Do not merge top-level modules with imports_granularity Module

Fix 6191

Top level modules are now always formatted as their own `use` statement when
setting `imports_granularity=Module`

Signed-off-by: mwlon <m.w.loncaric@gmail.com>

Author: mwlon

Configurations.md

@@ -2012,6 +2012,7 @@ use qux::{h, i};
 #### `Module`:
 
 Merge imports from the same module into a single `use` statement. Conversely, imports from different modules are split into separate statements.
+Does not merge top-level modules.
 
 ```rust
 use foo::b::{f, g};

src/imports.rs

@@ -33,6 +33,13 @@ pub(crate) fn path_to_imported_ident(path: &ast::Path) -> symbol::Ident {
     path.segments.last().unwrap().ident
 }
 
+/// Returns all but the last portion of the module path, except in the case of
+/// top-level modules (length 1), which remain unchanged. Used for Module-level
+/// imports_granularity.
+fn module_prefix(path: &[UseSegment]) -> &[UseSegment] {
+    &path[..(path.len() - 1).max(1)]
+}
+
 impl<'a> FmtVisitor<'a> {
     pub(crate) fn format_import(&mut self, item: &ast::Item, tree: &ast::UseTree) {
         let span = item.span();
@@ -683,9 +690,7 @@ impl UseTree {
         } else {
             match shared_prefix {
                 SharedPrefix::Crate => self.path[0] == other.path[0],
-                SharedPrefix::Module => {
-                    self.path[..self.path.len() - 1] == other.path[..other.path.len() - 1]
-                }
+                SharedPrefix::Module => module_prefix(&self.path) == module_prefix(&other.path),
                 SharedPrefix::One => true,
             }
         }

tests/source/imports/imports_granularity_module.rs

@@ -45,3 +45,8 @@ use b::v::{
 };
 use b::t::{/* Before b::t::self */ self};
 use b::c;
+
+use c;
+use d;
+
+use {library1, library2 as lib2, library3};

tests/target/imports/imports_granularity_module.rs

@@ -53,3 +53,10 @@ use b::{
     /* Before b::l group */ l::{self, m, n::o, p::*},
     q,
 };
+
+use c;
+use d;
+
+use library1;
+use library2 as lib2;
+use library3;