learnrust.rs requires unstable / nightly build

[fekerr]

(First - apologies - I am new to reporting issues for open source and new to Rust, but otherwise experienced in programming. However, I am probably more likely to hit issues that new learners will hit.)

Lines 298 and 310 of learnrust.rs won't compile for me (without warnings).

$ rustc --version
rustc 1.67.1 (d5a82bbd2 2023-02-07)

Line 298 of learnrust.rs:
ref_var; // no-op, but counts as a use and keeps the borrow active
Line 310 of learnrust.rs:
ref_var2; // no-op, but counts as a use and keeps the borrow active until here

I set it up like this:

cargo new learnxy
cd learnxy

copy learnrust.rs here as reference file (won't change)

cd src
cp ../learnrust.rs .

add "mod learnrust;" to beginning of main.rs

cargo run

~/src/rust/learnxy/src (main)
$ cargo run
Compiling learnxy v0.1.0 (C:\Users\feker\src\rust\learnxy)
warning: path statement with no effect
--> src\learnrust.rs:298:5
|
298 | ref_var; // no-op, but counts as a use and keeps the borrow active
| ^^^^^^^^
|
= note: #[warn(path_statements)] on by default

warning: path statement with no effect
--> src\learnrust.rs:310:5
|
310 | ref_var2; // no-op, but counts as a use and keeps the borrow active until here
| ^^^^^^^^^

warning: learnxy (bin "learnxy") generated 2 warnings
Finished dev [unoptimized + debuginfo] target(s) in 0.36s
Running C:\Users\feker\src\rust\learnxy\target\debug\learnxy.exe
Hello, world!

I tried #[allow(path_statements)] before 298, which is apparently ignored.

$ cargo run
Compiling learnxy v0.1.0 (C:\Users\feker\src\rust\learnxy)
warning: path statement with no effect
--> src\learnrust.rs:299:5
|
299 | ref_var; // no-op, but counts as a use and keeps the borrow active
| ^^^^^^^^
|
= note: #[warn(path_statements)] on by default

...

I had also tried #[warn(path_statements)] after that line, to turn the warnings back on, but then got this:

$ cargo run
Compiling learnxy v0.1.0 (C:\Users\feker\src\rust\learnxy)
error[E0658]: attributes on expressions are experimental
--> src\learnrust.rs:300:1
|
300 | #[warn(path_statements)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #15701 rust-lang/rust#15701 for more information

For more information about this error, try rustc --explain E0658.
error: could not compile learnxy due to previous error