panic: `can't project out of`

[matthiaskrgr]

#[derive(Debug, PartialEq, Eq)]
pub enum Color {
    RGB(u8, u8, u8),
}

fn main() {
    let mut color = Color::RGB(0, 0, 0);
    let mut red = |v| {
        let Color::RGB(ref mut r, _, _) = color;
        *r = v;
    };
    assert_eq!(Color::RGB(1, 2, 3), color);
}

code compiles with rustc

rust-analyzer analysis-stats . --run-all-ide-things

Database loaded:     593.06ms, 224minstr, 48mb (metadata 307.62ms, 1011kinstr, 152kb; build 35.21ms, 179kinstr, 1744b)
  item trees: 1
  dependency lines of code: 1_462_633, item trees: 2_900
  dependency item stats: traits: 851, impl: 9_701, mods: 3_012, macro calls: 5_501, macro rules: 1_053
Item Tree Collection: 4.05s, 22ginstr, 425mb
  Total Statistics:
    crates: 1, mods: 1, decls: 2, bodies: 2, adts: 1, consts: 0, signatures: 2, variants: 1
  Workspace:
    traits: 0, macro_rules macros: 0, proc_macros: 0
    lines of code: 11, item trees: 1
    usages: traits: 0, impl: 0, mods: 0, macro calls: 0, macro rules: 0
  Dependencies:
    lines of code: 1_462_633, item trees: 2_900
    declarations: traits: 851, impl: 9_701, mods: 3_012, macro calls: 5_501, macro rules: 1_053
Item Collection:     2.41s, 11ginstr, 177mb
Crate lang items:    449.02ms, 2471minstr, 31mb
Expression Store Lowering: 271.44µs, 348kinstr, 8kb
  exprs: 14, ??ty: 0 (0%), ?ty: 0 (0%), !ty: 0
  pats: 8, ??ty: 0 (0%), ?ty: 0 (0%), !ty: 0
  panics: 0
Inference:           2.21ms, 973kinstr, 2622kb
MIR lowering:        330.91ms, 1243minstr, 33mb
Mir failed bodies: 0 (0%)
Data layouts:        44.63µs, 38kinstr, 96kb
Failed data layouts: 0 (0%)
Const evaluation:    29.68µs, 47kinstr, 208b
Failed const evals: 0 (100%)
0/1 0% diagnostics: /tmp/sn/dummy/src/main.rs
thread 'main' (2352877) panicked at src/tools/rust-analyzer/crates/hir-ty/src/mir.rs:1277:22:
can't project out of Color
stack backtrace:
   0:     0x7f1cb6e43416 - <<std[5264d755c47072bd]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[435d43ddfce3abfa]::fmt::Display>::fmt
   1:     0x7f1cb740ec88 - core[435d43ddfce3abfa]::fmt::write
   2:     0x7f1cb6e5867c - <std[5264d755c47072bd]::sys::stdio::unix::Stderr as core[435d43ddfce3abfa]::io::write::Write>::write_fmt
   3:     0x7f1cb6e1558a - std[5264d755c47072bd]::panicking::default_hook::{closure#0}
   4:     0x7f1cb6e35b33 - std[5264d755c47072bd]::panicking::default_hook
   5:     0x7f1cb6e35f9b - std[5264d755c47072bd]::panicking::panic_with_hook
   6:     0x7f1cb6e15642 - std[5264d755c47072bd]::panicking::panic_handler::{closure#0}
   7:     0x7f1cb6e0dfc9 - std[5264d755c47072bd]::sys::backtrace::__rust_end_short_backtrace::<std[5264d755c47072bd]::panicking::panic_handler::{closure#0}, !>
   8:     0x7f1cb6e16f9d - __rustc[efc60ea9c7e5e02b]::rust_begin_unwind
   9:     0x7f1cb3b3c8ec - core[435d43ddfce3abfa]::panicking::panic_fmt
  10:     0x55e2e2354c45 - <hir_ty[a11d2f9205227a88]::mir::PlaceTy>::field_ty
  11:     0x55e2e20b4f15 - <hir_ty[a11d2f9205227a88]::mir::PlaceTy>::projection_ty::<la_arena[3980ab47ed97dde9]::Idx<hir_ty[a11d2f9205227a88]::mir::Local>>
  12:     0x55e2e21ea808 - <hir_ty[a11d2f9205227a88]::mir::borrowck::_::borrowck_query_Configuration_ as salsa[bfa5eed162dfe307]::function::Configuration>::execute::inner_::{closure#0}
  13:     0x55e2e241267d - <hir_ty[a11d2f9205227a88]::mir::borrowck::_::borrowck_query_Configuration_ as salsa[bfa5eed162dfe307]::function::Configuration>::execute
  14:     0x55e2e225522e - <salsa[bfa5eed162dfe307]::function::IngredientImpl<hir_ty[a11d2f9205227a88]::mir::borrowck::_::borrowck_query_Configuration_>>::execute
  15:     0x55e2e2398246 - hir_ty[a11d2f9205227a88]::mir::borrowck::borrowck_query
  16:     0x55e2e2b90557 - <ide_db[9214579b98daef8b]::RootDatabase as hir_ty[a11d2f9205227a88]::db::HirDatabase>::borrowck
  17:     0x55e2e1cc098e - <hir[dc7c3d5fb10c5bc]::DefWithBody>::diagnostics
  18:     0x55e2e1c734be - <hir[dc7c3d5fb10c5bc]::ModuleDef>::diagnostics
  19:     0x55e2e1c7da5c - <hir[dc7c3d5fb10c5bc]::Module>::diagnostics
  20:     0x55e2e2b4c0ee - ide_diagnostics[3f577ca5efe14601]::semantic_diagnostics
  21:     0x55e2e2b46722 - ide_diagnostics[3f577ca5efe14601]::full_diagnostics
  22:     0x55e2e258fc79 - <ide[9e8b99a0b3725d5c]::Analysis>::full_diagnostics
  23:     0x55e2e2fd729f - <rust_analyzer[54a442221c605e5]::cli::flags::AnalysisStats>::run
  24:     0x55e2e3280272 - rust_analyzer[25166d87b799c29f]::main
  25:     0x55e2e3216aa3 - std[5264d755c47072bd]::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>, core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>>
  26:     0x55e2e3279412 - std[5264d755c47072bd]::rt::lang_start::<core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>>::{closure#0}
  27:     0x7f1cb8614e99 - std[5264d755c47072bd]::rt::lang_start_internal
  28:     0x55e2e32994f8 - main
  29:     0x7f1cb1e27741 - <unknown>
  30:     0x7f1cb1e27879 - __libc_start_main
  31:     0x55e2e1b8b369 - <unknown>
  32:                0x0 - <unknown>

[matthiaskrgr]

pub enum Request {
    Resolve { url: String },
}

pub fn handle_event(event: Request) {
    (move || {
        let Request::Resolve { url: _url } = event;
    })();
}

[MatthiasvB]

I ran into a different issue and had AI analyze it. It got distracted by running into this issue (according to its analysis) as well. It spent a good amount of tokens on a deep analysis, so I thought I'd paste that here as additional hints instead of letting it go to waste:

Still reproduces on rust-analyzer 0.3.2989 (release 2026-07-27), and I hit it on a real workspace where it kills full_diagnostics for the whole crate. Some extra data that may narrow it down.

Smaller repro

pub enum E {
    V { a: String },
}

pub fn f(e: E) {
    let _ = (|| match e {
        E::V { a } => a.len(),
    })();
}

cargo check is clean; rust-analyzer diagnostics . panics with can't project out of E.

Required ingredients

I bisected the repro. All four are necessary:

variation	panics
as above	yes
E::V(String) (tuple variant instead of struct variant)	yes
add a second variant W to E	no
field is u64 instead of String (i.e. Copy)	no
same match without the closure	no
struct S { a: String } + let S { a } = s in a closure	no

So it needs: a single-variant enum, a non-Copy field, destructured inside a closure.

Edition-dependent

Same crate, only edition changed:

• 2015 / 2018 — clean
• 2021 / 2024 — panics

That points at RFC 2229 precise closure captures. Per the rule described in rust-lang/rust#138961, matching an enum against a constructor only counts as requiring a discriminant read when the enum has more than one variant — so for a single-variant enum the capture gets refined from e down to e.a. That yields a Field projection whose base place type is the enum E with no preceding Downcast, and PlaceTy::field_ty has no arm for that. Which lines up exactly with the "two variants ⇒ no panic" row above.

Stack

Mine names the moved_out_of_ref frame, which I didn't see in the original trace:

thread 'BIG_STACK_THREAD' panicked at crates/hir-ty/src/mir.rs:1271:22:
can't project out of E
   2: <hir_ty::mir::PlaceTy>::field_ty
   3: <hir_ty::mir::PlaceTy>::projection_ty::<la_arena::Idx<hir_ty::mir::Local>>
   4: hir_ty::mir::borrowck::moved_out_of_ref::{closure#0}
   5: <<hir_ty::InferBodyId>::borrowck::_::borrowck_query_Configuration_ as salsa::function::Configuration>::execute::inner_::{closure#0}
   8: <hir_ty::InferBodyId>::borrowck
   9: <hir::DefWithBody>::diagnostics
  10: <hir::ModuleDef>::diagnostics
  11: <hir::Module>::diagnostics
  12: ide_diagnostics::semantic_diagnostics
  13: ide_diagnostics::full_diagnostics
  14: <ide::Analysis>::full_diagnostics

Same family as #22757 / #22758 (field projection off a slice) — field_ty panicking instead of bailing out.

[ChayimFriedman2]

@MatthiasvB That is not impossible but unlikely because:

• The code for closure captures was copied from rustc, it's very unlikely to have bugs.
• The panic is in borrow checking and not in inference, where closure captures are computed.

[MatthiasvB]

@ChayimFriedman2 could you explain what part is unlikely? The repro does produce a panic during rust-analyzer diagnostics .

[ChayimFriedman2]

could you explain what part is unlikely?

That edition 2024 closure captures are to blame.

[jiripospisil]

fn main() {
    let vec = vec![];
    let foo = vec.len
}

rust-analyzer analysis-stats . --run-all-ide-things

(...)

thread 'main' (23577) panicked at src/tools/rust-analyzer/crates/hir-ty/src/mir.rs:1271:22:
can't project out of [{type error}]
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <hir_ty::mir::PlaceTy>::field_ty
   3: <hir_ty::mir::PlaceTy>::projection_ty::<la_arena::Idx<hir_ty::mir::Local>>
   4: hir_ty::mir::borrowck::moved_out_of_ref::{closure#0}
   5: <<hir_ty::InferBodyId>::borrowck::_::borrowck_query_Configuration_ as salsa::function::Configuration>::execute::inner_::{closure#0}
   6: <<hir_ty::InferBodyId>::borrowck::_::borrowck_query_Configuration_ as salsa::function::Configuration>::execute
   7: <salsa::function::IngredientImpl<<hir_ty::InferBodyId>::borrowck::_::borrowck_query_Configuration_>>::execute
   8: <hir_ty::InferBodyId>::borrowck
   9: <hir::DefWithBody>::diagnostics
  10: <hir::ModuleDef>::diagnostics
  11: <hir::Module>::diagnostics
  12: ide_diagnostics::semantic_diagnostics
  13: ide_diagnostics::full_diagnostics
  14: <ide::Analysis>::full_diagnostics
  15: <rust_analyzer::cli::flags::AnalysisStats>::run
  16: rust_analyzer::main

rust-analyzer 1.99.0-nightly (7608eb7 2026-08-05)