unnecessary_cast false positives in macros

[gnzlbg]

Playground:

#![deny(clippy::unnecessary_cast)]

macro_rules! foo {
    ($a:ident, $b:ident) => {
        pub fn $a() -> $b {
            1 as $b
        }
    };
}

foo!(a, i32);
foo!(b, f32);
foo!(c, f64);

produces

error: casting integer literal to f32 is unnecessary
  --> src/lib.rs:6:13
   |
6  |             1 as $b
   |             ^^^^^^^ help: try: `1_f32`
...
12 | foo!(b, f32);
   | ------------- in this macro invocation
   |
note: lint level defined here
  --> src/lib.rs:1:9
   |
1  | #![deny(clippy::unnecessary_cast)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast

error: casting integer literal to f64 is unnecessary
  --> src/lib.rs:6:13
   |
6  |             1 as $b
   |             ^^^^^^^ help: try: `1_f64`
...
13 | foo!(c, f64);
   | ------------- in this macro invocation
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast

error: aborting due to 2 previous errors

[cemiloten]

Hi, I would like to try to solve this, but I am very new to Rust, is that ok ?
If somebody could lead me towards a good way to getting started on it. Thanks :)

[cemiloten]

I see the same errors as you in the playground, but I cannot reproduce it in my own machine.

cargo new test_project
cd test_project/
mv src/main.rs src/lib.rs

... copy paste your code in src/lib.rs and save

cargo clippy

results in

Cemils-MBP:test_project cemil$ cargo clippy
    Checking test_project v0.1.0 (/Users/cemil/dev/rust/test_project)
warning: casting i32 to f64 may become silently lossy if types change
  --> src/lib.rs:6:13
   |
6  |             1 as $b
   |             ^^^^^^^ help: try: `f64::from(1)`
...
13 | foo!(c, f64);
   | ------------- in this macro invocation
   |
   = note: #[warn(clippy::cast_lossless)] on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless

    Finished dev [unoptimized + debuginfo] target(s) in 0.59s

[gnzlbg]

Can you reproduce with cargo clippy -- -D clippy::pedantic ?

[cemiloten]

Here is what I got (--verbose)

Cemils-MBP:test_project cemil$ cargo clippy --verbose -- -D clippy::pedantic
    Checking test_project v0.1.0 (/Users/cemil/dev/rust/test_project)
     Running `/Users/cemil/.rustup/toolchains/stable-x86_64-apple-darwin/bin/clippy-driver rustc --edition=2018 --crate-name test_project src/lib.rs --col
or always --crate-type lib --emit=dep-info,metadata -C debuginfo=2 -C metadata=c7c978c2d835e2e8 -C extra-filename=-c7c978c2d835e2e8 --out-dir /Users/cemil
/dev/rust/test_project/target/debug/deps -C incremental=/Users/cemil/dev/rust/test_project/target/debug/incremental -L dependency=/Users/cemil/dev/rust/te
st_project/target/debug/deps`
error: casting i32 to f32 causes a loss of precision (i32 is 32 bits wide, but f32's mantissa is only 23 bits wide)
  --> src/lib.rs:6:13
   |
6  |             1 as $b
   |             ^^^^^^^
...
12 | foo!(b, f32);
   | ------------- in this macro invocation
   |
   = note: `-D clippy::cast-precision-loss` implied by `-D clippy::pedantic`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_precision_loss

warning: casting i32 to f64 may become silently lossy if types change
  --> src/lib.rs:6:13
   |
6  |             1 as $b
   |             ^^^^^^^ help: try: `f64::from(1)`
...
13 | foo!(c, f64);
   | ------------- in this macro invocation
   |
   = note: #[warn(clippy::cast_lossless)] on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless

error: aborting due to previous error

error: Could not compile `test_project`.

Caused by:
  process didn't exit successfully: `/Users/cemil/.rustup/toolchains/stable-x86_64-apple-darwin/bin/clippy-driver rustc --edition=2018 --crate-name test_p
roject src/lib.rs --color always --crate-type lib --emit=dep-info,metadata -C debuginfo=2 -C metadata=c7c978c2d835e2e8 -C extra-filename=-c7c978c2d835e2e8
 --out-dir /Users/cemil/dev/rust/test_project/target/debug/deps -C incremental=/Users/cemil/dev/rust/test_project/target/debug/incremental -L dependency=/
Users/cemil/dev/rust/test_project/target/debug/deps` (exit code: 1)

Still not getting these errors, are you?

[flip1995]

Oh, what version of clippy do you have? (cargo clippy --version)
Try

$ rustup update nightly
($ rustup component add clippy --toolchain=nightly) # only necessary if you haven't installed Clippy in nightly before
$ cargo +nightly clippy

[cemiloten]

Ok with nightly I am seeing these errors now. How would you suggest getting started on this ? Is there a way to use a debugger when running clippy from a build of the project ? (I am using VSCode) In any case I'm not sure how to check my work. Thanks :)

[flip1995]

You can use the example code @gnzlbg provided and add it somewhere here:

rust-clippy/tests/ui/cast.rs

Line 46 in 9897442

// Test cast_unnecessary

After that you can test it with

$ TESTNAME=ui/cast cargo +master uitest

(You can install the master toolchain with RTIM or you can try if the nightly toolchain works by swapping master with nightly)

To fix this issue you need to add a macro check with the utils::in_macro function. You can grep the clippy_lints codebase to find examples how to do that.

After that the above command should run without errors. (Maybe some lines in the *.stderr file need to be adapted)

Also have a look at https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md. It's a really good documentation on how to work on Clippy.