The asker is trying to build a "blinky" style program in Rust for the original Raspberry Pi Pico (RP2040, Cortex-M0+). Their Cargo configuration looks textbook-correct: thumbv6m-none-eabi target, the linker flags that preserve the .boot2 section, and a runner that pipes the ELF through elf2uf2-rs so the resulting UF2 can be dropped onto the bootloader mass-storage device. Yet every build ends with elf2uf2-rs bailing out with "Unrecognized ABI".

What makes this one interesting is that the error message is misleading. elf2uf2-rs doesn't actually care about C ABI variants — it looks at the ELF's e_flags and the program headers and validates that the binary's load addresses fall inside the Pico's expected flash/SRAM regions. The "Unrecognized ABI" branch fires when the tool can't reconcile what it sees with the layout it knows for the RP2040. The common root causes are:

• Wrong memory.x: if FLASH ORIGIN isn't 0x10000000 with the boot2 stub at offset 0, or RAM isn't 0x20000000, the load segments end up outside the known windows.
• Missing or stale rp2040-boot2: without the second-stage bootloader in .boot2, the first 256 bytes of flash are empty, so the entry-point heuristic fails.
• Targeting the Pico 2 (RP2350) by accident: thumbv6m-none-eabi is correct for RP2040, but if the asker actually has a Pico 2, they need thumbv8m.main-none-eabihf and a newer elf2uf2-rs (or better, picotool). The error message looks identical.
• Cortex-M runtime mismatch: pulling in cortex-m-rt without a board-support crate like rp-pico or rp2040-hal often leaves memory.x undefined, and Cargo silently falls back to a default that doesn't match.

The direction I'd point them: first run arm-none-eabi-readelf -l target/thumbv6m-none-eabi/debug/<binary> and verify that LOAD segments start at 0x10000000 and that a .boot2 section exists and is non-empty. If .boot2 is empty, they're missing the #[link_section = ".boot2"] static from rp2040-boot2. If the load address is wrong, their memory.x is wrong or absent.

Gotcha worth flagging: elf2uf2-rs has been functionally superseded by picotool for newer setups, and the RP2040 vs RP2350 distinction trips up almost everyone buying a Pico in 2026 — the SKUs look nearly identical on the shelf.