if-let guard point to let_chains issue

[tesuji]

I tried this code:

pub fn foo(a: &[u8], b: bool) -> bool {
    match b {
        true if let [1, 2, 3, ..] = a => true,
        _ => false,
    }
}

I expected to see this happen:
The compilation fails and error points out that if-let guard is not implemented
and being tracked at #51114

Instead, this happened:
The error points to let_chains feature, which is wrong and adding that feature gate
doesn't make the code compile.

error[E0658]: `let` expressions in this position are experimental
 --> <source>:3:17
  |
3 |         true if let [1, 2, 3, ..] = a => true,
  |                 ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
  = help: add `#![feature(let_chains)]` to the crate attributes to enable

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (8aa18cbdc 2020-07-08)
binary: rustc
commit-hash: 8aa18cbdc5d4bc33bd61e2d9a4b643d87f5d21de
commit-date: 2020-07-08
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0
Compiler returned: 0