Auto merge of rust-lang#8857 - smoelius:fix-8855, r=flip1995

Add test for rust-lang#8855

Fix rust-lang#8855

Here is what I think is going on.

First, the expression `format!("{:>6} {:>6}", a, b.to_string())` expands to:
```rust
{
    let res =
        ::alloc::fmt::format(::core::fmt::Arguments::new_v1_formatted(&["",
                            " "],
                &[::core::fmt::ArgumentV1::new_display(&a),
                            ::core::fmt::ArgumentV1::new_display(&b.to_string())],
                &[::core::fmt::rt::v1::Argument {
                                position: 0usize,
                                format: ::core::fmt::rt::v1::FormatSpec {
                                    fill: ' ',
                                    align: ::core::fmt::rt::v1::Alignment::Right,
                                    flags: 0u32,
                                    precision: ::core::fmt::rt::v1::Count::Implied,
                                    width: ::core::fmt::rt::v1::Count::Is(6usize),
                                },
                            },
                            ::core::fmt::rt::v1::Argument {
                                position: 1usize,
                                format: ::core::fmt::rt::v1::FormatSpec {
                                    fill: ' ',
                                    align: ::core::fmt::rt::v1::Alignment::Right,
                                    flags: 0u32,
                                    precision: ::core::fmt::rt::v1::Count::Implied,
                                    width: ::core::fmt::rt::v1::Count::Is(6usize),
                                },
                            }], unsafe { ::core::fmt::UnsafeArg::new() }));
    res
}
```
When I dump the expressions that get past the call to `has_string_formatting` [here](https://github.com/rust-lang/rust-clippy/blob/b312ad7d0cf0f30be2bd4658b71a3520a2e76709/clippy_lints/src/format_args.rs#L83), I see more than I would expect.

In particular, I see this subexpression of the above:
```
                &[::core::fmt::ArgumentV1::new_display(&a),
                            ::core::fmt::ArgumentV1::new_display(&b.to_string())],
```

This suggests to me that more expressions are getting past [this call](https://github.com/rust-lang/rust-clippy/blob/b312ad7d0cf0f30be2bd4658b71a3520a2e76709/clippy_lints/src/format_args.rs#L71) to `FormatArgsExpn::parse` than should.

Those expressions are then visited, but no `::core::fmt::rt::v1::Argument`s are found and pushed [here](https://github.com/rust-lang/rust-clippy/blob/b312ad7d0cf0f30be2bd4658b71a3520a2e76709/clippy_utils/src/macros.rs#L407).

As a result, the expressions appear unformatted, hence, the false positive.

My proposed fix is to restrict `FormatArgsExpn::parse` so that it only matches `Call` expressions.

cc: `@akanalytics`

changelog: none

Author: bors

tests/ui/format_args.fixed

@@ -122,3 +122,27 @@ fn issue8643(vendor_id: usize, product_id: usize, name: &str) {
         name
     );
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/8855
+mod issue_8855 {
+    #![allow(dead_code)]
+
+    struct A {}
+
+    impl std::fmt::Display for A {
+        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+            write!(f, "test")
+        }
+    }
+
+    fn main() {
+        let a = A {};
+        let b = A {};
+
+        let x = format!("{} {}", a, b);
+        dbg!(x);
+
+        let x = format!("{:>6} {:>6}", a, b.to_string());
+        dbg!(x);
+    }
+}

tests/ui/format_args.rs

@@ -122,3 +122,27 @@ fn issue8643(vendor_id: usize, product_id: usize, name: &str) {
         name
     );
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/8855
+mod issue_8855 {
+    #![allow(dead_code)]
+
+    struct A {}
+
+    impl std::fmt::Display for A {
+        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+            write!(f, "test")
+        }
+    }
+
+    fn main() {
+        let a = A {};
+        let b = A {};
+
+        let x = format!("{} {}", a, b.to_string());
+        dbg!(x);
+
+        let x = format!("{:>6} {:>6}", a, b.to_string());
+        dbg!(x);
+    }
+}

tests/ui/format_args.stderr

@@ -126,5 +126,11 @@ error: `to_string` applied to a type that implements `Display` in `println!` arg
 LL |     println!("{foo}{bar}", bar = "bar", foo = "foo".to_string());
    |                                                    ^^^^^^^^^^^^ help: remove this
 
-error: aborting due to 21 previous errors
+error: `to_string` applied to a type that implements `Display` in `format!` args
+  --> $DIR/format_args.rs:142:38
+   |
+LL |         let x = format!("{} {}", a, b.to_string());
+   |                                      ^^^^^^^^^^^^ help: remove this
+
+error: aborting due to 22 previous errors