Panic when completing Vec<{unknown}> (nightly regression)

[iDawer]

fn main() {
    let x: Vec<_>;
    x.$0
}

Panic context:
> 
version: b21f15a23 2021-09-30 nightly
request: textDocument/completion CompletionParams {
    text_document_position: TextDocumentPositionParams {
        text_document: TextDocumentIdentifier {
            uri: Url {
                scheme: "file",
                cannot_be_a_base: false,
                username: "",
                password: None,
                host: None,
                port: None,
                path: "/tmp/play/src/main.rs",
                query: None,
                fragment: None,
            },
        },
        position: Position {
            line: 2,
            character: 6,
        },
    },
    work_done_progress_params: WorkDoneProgressParams {
        work_done_token: None,
    },
    partial_result_params: PartialResultParams {
        partial_result_token: None,
    },
    context: Some(
        CompletionContext {
            trigger_kind: Invoked,
            trigger_character: None,
        },
    ),
}

thread '<unnamed>' panicked at 'index out of bounds: the len is 0 but the index is 0', /home/runner/.cargo/registry/src/gb.xjqchip.workers.dev-1ecc6299db9ec823/chalk-ir-0.71.0/src/lib.rs:2752:10
stack backtrace:
   0: rust_begin_unwind
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/std/src/panicking.rs:515:5
   1: core::panicking::panic_fmt
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:92:14
   2: core::panicking::panic_bounds_check
             at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:69:5
   3: <&chalk_ir::SubstFolder<I,A> as chalk_ir::fold::Folder<I>>::fold_free_var_ty
   4: <chalk_ir::Ty<I> as chalk_ir::fold::SuperFold<I>>::super_fold_with
   5: chalk_ir::fold::boring_impls::<impl chalk_ir::fold::Fold<I> for chalk_ir::GenericArg<I>>::fold_with
   6: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
   7: chalk_ir::fold::boring_impls::<impl chalk_ir::fold::Fold<I> for chalk_ir::Substitution<I>>::fold_with
   8: <chalk_ir::Ty<I> as chalk_ir::fold::SuperFold<I>>::super_fold_with
   9: chalk_ir::fold::boring_impls::<impl chalk_ir::fold::Fold<I> for chalk_ir::GenericArg<I>>::fold_with
  10: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
  11: chalk_ir::fold::boring_impls::<impl chalk_ir::fold::Fold<I> for chalk_ir::Substitution<I>>::fold_with
  12: chalk_ir::_::<impl chalk_ir::fold::Fold<I> for chalk_ir::WhereClause<I>>::fold_with
  13: chalk_ir::_::<impl chalk_ir::fold::Fold<I> for chalk_ir::DomainGoal<I>>::fold_with
  14: chalk_ir::_::<impl chalk_ir::fold::Fold<_I> for chalk_ir::InEnvironment<G>>::fold_with
  15: chalk_solve::infer::InferenceTable<I>::from_canonical
  16: chalk_recursive::solve::SolveIteration::solve_iteration
  17: chalk_recursive::fixed_point::RecursiveContext<K,V>::solve_goal
  18: chalk_recursive::fixed_point::RecursiveContext<K,V>::solve_root_goal
  19: hir_ty::traits::trait_solve_query
  20: salsa::runtime::Runtime::execute_query_implementation
  21: salsa::derived::slot::Slot<Q,MP>::read_upgrade
  22: salsa::derived::slot::Slot<Q,MP>::read
  23: <salsa::derived::DerivedStorage<Q,MP> as salsa::plumbing::QueryStorageOps<Q>>::try_fetch
  24: <DB as hir_ty::db::HirDatabase>::trait_solve_query::__shim
  25: <DB as hir_ty::db::HirDatabase>::trait_solve_query
  26: hir_ty::db::trait_solve_wait
  27: <DB as hir_ty::db::HirDatabase>::trait_solve
  28: hir_ty::autoderef::deref_by_trait
  29: hir_ty::autoderef::deref
  30: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::next
  31: ide_completion::completions::dot::complete_dot
  32: ide_completion::completions
  33: std::panicking::try
  34: rust_analyzer::handlers::handle_completion
  35: std::panicking::try
  36: <F as threadpool::FnBox>::call_box
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
[Error - 1:26:20 PM] Request textDocument/completion failed.
  Message: server panicked: index out of bounds: the len is 0 but the index is 0
  Code: -32603 

This is even broader. It panics at let _ = Vec::$0, let _ = std::collections::HashMap::$0

CC #10373 This regressed after that PR I suppose. I simply tried examples from it and got the panic.

[lnicola]

CC #10390

Are you sure it was caused by #10373?

[iDawer]

Let me recheck with revert

[Nima155]

This is also happening to me!

[iDawer]

Are you sure it was caused by #10373?

I think so: just applied reverts of #10373 to b21f15a nightly tag, dev build didn't panic on the examples.

[iDawer]

Oops

https://github.com/rust-analyzer/rust-analyzer/blob/629db286d1ac8f43d67c65827ff8f5a50c095e04/crates/hir/src/lib.rs#L2505-L2508
I dropped binders here for no reason!

I'll post a fix soon