fails to compile clap 2.33.3 -- 'if' is not allowed in a 'const fn'

[jtmoon79]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Rust Version

1.42.1

Specifically rustc version 1.41.1 and cargo version 1.42.1.

Clap Version

2.33.3

Minimal reproducible code

src/main.rs

// src/main.rs
fn main() {}

Cargo.toml

[package]
name = "cargo1"
version = "0.0.1"

[dependencies]
clap = "2.33.3"

Steps to reproduce the bug with the above code

cargo build

Actual Behaviour

$ cargo build
   Compiling clap v2.33.3
error[E0658]: `if` is not allowed in a `const fn`
  --> /home/user1/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-2.33.3/src/app/settings.rs:7:1
   |
7  | / bitflags! {
8  | |     struct Flags: u64 {
9  | |         const SC_NEGATE_REQS       = 1;
10 | |         const SC_REQUIRED          = 1 << 1;
...  |
51 | |     }
52 | | }
   | |_^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error[E0658]: `if` is not allowed in a `const fn`
  --> /home/user1/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-2.33.3/src/args/settings.rs:6:1
   |
6  | / bitflags! {
7  | |     struct Flags: u32 {
8  | |         const REQUIRED         = 1;
9  | |         const MULTIPLE         = 1 << 1;
...  |
28 | |     }
29 | | }
   | |_^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/49146
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.
error: could not compile `clap`.

Expected Behaviour

$ cargo build

   Compiling cargo1 v0.0.1 (/tmp/project1)
    Finished dev [unoptimized + debuginfo] target(s) in 2.34s

Additional Context

Run on Debian 9 Linux.

Debug Output

No difference in cargo build output.

[jtmoon79]

The same minimal reproduction setup passes when using cargo version 1.51.0 and rustc version 1.51.0 on Ubuntu 20.04.

The versions reported 1.42.1 and 1.41.1 are the supported versions in Debian 9 which is supported by Debian until July 2022.

[epage]

This is a duplicate of #2691. Users are subject to how the dependency resolver selects all crates and how that impacts MSRV. By default, it will pick the latest for everything, so with a new bitflags changing its MSRV, people are now picking that up. The flag -Z minimal-versions is an option to get around this but the challenge is not everyone tests with it, so people will have too-low of dependencies. The other option is to patch dependencies.

What clap shouldn't do is constrain the dependency for everyone for the sake of MSRV. That would be inaccurate and can lead to other problems like what happened to nom