fix: skip removing self in imports if they are stacked like use self::self; (#6573)

This prevents an idempotence issue where each run of rustfmt removed another `self` from the import path.
For example, `use self::self::self;` -> `use self::self;` -> `use self;` -> `{import removed}`.

Now `use self::self;`, `use self::self::self`, etc. are all left unchanged, and users will get a compiler error if they try to compile code with these invalid imports.

This change does not impact normalizing `use foo::self;` -> `use foo;`.

Author: MarcusGrass

src/imports.rs

@@ -566,8 +566,13 @@ impl UseTree {
 
         // Normalise foo::self -> foo.
         if let UseSegmentKind::Slf(None) = last.kind {
-            if !self.path.is_empty() {
-                return self;
+            if let Some(second_last) = self.path.pop() {
+                if matches!(second_last.kind, UseSegmentKind::Slf(_)) {
+                    self.path.push(second_last);
+                } else {
+                    self.path.push(second_last);
+                    return self;
+                }
             }
         }
 

tests/source/issue_6558.rs

@@ -0,0 +1,5 @@
+// Removing `self::self` breaks idempotence, leaving it causes a compilation error
+// which the user should be made aware of #6558
+use self;
+use self::self;
+use self::self::self;

tests/target/issue_6558.rs

@@ -0,0 +1,4 @@
+// Removing `self::self` breaks idempotence, leaving it causes a compilation error
+// which the user should be made aware of #6558
+use self::self;
+use self::self::self;