ESP32-C6 is RISC-V instead, so stock stable Rust builds for it without any fork.
See also