Can rust provide a warning on unreachable `if` patterns?

[farcaller]

Here's a bug generated by a simple copy-paste mistake:

fn init_flash_access(freq: u32) {
  let num_clocks: u32 = if freq > 100_000_000 { 6 } else
                        if freq > 80_000_000 { 5 } else
                        if freq > 60_000_000 { 4 } else
                        if freq > 40_000_000 { 3 } else
                        if freq > 40_000_000 { 2 } else
                        { 1 };
  let val = (num_clocks - 1) << 12;
  reg::FLASHCFG.set_value(val);
}

This function will never return 2, as 40_000_000 in the last line should actually be 20_000_000. Is there any way rust could warn me about that or any other applicable pattern to do it?

[huonw]

It seems reasonable to have a lint that can check for copy-paste errors (this is probably quite simple, just compare ASTs for equality), more general range analysis etc. is presumably NP-hard (seems like it would be easy to encode 3-SAT) or just plain undecidable, meaning it wouldn't be possible pick up every case.

I will note that lints can now loaded as plugins (even via cargo, e.g., https://github.com/huonw/spellck), so it would be possible to have this as an external addition.

[kud1ing]

Just for reference, PVS-Studio has a check for this: http://www.viva64.com/en/d/0106/

[ftxqxd]

While attempting an implementation of such a lint, I encountered a problem: cfg! macros are often used in if conditions, and a lint would be triggered by a chain like:

if cfg!(target_os = "windows") { ... }
else if cfg!(target_os = "macos") { ... }
else if cfg!(target_os = "linux") { ... }

because at compile-time, at least two of the cfg!s would expand to false. Should this lint just ignore bare trues and falses (because presumably you know what you’re doing), or should such places manually disable the lint?

Also, this lint discovered what may be a bug in std: over here, $RP$ presumably stands for ‘raw pointer’, but a few lines later it’s used to mean ‘right parenthesis’. Assuming this is a bug, this really does show the use of such a lint.

[huonw]

@P1start oh, you tried implementing it too? Cool!

I got something vaguely working here: https://github.com/huonw/copypasteck (requires #15737 to work). (Although I haven't run it on any non-trivial code, so I didn't encounter the cfg! thing you describe.)

@huonw That looks very interesting but this issue seems nonetheless out of scope of rustc? Could this be closed?

I am going to close this issue. It is not feasible for the compiler to reason about arbitrary boolean expressions to determine which if clauses are not reachable. In addition, the narrow use case of warning on accidentally copy-pasted if clauses seems to be best served by a pluggable lint rather than a built-in one.