Clippy reports map with identity function on functions that perform implicit conversion

[nihohit]

Summary

Functions that seem like identity functions, but contain implicit conversions such as &(A,B) -> (&A, &B) are warned against, even if they're required for compilation.

Example - line 7 cannot be removed, but clippy warns against it.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c27e3a77ebb1c965883045a67a8375b3

Lint Name

map_identity

Reproducer

I tried this code:

fn main() {
    let vec = [(1,2),(3,4)];
    let (k,v):(Vec<&usize>, Vec<&usize>) = vec
        .iter()
        .map(|(k,v)|(k,v)) // this isn't the identity function - it's a conversion &(K,V) -> (&K, &V)
        .unzip();
    println!("{k:?}, {v:?}");
}

I saw this happen:

    Checking playground v0.0.1 (/playground)
warning: unnecessary map of the identity function
 --> src/main.rs:6:16
  |
6 |           .iter()
  |  ________________^
7 | |         .map(|(k,v)|(k,v)) // this isn't the identity function - it's a conversion &(K,V) -> (&K, &V)
  | |__________________________^ help: remove the call to `map`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_identity
  = note: `#[warn(clippy::map_identity)]` on by default

I expected to see this happen:
no lint.

Version

rustc 1.75.0 (82e1608df 2023-12-21)
binary: rustc
commit-hash: 82e1608dfa6e0b5569232559e3d385fea5a93112
commit-date: 2023-12-21
host: x86_64-unknown-linux-gnu
release: 1.75.0
LLVM version: 17.0.6

Additional Labels

No response

[dannycjones]

Looks similar to #11764 which has a fix but I'm unsure when that'll be released into stable.

[PartiallyUntyped]

@dannycjones is correct, this has been fixed with #11792

Running uibless on latest commit:

TESTNAME=map_identity cargo uibless && git diff

--- a/tests/ui/map_identity.fixed
+++ b/tests/ui/map_identity.fixed
@@ -61,3 +61,12 @@ fn issue11764() {
     // no match ergonomics for `(i32, i32)`
     let _ = x.iter().copied();
 }
+
+fn issue12043() {
+    let vec = [(1,2),(3,4)];
+    let (k,v):(Vec<&usize>, Vec<&usize>) = vec
+        .iter()
+        .map(|(k,v)|(k,v)) // this isn't the identity function - it's a conversion &(K,V) -> (&K, &V)
+        .unzip();
+}
+
diff --git a/tests/ui/map_identity.rs b/tests/ui/map_identity.rs
index c646c0568..e0f941e6c 100644
--- a/tests/ui/map_identity.rs
+++ b/tests/ui/map_identity.rs
@@ -65,3 +65,12 @@ fn issue11764() {
     // no match ergonomics for `(i32, i32)`
     let _ = x.iter().copied().map(|(x, y)| (x, y));
 }
+
+fn issue12043() {
+    let vec = [(1,2),(3,4)];
+    let (k,v):(Vec<&usize>, Vec<&usize>) = vec
+        .iter()
+        .map(|(k,v)|(k,v)) // this isn't the identity function - it's a conversion &(K,V) -> (&K, &V)
+        .unzip();
+}
+

[blyxyas]

I'll close this then ~(=^‥^)/