Redundant closue lint can lose type annotation

[neverRare]

Summary

Redundant closure lint can lose type annotation. This can lead to type annotation needed error.

Lint Name

redundant_closure

Reproducer

In this rather contrived code

fn id<T>(x: T) -> T {
    x
}
fn main() {
    let _ = None.map(|x: i32| id(x));
}

Clippy reports the following

warning: redundant closure
 [--> src/main.rs:5:22
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
5 |     let _ = None.map(|x: i32| id(x));
  |                      ^^^^^^^^^^^^^^ help: replace the closure with the function itself: `id`
  |
  = note: `#[warn(clippy::redundant_closure)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

But if I do replace it, I get the following error

error[[E0282]](https://doc.rust-lang.org/nightly/error-index.html#E0282): type annotations needed for `Option<T>`
 [--> src/lib.rs:5:13
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
5 |     let _ = None.map(id);
  |         -   ^^^^ cannot infer type for type parameter `T` declared on the enum `Option`
  |         |
  |         consider giving this pattern the explicit type `Option<T>`, where the type parameter `T` is specified

For more information about this error, try `rustc --explain E0282`.

Version

No response

Additional Labels

No response

[dswij]

@rustbot claim

[dswij]

@rustbot release-assignment

[IceTDrinker]

Same kind of issue with a function pointer coming from C through the FFI that is passed as a Fn(u64) -> u64 through a wrapping closure (triggering the lint), correcting the lint by passing the function extern "C" fn(u64) -> u64 results in a compiler error expected an ``Fn<(u64,)>`` closure, found extern "C" fn(u64) -> u64

[profi248]

I've ran into this issue with clippy --fix, where Clippy replaced the closure with a direct mapping and broke the code.

This code:

pub async fn get_backup_path(&mut self) -> anyhow::Result<Option<PathBuf>> {
    let path = sqlx::query("select value from config where key = 'backup_path'")
        .fetch_optional(&mut self.transaction)
        .await?
        .map(|row| row.get(0));

    Ok(path.map(|path: String| PathBuf::from(path)))
}

Was "fixed" by Clippy by doing this:

pub async fn get_backup_path(&mut self) -> anyhow::Result<Option<PathBuf>> {
    let path = sqlx::query("select value from config where key = 'backup_path'")
        .fetch_optional(&mut self.transaction)
        .await?
        .map(|row| row.get(0));

    Ok(path.map(PathBuf::from))
}

Which produced this error:

error[E0282]: type annotations needed for `std::option::Option<_>`
  --> client/src/config/backup.rs:63:13
   |
63 |         let path = sqlx::query("select value from config where key = 'backup_path'")
   |             ^^^^
   |
help: consider giving `path` an explicit type, where the type for type parameter `U` is specified
   |
63 |         let path: std::option::Option<_> = sqlx::query("select value from config where key = 'backup_path'")
   |                 ++++++++++++++++++++++++

error[E0283]: type annotations needed for `std::option::Option<_>`
  --> client/src/config/backup.rs:63:13
   |
63 |         let path = sqlx::query("select value from config where key = 'backup_path'")
   |             ^^^^
...
68 |         Ok(path.map(PathBuf::from))
   |                     ------------- type must be known at this point
   |
   = note: multiple `impl`s satisfying `std::path::PathBuf: std::convert::From<_>` found in the following crates: `core`, `std`:
           - impl std::convert::From<std::boxed::Box<std::path::Path>> for std::path::PathBuf;
           - impl std::convert::From<std::ffi::OsString> for std::path::PathBuf;
           - impl std::convert::From<std::string::String> for std::path::PathBuf;
           - impl<'a> std::convert::From<std::borrow::Cow<'a, std::path::Path>> for std::path::PathBuf;
           - impl<T> std::convert::From<!> for T;
           - impl<T> std::convert::From<&T> for std::path::PathBuf
             where T: std::convert::AsRef<std::ffi::OsStr>, T: ?Sized;
           - impl<T> std::convert::From<T> for T;
help: consider giving `path` an explicit type, where the type for type parameter `T` is specified
   |
63 |         let path: std::option::Option<_> = sqlx::query("select value from config where key = 'backup_path'")
   |                 ++++++++++++++++++++++++

error[E0283]: type annotations needed for `std::option::Option<_>`
  --> client/src/config/backup.rs:63:13
   |
63 |         let path = sqlx::query("select value from config where key = 'backup_path'")
   |             ^^^^
...
68 |         Ok(path.map(PathBuf::from))
   |            ----------------------- type must be known at this point
   |
   = note: multiple `impl`s satisfying `std::path::PathBuf: std::convert::From<_>` found in the following crates: `core`, `std`:
           - impl std::convert::From<std::boxed::Box<std::path::Path>> for std::path::PathBuf;
           - impl std::convert::From<std::ffi::OsString> for std::path::PathBuf;
           - impl std::convert::From<std::string::String> for std::path::PathBuf;
           - impl<'a> std::convert::From<std::borrow::Cow<'a, std::path::Path>> for std::path::PathBuf;
           - impl<T> std::convert::From<!> for T;
           - impl<T> std::convert::From<&T> for std::path::PathBuf
             where T: std::convert::AsRef<std::ffi::OsStr>, T: ?Sized;
           - impl<T> std::convert::From<T> for T;
help: consider giving `path` an explicit type, where the type for type parameter `T` is specified
   |
63 |         let path: std::option::Option<_> = sqlx::query("select value from config where key = 'backup_path'")
   |                 ++++++++++++++++++++++++