build: release build fails on esp toolchains 1.94.0.0+ (LLVM 21 "Cannot select" regression)

[rkuester]

Summary

cargo build --release fails during LLVM instruction selection on the
current esp Rust toolchains (1.94.0.0 through 1.95.0.0). The build aborts
before linking with:

rustc-LLVM ERROR: Cannot select: ... i32 = XtensaISD::PCREL_WRAPPER
  TargetConstantPool:i32<@anon... = private unnamed_addr constant
  [14 x i8] c"ADC Read Error", align 1> 0
In function: ..._usb_task_task0...poll

The Xtensa backend can't lower a PC-relative reference to an aggregate
constant in the constant pool. Here that's the "ADC Read Error" string
literal in src/control/adc.rs, pulled into the USB task by inlining.

Details

• Only optimized codegen is affected. cargo check and cargo clippy
  pass; debug builds (opt-level 0) get through codegen and only fail at
  link for unrelated reasons.
• Independent of the release profile knobs: reproduces with lto set to
  fat, thin, or off, and at opt-level 2, 3, and s.
• Target: xtensa-esp32s3-none-elf (our no_std firmware).

Toolchain bisect

esp toolchain	bundled LLVM	result
1.93.0.0	20.1.1	builds
1.94.0.2	21.1.3	fails
1.95.0.0	21.1.3	fails

The regression tracks the esp-rs fork's LLVM 20 to 21 bump.

Known upstream regression

• [ICE]: Cannot select XtensaISD::PCREL_WRAPPER for serde Vec deserialize on xtensa-esp32s3-espidf (regression in 1.94.0.0+) esp-rs/rust#277 reports the same Cannot select XtensaISD::PCREL_WRAPPER
  failure, also a regression in 1.94.0.0+, from the LLVM 20 to 21 bump.
  That reporter hit it on the std target with a [2 x float] constant and
  said it did not reproduce on no_std for them. This case shows it does
  affect no_std too, via a string ([14 x i8]) constant.
• A candidate fix is committed to the LLVM fork:
  espressif/llvm-project@39a5993,
  "[Xtensa] Fix ConstantPool lowering for aggregate constants"
  (2026-05-13). Not confirmed in a released toolchain yet.
• Two more open "Cannot select" reports against the same LLVM 21 backend:
  rustc-LLVM ERROR: Cannot select: 0x11c366ee0: i32 = Constant<4096> esp-rs/rust#275, Another cannot select issue esp-rs/rust#281.

Workaround

Pin the esp toolchain to the last good version:

espup install --toolchain-version 1.93.0.0

[rkuester]

Anyone else hitting this on esp 1.94.x or 1.95.x?

#17 pins the toolchain to 1.93.0.0 as a temporary fix: the README install step now requests that version, and a build.rs check stops the build with install instructions if it sees LLVM 21 or newer. Happy to drop it once a fixed toolchain is released.

[winterrdog]

Anyone else hitting this on esp 1.94.x or 1.95.x?

yes, i did. i was running esp 1.95.0.0

Pin the esp toolchain to the last good version:

espup install --toolchain-version 1.93.0.0

this worked great