`RustFmt` error: "let chains are only allowed in Rust 2024 or later" but I'm on 2024

[marcianx]

Project setup: I have a workspace that specifies the project versions, rust editions, and dependencies in the workspace Cargo.toml, and the crates contained within use:

version.workspace = true
edition.workspace = true
...

etc to refer to the configuration in the workspace Cargo.toml. I'm using edition "2024".

Vim setup: I'm on vim 8.2. rust.vim is already on its newest version. Here are the versions reported by :RustInfo.

rustfmt 1.8.0-stable (1159e78c47 2025-09-14)
rustc 1.90.0 (1159e78c4 2025-09-14)
cargo 1.90.0 (840b83a10 2025-07-30)  
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Apr 02 2025 12:39:01)
Included patches: 1-16, 647, 17-579, 1969, 580-647, 678, 648-1848, 4975, 5016, 5023, 5072, 2068, 1849-1854, 1857, 1855-1857, 1331, 1858, 1858-1859, 1873, 1860-1969, 1992, 1
970-1992, 2010, 1993-2068, 2106, 2069-2106, 2108, 2107-2109, 2109-3995, 4563, 4646, 4774, 4895, 4899, 4901, 4919, 213, 1840, 1846-1847, 2110-2112, 2121
Modified by team+vim@tracker.debian.org
Compiled by team+vim@tracker.debian.org  

Issue: Running :RustFmt on a rust file that's using let chains fails with a location list of errors saying:

error: let chains are only allowed in Rust 2024 or later

The issue still reproduces this issue by using edition = "2024" on the crate instead of referring to the workspace edition. But cargo fmt on the command line works just fine. cargo fmt will produce the error above if I change to edition = "2018", for example. So rust.vim seems to not see the version correctly.

Any insights on what the underlying issue is and how to resolve it?

[marcianx]

Thanks to this thread on the Rust users forum for a workaround: configure rustfmt itself via rustfmt.toml with edition = "2024".

So the main question for the issue above is: should RustFmt automatically derive the current edition based on the ambient Cargo.toml?