Implement One option for imports_granularity (rust-lang#4669)

This option merges all imports into a single `use` statement as long as
they have the same visibility.

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`, `Item`
+- **Possible values**: `Preserve`, `Crate`, `Module`, `Item`, `One`
 - **Stable**: No
 
 #### `Preserve` (default):
@@ -1764,6 +1764,23 @@ use qux::h;
 use qux::i;
 ```
 
+#### `One`:
+
+Merge all imports into a single `use` statement as long as they have the same visibility.
+
+```rust
+pub use foo::{x, y};
+use {
+    bar::{
+        a,
+        b::{self, f, g},
+        c,
+        d::e,
+    },
+    qux::{h, i},
+};
+```
+
 ## `merge_imports`
 
 This option is deprecated. Use `imports_granularity = "Crate"` instead.

src/config/options.rs

@@ -130,6 +130,8 @@ pub enum ImportGranularity {
     Module,
     /// Use one `use` statement per imported item.
     Item,
+    /// Use one `use` statement including all items.
+    One,
 }
 
 #[config_type]

src/formatting/imports.rs

@@ -143,6 +143,29 @@ impl UseSegment {
         }
     }
 
+    // Check if self == other with their aliases removed.
+    fn equal_except_alias(&self, other: &Self) -> bool {
+        match (self, other) {
+            (UseSegment::Ident(ref s1, _), UseSegment::Ident(ref s2, _)) => s1 == s2,
+            (UseSegment::Slf(_), UseSegment::Slf(_))
+            | (UseSegment::Super(_), UseSegment::Super(_))
+            | (UseSegment::Crate(_), UseSegment::Crate(_))
+            | (UseSegment::Glob, UseSegment::Glob) => true,
+            (UseSegment::List(ref list1), UseSegment::List(ref list2)) => list1 == list2,
+            _ => false,
+        }
+    }
+
+    fn get_alias(&self) -> Option<&str> {
+        match self {
+            UseSegment::Ident(_, a)
+            | UseSegment::Slf(a)
+            | UseSegment::Super(a)
+            | UseSegment::Crate(a) => a.as_deref(),
+            _ => None,
+        }
+    }
+
     fn from_path_segment(
         context: &RewriteContext<'_>,
         path_seg: &ast::PathSegment,
@@ -579,6 +602,7 @@ impl UseTree {
                 SharedPrefix::Module => {
                     self.path[..self.path.len() - 1] == other.path[..other.path.len() - 1]
                 }
+                SharedPrefix::One => true,
             }
         }
     }
@@ -616,7 +640,7 @@ impl UseTree {
     fn merge(&mut self, other: &UseTree, merge_by: SharedPrefix) {
         let mut prefix = 0;
         for (a, b) in self.path.iter().zip(other.path.iter()) {
-            if *a == *b {
+            if a.equal_except_alias(b) {
                 prefix += 1;
             } else {
                 break;
@@ -651,14 +675,20 @@ fn merge_rest(
             return Some(new_path);
         }
     } else if len == 1 {
-        let rest = if a.len() == len { &b[1..] } else { &a[1..] };
-        return Some(vec![
-            b[0].clone(),
-            UseSegment::List(vec![
-                UseTree::from_path(vec![UseSegment::Slf(None)], DUMMY_SP),
-                UseTree::from_path(rest.to_vec(), DUMMY_SP),
-            ]),
-        ]);
+        let (common, rest) = if a.len() == len {
+            (&a[0], &b[1..])
+        } else {
+            (&b[0], &a[1..])
+        };
+        let mut list = vec![UseTree::from_path(
+            vec![UseSegment::Slf(common.get_alias().map(ToString::to_string))],
+            DUMMY_SP,
+        )];
+        match rest {
+            [UseSegment::List(rest_list)] => list.extend(rest_list.clone()),
+            _ => list.push(UseTree::from_path(rest.to_vec(), DUMMY_SP)),
+        }
+        return Some(vec![b[0].clone(), UseSegment::List(list)]);
     } else {
         len -= 1;
     }
@@ -673,18 +703,55 @@ fn merge_rest(
 }
 
 fn merge_use_trees_inner(trees: &mut Vec<UseTree>, use_tree: UseTree, merge_by: SharedPrefix) {
-    let similar_trees = trees
-        .iter_mut()
-        .filter(|tree| tree.share_prefix(&use_tree, merge_by));
+    struct SimilarTree<'a> {
+        similarity: usize,
+        path_len: usize,
+        tree: &'a mut UseTree,
+    }
+
+    let similar_trees = trees.iter_mut().filter_map(|tree| {
+        if tree.share_prefix(&use_tree, merge_by) {
+            // In the case of `SharedPrefix::One`, `similarity` is used for deciding with which
+            // tree `use_tree` should be merge.
+            // In other cases `similarity` won't be used, so set it to `0` as a dummy value.
+            let similarity = if merge_by == SharedPrefix::One {
+                tree.path
+                    .iter()
+                    .zip(&use_tree.path)
+                    .take_while(|(a, b)| a.equal_except_alias(b))
+                    .count()
+            } else {
+                0
+            };
+
+            let path_len = tree.path.len();
+            Some(SimilarTree {
+                similarity,
+                tree,
+                path_len,
+            })
+        } else {
+            None
+        }
+    });
+
     if use_tree.path.len() == 1 && merge_by == SharedPrefix::Crate {
-        if let Some(tree) = similar_trees.min_by_key(|tree| tree.path.len()) {
-            if tree.path.len() == 1 {
+        if let Some(tree) = similar_trees.min_by_key(|tree| tree.path_len) {
+            if tree.path_len == 1 {
                 return;
             }
         }
-    } else if let Some(tree) = similar_trees.max_by_key(|tree| tree.path.len()) {
-        if tree.path.len() > 1 {
-            tree.merge(&use_tree, merge_by);
+    } else if merge_by == SharedPrefix::One {
+        if let Some(sim_tree) = similar_trees.max_by_key(|tree| tree.similarity) {
+            if sim_tree.similarity > 0 {
+                sim_tree.tree.merge(&use_tree, merge_by);
+                return;
+            }
+        }
+    } else if let Some(sim_tree) = similar_trees.max_by_key(|tree| (tree.path_len, tree.similarity))
+    {
+        if sim_tree.path_len > 1 {
+            sim_tree.tree.merge(&use_tree, merge_by);
             return;
         }
     }
@@ -897,6 +964,7 @@ impl Rewrite for UseTree {
 pub(crate) enum SharedPrefix {
     Crate,
     Module,
+    One,
 }
 
 #[cfg(test)]
@@ -921,7 +989,7 @@ mod test {
             }
 
             fn eat(&mut self, c: char) {
-                assert!(self.input.next().unwrap() == c);
+                assert_eq!(self.input.next().unwrap(), c);
             }
 
             fn push_segment(
@@ -1111,6 +1179,49 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_use_tree_merge_one() {
+        test_merge!(One, ["a", "b"], ["{a, b}"]);
+
+        test_merge!(One, ["a::{aa, ab}", "b", "a"], ["{a::{self, aa, ab}, b}"]);
+
+        test_merge!(One, ["a as x", "b as y"], ["{a as x, b as y}"]);
+
+        test_merge!(
+            One,
+            ["a::{aa as xa, ab}", "b", "a"],
+            ["{a::{self, aa as xa, ab}, b}"]
+        );
+
+        test_merge!(
+            One,
+            ["a", "a::{aa, ab::{aba, abb}}"],
+            ["a::{self, aa, ab::{aba, abb}}"]
+        );
+
+        test_merge!(One, ["a", "b::{ba, *}"], ["{a, b::{ba, *}}"]);
+
+        test_merge!(One, ["a", "b", "a::aa"], ["{a::{self, aa}, b}"]);
+
+        test_merge!(
+            One,
+            ["a::aa::aaa", "a::ac::aca", "a::aa::*"],
+            ["a::{aa::{aaa, *}, ac::aca}"]
+        );
+
+        test_merge!(
+            One,
+            ["a", "b::{ba, bb}", "a::{aa::*, ab::aba}"],
+            ["{a::{self, aa::*, ab::aba}, b::{ba, bb}}"]
+        );
+
+        test_merge!(
+            One,
+            ["b", "a::ac::{aca, acb}", "a::{aa::*, ab}"],
+            ["{a::{aa::*, ab, ac::{aca, acb}}, b}"]
+        );
+    }
+
     #[test]
     fn test_flatten_use_trees() {
         assert_eq!(

src/formatting/reorder.rs

@@ -234,6 +234,7 @@ fn rewrite_reorderable_or_regroupable_items(
                     merge_use_trees(normalized_items, SharedPrefix::Module)
                 }
                 ImportGranularity::Item => flatten_use_trees(normalized_items),
+                ImportGranularity::One => merge_use_trees(normalized_items, SharedPrefix::One),
                 ImportGranularity::Preserve => normalized_items,
             };
 

tests/source/imports_granularity_one.rs

@@ -0,0 +1,60 @@
+// rustfmt-imports_granularity: One
+
+use b;
+use a::ac::{aca, acb};
+use a::{aa::*, ab};
+
+use a as x;
+use b::ba;
+use a::{aa, ab};
+
+use a::aa::aaa;
+use a::ab::aba as x;
+use a::aa::*;
+
+use a::aa;
+use a::ad::ada;
+#[cfg(test)]
+use a::{ab, ac::aca};
+use b;
+#[cfg(test)]
+use b::{
+    ba, bb,
+    bc::bca::{bcaa, bcab},
+};
+
+pub use a::aa;
+pub use a::ae;
+use a::{ab, ac, ad};
+use b::ba;
+pub use b::{bb, bc::bca};
+
+use a::aa::aaa;
+use a::ac::{aca, acb};
+use a::{aa::*, ab};
+use b::{
+    ba,
+    bb::{self, bba},
+};
+
+use crate::a;
+use crate::b::ba;
+use c::ca;
+
+use super::a;
+use c::ca;
+use super::b::ba;
+
+use crate::a;
+use super::b;
+use c::{self, ca};
+
+use a::{
+    // some comment
+    aa::{aaa, aab},
+    ab,
+    // another comment
+    ac::aca,
+};
+use b as x;
+use a::ad::ada;

tests/target/imports_granularity_one.rs

@@ -0,0 +1,79 @@
+// rustfmt-imports_granularity: One
+
+use {
+    a::{
+        aa::*,
+        ab,
+        ac::{aca, acb},
+    },
+    b,
+};
+
+use {
+    a::{self as x, aa, ab},
+    b::ba,
+};
+
+use a::{
+    aa::{aaa, *},
+    ab::aba as x,
+};
+
+#[cfg(test)]
+use a::{ab, ac::aca};
+#[cfg(test)]
+use b::{
+    ba, bb,
+    bc::bca::{bcaa, bcab},
+};
+use {
+    a::{aa, ad::ada},
+    b,
+};
+
+pub use {
+    a::{aa, ae},
+    b::{bb, bc::bca},
+};
+use {
+    a::{ab, ac, ad},
+    b::ba,
+};
+
+use {
+    a::{
+        aa::{aaa, *},
+        ab,
+        ac::{aca, acb},
+    },
+    b::{
+        ba,
+        bb::{self, bba},
+    },
+};
+
+use {
+    crate::{a, b::ba},
+    c::ca,
+};
+
+use {
+    super::{a, b::ba},
+    c::ca,
+};
+
+use {
+    super::b,
+    crate::a,
+    c::{self, ca},
+};
+
+use {
+    a::{
+        aa::{aaa, aab},
+        ab,
+        ac::aca,
+        ad::ada,
+    },
+    b as x,
+};