Add imports_granularity="Flatten".

This option splits all imports into their own `use` statement.

Configurations.md

@@ -1710,7 +1710,7 @@ pub enum Foo {}
 Merge together related imports based on their paths.
 
 - **Default value**: `Preserve`
-- **Possible values**: `Preserve`, `Crate`, `Module`
+- **Possible values**: `Preserve`, `Crate`, `Module`, `Flatten`
 - **Stable**: No
 
 #### `Preserve` (default):
@@ -1749,6 +1749,21 @@ use foo::{a, b, c};
 use qux::{h, i};
 ```
 
+#### `Flatten`:
+
+Flatten imports so that each has its own `use` statement.
+
+```rust
+use foo::a;
+use foo::b;
+use foo::b::f;
+use foo::b::g;
+use foo::c;
+use foo::d::e;
+use qux::h;
+use qux::i;
+```
+
 ## `merge_imports`
 
 This option is deprecated. Use `imports_granularity = "Crate"` instead.

src/config/options.rs

@@ -128,6 +128,8 @@ pub enum ImportGranularity {
     Crate,
     /// Use one `use` statement per module.
     Module,
+    /// Use one `use` statement per imported item.
+    Flatten,
 }
 
 #[config_type]

src/formatting/imports.rs

@@ -542,6 +542,7 @@ impl UseTree {
                 SharedPrefix::Module => {
                     self.path[..self.path.len() - 1] == other.path[..other.path.len() - 1]
                 }
+                SharedPrefix::NoPrefix => false,
             }
         }
     }
@@ -854,6 +855,7 @@ impl Rewrite for UseTree {
 pub(crate) enum SharedPrefix {
     Crate,
     Module,
+    NoPrefix,
 }
 
 #[cfg(test)]
@@ -1068,6 +1070,21 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_use_tree_merge_no_prefix() {
+        test_merge!(
+            NoPrefix,
+            ["foo::{a::{b, c}, d::e}"],
+            ["foo::a::b", "foo::a::c", "foo::d::e"]
+        );
+
+        test_merge!(
+            NoPrefix,
+            ["foo::{self, a, b::{c, d}, e::*}"],
+            ["foo::self", "foo::a", "foo::b::c", "foo::b::d", "foo::e::*"]
+        )
+    }
+
     #[test]
     fn test_use_tree_flatten() {
         assert_eq!(

src/formatting/reorder.rs

@@ -228,15 +228,16 @@ fn rewrite_reorderable_or_regroupable_items(
             for (item, list_item) in normalized_items.iter_mut().zip(list_items) {
                 item.list_item = Some(list_item.clone());
             }
-            match context.config.imports_granularity() {
-                ImportGranularity::Crate => {
-                    normalized_items = merge_use_trees(normalized_items, SharedPrefix::Crate)
-                }
+            normalized_items = match context.config.imports_granularity() {
+                ImportGranularity::Crate => merge_use_trees(normalized_items, SharedPrefix::Crate),
                 ImportGranularity::Module => {
-                    normalized_items = merge_use_trees(normalized_items, SharedPrefix::Module)
+                    merge_use_trees(normalized_items, SharedPrefix::Module)
                 }
-                ImportGranularity::Preserve => {}
-            }
+                ImportGranularity::Flatten => {
+                    merge_use_trees(normalized_items, SharedPrefix::NoPrefix)
+                }
+                ImportGranularity::Preserve => normalized_items,
+            };
 
             let mut regrouped_items = match context.config.group_imports() {
                 GroupImportsTactic::Preserve => vec![normalized_items],

tests/target/imports_granularity_flatten.rs

@@ -0,0 +1,23 @@
+// rustfmt-imports_granularity: Separate
+
+use a::b::c;
+use a::d::e;
+use a::f;
+use a::g::h;
+use a::g::i;
+use a::j;
+use a::j::k;
+use a::j::k::l;
+use a::j::m;
+use a::n::o::p;
+use a::n::q;
+pub use a::r::s;
+pub use a::t;
+
+use foo::e;
+#[cfg(test)]
+use foo::{a::b, c::d};
+
+use bar::a::b;
+use bar::c::d;
+use bar::e::f;