Inner attributes on block expressions on match arms are not allowed

[lukas-code]

I tried this code:

fn main() {
    match 0 {
        _ => {
            #![allow(unused_variables)]
        }
    }
}

I expected to see this happen: The code compiles.

Instead, this happened: The code fails to compile with the following error:

error[E0658]: attributes on expressions are experimental
 --> test.rs:4:13
  |
4 |             #![allow(unused_variables)]
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

The following code compiles as expected:

fn main() {
    match 0 {
        _ => {{ // <-- double braces
            #![allow(unused_variables)]
        }}
    }
}

Meta

rustc --version --verbose:

binary: rustc
commit-hash: d3c79346a3e7ddbb5fb417810f226ac5a9209007
commit-date: 2020-02-29
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

[LeSeulArtichaut]

@rustbot modify labels: A-parser

[ehuss]

I believe this is working as expected. Attributes can be applied to blocks as statements, but not as expressions. In some cases, attributes can be used in expressions, but those were accidentally stabilized in #36995, and nobody noticed for a long time. The current stable rules are detailed in https://doc.rust-lang.org/reference/expressions/block-expr.html#attributes-on-block-expressions. Stabilization in other expression positions is tracked in the linked issue (#15701).

[Centril]

@ehuss's summary is accurate. Closing as not-a-bug.