Update comment search in ControlFlow::rewrite_pat_expr for for loops

Resolves 5009

For loops represented by a ControlFlow object use " in" as their connector.
In order to find the span where comments might be located, rustfmt
searches for the first uncommented occurrence of the word "in" within the
current span and adjusts it's starting point to look for comments right after that.
visually this looks like this:

    ruftfmt starts looking for comments here
            |
            V
    for x in /* ... */ 0..1 {}

This works well in most cases, however when the pattern also contains
the word "in", this leads to issues.

    ruftfmt starts looking for comments here
          |
          V
    for in_here in /* ... */ 0..1 {}
        -------
        pattern

In order to correctly identify the connector, the new approach first
updates the span to start after the pattern and then searches for the
first uncommented occurrence of "in".

Author: ytmimi

src/expr.rs

@@ -820,9 +820,20 @@ impl<'a> ControlFlow<'a> {
                 .offset_left(matcher.len())?
                 .sub_width(self.connector.len())?;
             let pat_string = pat.rewrite(context, pat_shape)?;
-            let comments_lo = context
-                .snippet_provider
-                .span_after(self.span, self.connector.trim());
+            // When handling for loops we want to avoid incorrectly finding "in"
+            // inside the pattern so we first offset the span to start after the pattern
+            // see https://github.com/rust-lang/rustfmt/issues/5009
+            let comments_lo = if self.keyword == "for" {
+                let lo_after_pat = context.snippet_provider.span_after(self.span, &pat_string);
+
+                context
+                    .snippet_provider
+                    .span_after(self.span.with_lo(lo_after_pat), self.connector.trim())
+            } else {
+                context
+                    .snippet_provider
+                    .span_after(self.span, self.connector.trim())
+            };
             let comments_span = mk_sp(comments_lo, expr.span.lo());
             return rewrite_assign_rhs_with_comments(
                 context,

tests/target/issue-5009/1_minimum_example.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // the "in" inside the pattern produced invalid syntax
+    for variable_in_here /* ... */ in 0..1 {}
+}

tests/target/issue-5009/2_many_in_connectors_in_pattern.rs

@@ -0,0 +1,3 @@
+fn main() {
+    for in_in_in_in_in_in_in_in /* ... */ in 0..1 {}
+}

tests/target/issue-5009/3_nested_for_loop_with_connector_in_pattern.rs

@@ -0,0 +1,5 @@
+fn main() {
+    for variable_in_x /* ... */ in 0..1 {
+        for variable_in_y /* ... */ in 0..1 {}
+    }
+}

tests/target/issue-5009/4_nested_for_loop_with_if_elseif_else.rs

@@ -0,0 +1,13 @@
+fn main() {
+    for variable_in_x /* ... */ in 0..1 {
+        for variable_in_y /* ... */ in 0..1 {
+            if false {
+
+            } else if false {
+
+            } else {
+
+            }
+        }
+    }
+}

tests/target/issue-5009/5_nested_for_loop_with_connector_in_if_elseif_else.rs

@@ -0,0 +1,15 @@
+fn main() {
+    let in_ = false;
+
+    for variable_in_x /* ... */ in 0..1 {
+        for variable_in_y /* ... */ in 0..1 {
+            if in_ {
+
+            } else if in_ {
+
+            } else {
+
+            }
+        }
+    }
+}

tests/target/issue-5009/6_deeply_nested_for_loop_with_connector_in_pattern.rs

@@ -0,0 +1,32 @@
+fn main() {
+    for variable_in_a /* ... */ in 0..1 {
+        for variable_in_b /* ... */ in 0..1 {
+            for variable_in_c /* ... */ in 0..1 {
+                for variable_in_d /* ... */ in 0..1 {
+                    for variable_in_e /* ... */ in 0..1 {
+                        for variable_in_f /* ... */ in 0..1 {
+                            for variable_in_g /* ... */ in 0..1 {
+                                for variable_in_h /* ... */ in 0..1 {
+                                    for variable_in_i /* ... */ in 0..1 {
+                                        for variable_in_j /* ... */ in 0..1 {
+                                            for variable_in_k /* ... */ in 0..1 {
+                                                for variable_in_l /* ... */ in 0..1 {
+                                                    for variable_in_m /* ... */ in 0..1 {
+                                                        for variable_in_n /* ... */ in 0..1 {
+                                                            for variable_in_o /* ... */ in 0..1 {
+                                                            }
+                                                        }
+                                                    }
+                                                }
+                                            }
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

tests/target/issue-5009/7_for_loop_without_connector_in_pattern.rs

@@ -0,0 +1,4 @@
+// should not reformat
+fn main() {
+    for x /* ... */ in 0..1 {}
+}

tests/target/issue-5009/8_nested_for_loop_without_connector_in_pattern.rs

@@ -0,0 +1,6 @@
+// should not reformat
+fn main() {
+    for x /* ... */ in 0..1 {
+        for y /* ... */ in 0..1 {}
+    }
+}