Auto merge of #11070 - y21:issue11065, r=flip1995

[`useless_conversion`]: only lint on paths to fn items and fix FP in macro

Fixes #11065 (which is actually two issues: an ICE and a false positive)

It now makes sure that the function call path points to a function-like item (and not e.g. a `const` like in the linked issue), so that calling `TyCtxt::fn_sig` later in the lint does not ICE (fixes https://github.com/rust-lang/rust-clippy/issues/11065#issuecomment-1616836099).
It *also* makes sure that the expression is not part of a macro call (fixes https://github.com/rust-lang/rust-clippy/issues/11065#issuecomment-1616919639). ~~I'm not sure if there's a better way to check this other than to walk the parent expr chain and see if any of them are expansions.~~ (edit: it doesn't do this anymore)

changelog: [`useless_conversion`]: fix ICE when call receiver is a non-fn item
changelog: [`useless_conversion`]: don't lint if argument is a macro argument (fixes a FP)

r? `@llogiq` (reviewed #10814, which introduced these issues)

tests/ui/crashes/ice-11065.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::useless_conversion)]
+
+use std::iter::FromIterator;
+use std::option::IntoIter as OptionIter;
+
+fn eq<T: Eq>(a: T, b: T) -> bool {
+    a == b
+}
+
+macro_rules! tests {
+    ($($expr:expr, $ty:ty, ($($test:expr),*);)+) => (pub fn main() {$({
+        const C: $ty = $expr;
+        assert!(eq(C($($test),*), $expr($($test),*)));
+    })+})
+}
+
+tests! {
+    FromIterator::from_iter, fn(OptionIter<i32>) -> Vec<i32>, (Some(5).into_iter());
+}

tests/ui/useless_conversion.fixed

@@ -153,6 +153,20 @@ fn main() {
     let _ = vec![s4, s4, s4].into_iter();
 }
 
+#[allow(dead_code)]
+fn issue11065_fp() {
+    use std::option::IntoIter;
+    fn takes_into_iter(_: impl IntoIterator<Item = i32>) {}
+
+    macro_rules! x {
+        ($e:expr) => {
+            takes_into_iter($e);
+            let _: IntoIter<i32> = $e; // removing `.into_iter()` leads to a type error here
+        };
+    }
+    x!(Some(5).into_iter());
+}
+
 #[allow(dead_code)]
 fn explicit_into_iter_fn_arg() {
     fn a<T>(_: T) {}

tests/ui/useless_conversion.rs

@@ -153,6 +153,20 @@ fn main() {
     let _ = vec![s4, s4, s4].into_iter().into_iter();
 }
 
+#[allow(dead_code)]
+fn issue11065_fp() {
+    use std::option::IntoIter;
+    fn takes_into_iter(_: impl IntoIterator<Item = i32>) {}
+
+    macro_rules! x {
+        ($e:expr) => {
+            takes_into_iter($e);
+            let _: IntoIter<i32> = $e; // removing `.into_iter()` leads to a type error here
+        };
+    }
+    x!(Some(5).into_iter());
+}
+
 #[allow(dead_code)]
 fn explicit_into_iter_fn_arg() {
     fn a<T>(_: T) {}

tests/ui/useless_conversion.stderr

@@ -119,61 +119,61 @@ LL |     let _ = vec![s4, s4, s4].into_iter().into_iter();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![s4, s4, s4].into_iter()`
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> $DIR/useless_conversion.rs:169:7
+  --> $DIR/useless_conversion.rs:183:7
    |
 LL |     b(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> $DIR/useless_conversion.rs:159:13
+  --> $DIR/useless_conversion.rs:173:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> $DIR/useless_conversion.rs:170:7
+  --> $DIR/useless_conversion.rs:184:7
    |
 LL |     c(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> $DIR/useless_conversion.rs:160:18
+  --> $DIR/useless_conversion.rs:174:18
    |
 LL |     fn c(_: impl IntoIterator<Item = i32>) {}
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> $DIR/useless_conversion.rs:171:7
+  --> $DIR/useless_conversion.rs:185:7
    |
 LL |     d(vec![1, 2].into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> $DIR/useless_conversion.rs:163:12
+  --> $DIR/useless_conversion.rs:177:12
    |
 LL |         T: IntoIterator<Item = i32>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> $DIR/useless_conversion.rs:174:7
+  --> $DIR/useless_conversion.rs:188:7
    |
 LL |     b(vec![1, 2].into_iter().into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`s: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> $DIR/useless_conversion.rs:159:13
+  --> $DIR/useless_conversion.rs:173:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
-  --> $DIR/useless_conversion.rs:175:7
+  --> $DIR/useless_conversion.rs:189:7
    |
 LL |     b(vec![1, 2].into_iter().into_iter().into_iter());
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`s: `vec![1, 2]`
    |
 note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
-  --> $DIR/useless_conversion.rs:159:13
+  --> $DIR/useless_conversion.rs:173:13
    |
 LL |     fn b<T: IntoIterator<Item = i32>>(_: T) {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^