useless_conversion: also fire inside compiler desugarings (#16594)

Using `e.span.from_expansion()` alone prevented the lint from firing
within compiler-desugared expressions, e.g. `async fn`, `for` loops, or
`?`.

changelog: [`useless_conversion`]: also fire inside compiler desugarings

close rust-lang/rust-clippy#16590

clippy_lints/src/useless_conversion.rs

@@ -157,7 +157,9 @@ fn into_iter_deep_call<'hir>(cx: &LateContext<'_>, mut expr: &'hir Expr<'hir>) -
 impl<'tcx> LateLintPass<'tcx> for UselessConversion {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
         if e.span.from_expansion() {
-            self.expn_depth += 1;
+            if e.span.desugaring_kind().is_none() {
+                self.expn_depth += 1;
+            }
             return;
         }
 
@@ -437,7 +439,7 @@ fn check_expr_post(&mut self, _: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
         if Some(&e.hir_id) == self.try_desugar_arm.last() {
             self.try_desugar_arm.pop();
         }
-        if e.span.from_expansion() {
+        if e.span.from_expansion() && e.span.desugaring_kind().is_none() {
             self.expn_depth -= 1;
         }
     }

tests/ui/useless_conversion.fixed

@@ -463,3 +463,34 @@ fn issue16165() {
     for _ in mac!(iter [1, 2]) {}
     //~^ useless_conversion
 }
+
+fn takes_into_iter_usize(_: impl IntoIterator<Item = usize>) {}
+fn takes_into_iter_usize_result(_: impl IntoIterator<Item = usize>) -> Result<(), ()> {
+    Ok(())
+}
+
+async fn issue16590() {
+    let a: Vec<usize> = vec![];
+    let b: Vec<usize> = vec![];
+
+    takes_into_iter_usize(b);
+    //~^ useless_conversion
+}
+
+fn in_for_loop() {
+    let a: Vec<usize> = vec![1, 2, 3];
+    let b: Vec<usize> = vec![4, 5, 6];
+
+    for _ in &a {
+        takes_into_iter_usize(b.clone());
+        //~^ useless_conversion
+    }
+}
+
+fn after_question_mark() -> Result<(), ()> {
+    let b: Vec<usize> = vec![4, 5, 6];
+
+    takes_into_iter_usize_result(b.clone())?;
+    //~^ useless_conversion
+    Ok(())
+}

tests/ui/useless_conversion.rs

@@ -463,3 +463,34 @@ macro_rules! mac {
     for _ in mac!(iter [1, 2]).into_iter() {}
     //~^ useless_conversion
 }
+
+fn takes_into_iter_usize(_: impl IntoIterator<Item = usize>) {}
+fn takes_into_iter_usize_result(_: impl IntoIterator<Item = usize>) -> Result<(), ()> {
+    Ok(())
+}
+
+async fn issue16590() {
+    let a: Vec<usize> = vec![];
+    let b: Vec<usize> = vec![];
+
+    takes_into_iter_usize(b.into_iter());
+    //~^ useless_conversion
+}
+
+fn in_for_loop() {
+    let a: Vec<usize> = vec![1, 2, 3];
+    let b: Vec<usize> = vec![4, 5, 6];
+
+    for _ in &a {
+        takes_into_iter_usize(b.clone().into_iter());
+        //~^ useless_conversion
+    }
+}
+
+fn after_question_mark() -> Result<(), ()> {
+    let b: Vec<usize> = vec![4, 5, 6];
+
+    takes_into_iter_usize_result(b.clone().into_iter())?;
+    //~^ useless_conversion
+    Ok(())
+}

tests/ui/useless_conversion.stderr

@@ -412,5 +412,47 @@ error: useless conversion to the same type: `std::slice::Iter<'_, i32>`
 LL |     for _ in mac!(iter [1, 2]).into_iter() {}
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `mac!(iter [1, 2])`
 
-error: aborting due to 45 previous errors
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> tests/ui/useless_conversion.rs:476:27
+   |
+LL |     takes_into_iter_usize(b.into_iter());
+   |                           ^------------
+   |                            |
+   |                            help: consider removing the `.into_iter()`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> tests/ui/useless_conversion.rs:467:34
+   |
+LL | fn takes_into_iter_usize(_: impl IntoIterator<Item = usize>) {}
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> tests/ui/useless_conversion.rs:485:31
+   |
+LL |         takes_into_iter_usize(b.clone().into_iter());
+   |                               ^^^^^^^^^------------
+   |                                        |
+   |                                        help: consider removing the `.into_iter()`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> tests/ui/useless_conversion.rs:467:34
+   |
+LL | fn takes_into_iter_usize(_: impl IntoIterator<Item = usize>) {}
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> tests/ui/useless_conversion.rs:493:34
+   |
+LL |     takes_into_iter_usize_result(b.clone().into_iter())?;
+   |                                  ^^^^^^^^^------------
+   |                                           |
+   |                                           help: consider removing the `.into_iter()`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> tests/ui/useless_conversion.rs:468:41
+   |
+LL | fn takes_into_iter_usize_result(_: impl IntoIterator<Item = usize>) -> Result<(), ()> {
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 48 previous errors