Add ExternalCrate import grouping style

This creates two groups:

1. `std`, `core`, `alloc` and external crates,
2. `self`, `super` and `crate` imports.

Configurations.md

@@ -2229,7 +2229,7 @@ Controls the strategy for how consecutive imports are grouped together.
 Controls the strategy for grouping sets of consecutive imports. Imports may contain newlines between imports and still be grouped together as a single set, but other statements between imports will result in different grouping sets.
 
 - **Default value**: `Preserve`
-- **Possible values**: `Preserve`, `StdExternalCrate`, `One`
+- **Possible values**: `Preserve`, `StdExternalCrate`, `ExternalCrate`, `One`
 - **Stable**: No (tracking issue: [#5083](https://github.com/rust-lang/rustfmt/issues/5083))
 
 Each set of imports (one or more `use` statements, optionally separated by newlines) will be formatted independently. Other statements such as `mod ...` or `extern crate ...` will cause imports to not be grouped together.
@@ -2277,6 +2277,26 @@ use super::update::convert_publish_payload;
 use crate::models::Event;
 ```
 
+#### `ExternalCrate`:
+
+Discard existing import groups, and create two groups for:
+1. `std`, `core`, `alloc` and external crates,
+2. `self`, `super` and `crate` imports.
+
+```rust
+use alloc::alloc::Layout;
+use broker::database::PooledConnection;
+use chrono::Utc;
+use core::f32;
+use juniper::{FieldError, FieldResult};
+use std::sync::Arc;
+use uuid::Uuid;
+
+use super::schema::{Context, Payload};
+use super::update::convert_publish_payload;
+use crate::models::Event;
+```
+
 #### `One`:
 
 Discard existing import groups, and create a single group for everything

src/config/options.rs

@@ -112,6 +112,10 @@ pub enum GroupImportsTactic {
     ///  2. other imports
     ///  3. `self` / `crate` / `super` imports
     StdExternalCrate,
+    /// Discard existing groups, and create new groups for
+    ///  1. `std` / `core` / `alloc` / other imports
+    ///  2. `self` / `crate` / `super` imports
+    ExternalCrate,
     /// Discard existing groups, and create a single group for everything
     One,
 }

src/reorder.rs

@@ -116,7 +116,10 @@ fn rewrite_reorderable_or_regroupable_items(
                 GroupImportsTactic::Preserve | GroupImportsTactic::One => {
                     vec![normalized_items]
                 }
-                GroupImportsTactic::StdExternalCrate => group_imports(normalized_items),
+                GroupImportsTactic::StdExternalCrate => {
+                    group_imports_std_external_crate(normalized_items)
+                }
+                GroupImportsTactic::ExternalCrate => group_imports_external_crate(normalized_items),
             };
 
             if context.config.reorder_imports() {
@@ -172,7 +175,7 @@ fn contains_macro_use_attr(item: &ast::Item) -> bool {
 
 /// Divides imports into three groups, corresponding to standard, external
 /// and local imports. Sorts each subgroup.
-fn group_imports(uts: Vec<UseTree>) -> Vec<Vec<UseTree>> {
+fn group_imports_std_external_crate(uts: Vec<UseTree>) -> Vec<Vec<UseTree>> {
     let mut std_imports = Vec::new();
     let mut external_imports = Vec::new();
     let mut local_imports = Vec::new();
@@ -198,6 +201,30 @@ fn group_imports(uts: Vec<UseTree>) -> Vec<Vec<UseTree>> {
     vec![std_imports, external_imports, local_imports]
 }
 
+/// Divides imports into two groups, corresponding to external crates
+/// and local imports. Sorts each subgroup.
+fn group_imports_external_crate(uts: Vec<UseTree>) -> Vec<Vec<UseTree>> {
+    let mut external_imports = Vec::new();
+    let mut local_imports = Vec::new();
+
+    for ut in uts.into_iter() {
+        if ut.path.is_empty() {
+            external_imports.push(ut);
+            continue;
+        }
+        match &ut.path[0].kind {
+            UseSegmentKind::Ident(..) => external_imports.push(ut),
+            UseSegmentKind::Slf(_) | UseSegmentKind::Super(_) | UseSegmentKind::Crate(_) => {
+                local_imports.push(ut)
+            }
+            // These are probably illegal here
+            UseSegmentKind::Glob | UseSegmentKind::List(_) => external_imports.push(ut),
+        }
+    }
+
+    vec![external_imports, local_imports]
+}
+
 /// A simplified version of `ast::ItemKind`.
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 enum ReorderableItemKind {

tests/source/configs/group_imports/ExternalCrate.rs

@@ -0,0 +1,15 @@
+// rustfmt-group_imports: ExternalCrate
+use chrono::Utc;
+use super::update::convert_publish_payload;
+
+use juniper::{FieldError, FieldResult};
+use uuid::Uuid;
+use alloc::alloc::Layout;
+
+use std::sync::Arc;
+
+use broker::database::PooledConnection;
+
+use super::schema::{Context, Payload};
+use core::f32;
+use crate::models::Event;

tests/target/configs/group_imports/ExternalCrate.rs

@@ -0,0 +1,12 @@
+// rustfmt-group_imports: ExternalCrate
+use alloc::alloc::Layout;
+use broker::database::PooledConnection;
+use chrono::Utc;
+use core::f32;
+use juniper::{FieldError, FieldResult};
+use std::sync::Arc;
+use uuid::Uuid;
+
+use super::schema::{Context, Payload};
+use super::update::convert_publish_payload;
+use crate::models::Event;