Add a split-use pass.

It transforms "use std::io::{Read, Write}" into
"use std::io::Read; use std::io::Write"

Which later allows to remove just precisely the statements
we do not need, and leave rest be.

The test from the last commit does not pass, but that
is seemingly to do with the test harness setup, since it
works fine locally.

src/lib.rs

@@ -142,6 +142,7 @@ pub fn minimize(options: Options, stop: Arc<AtomicBool>) -> Result<()> {
 
     minimizer.run_passes([
         passes::EverybodyLoops::default().boxed(),
+        passes::SplitUse::default().boxed(),
         passes::FieldDeleter::default().boxed(),
         passes::Privatize::default().boxed(),
     ])?;

src/passes/mod.rs

@@ -2,8 +2,9 @@ mod everybody_loops;
 mod field_deleter;
 mod item_deleter;
 mod privatize;
+mod split_use;
 
 pub use self::{
     everybody_loops::EverybodyLoops, field_deleter::FieldDeleter, item_deleter::ItemDeleter,
-    privatize::Privatize,
+    privatize::Privatize, split_use::SplitUse,
 };

src/passes/split_use.rs

@@ -0,0 +1,139 @@
+use crate::processor::{tracking, Pass, PassController, ProcessState, SourceFile};
+use quote::ToTokens;
+
+use syn::{visit_mut::VisitMut, Item, ItemUse};
+
+struct Visitor<'a> {
+    process_state: ProcessState,
+    current_path: Vec<String>,
+    checker: &'a mut PassController,
+}
+
+impl<'a> Visitor<'a> {
+    fn new(checker: &'a mut PassController) -> Self {
+        Self {
+            process_state: ProcessState::NoChange,
+            current_path: Vec::new(),
+            checker,
+        }
+    }
+
+    // given a "some::group::{a, b::{c,d}, e}" tree, and assuming checker allows processing of (only) "some::group",
+    // returns a ["some::group::a", "some::group::b::{c,d}", "some::group::e"] list of trees.
+    fn expand_use_groups(&mut self, top: &syn::ItemUse, tree: &syn::UseTree) -> Vec<syn::UseTree> {
+        // It would probably be nice if instead of *expanding* the whole "some::group" group, we could instead
+        // *extract* individual items ("some::group::a"), but that makes code much more convoluted, sadly
+        match tree {
+            syn::UseTree::Path(p) => {
+                self.current_path.push(p.ident.to_string());
+
+                let out = self
+                    .expand_use_groups(top, &p.tree)
+                    .into_iter()
+                    .map(|x| {
+                        let mut new = p.clone();
+                        new.tree = Box::new(x);
+                        syn::UseTree::Path(new)
+                    })
+                    .collect();
+
+                self.current_path.pop();
+                out
+            }
+            syn::UseTree::Group(g) => {
+                let new_trees = g
+                    .items
+                    .iter()
+                    .map(|subtree| self.expand_use_groups(top, subtree))
+                    .flatten()
+                    .collect::<Vec<_>>();
+
+                self.current_path.push("{{group}}".to_string());
+                let can_process = self.checker.can_process(&self.current_path);
+                self.current_path.pop();
+                if can_process {
+                    self.process_state = ProcessState::Changed;
+                    return new_trees;
+                } else {
+                    // Do not expand the group.
+                    // recreate the UseTree::Group item (but with new subtrees), and return a single-element list
+                    let mut g = g.clone();
+                    g.items.clear();
+                    g.items.extend(new_trees);
+                    return vec![syn::UseTree::Group(g)];
+                }
+            }
+            _ => return vec![tree.clone()],
+        }
+    }
+
+    fn visit_item_list(&mut self, items: &mut Vec<syn::Item>) {
+        let mut pos = 0; // index into the `items` list
+        while pos < items.len() {
+            let item_use: ItemUse = {
+                match &items[pos] {
+                    Item::Use(u) => u.clone(),
+                    _ => {
+                        pos += 1; // if it's not a `use`` - simply advance to the next item
+                        continue;
+                    }
+                }
+            };
+
+            let new_use_trees = self.expand_use_groups(&item_use, &item_use.tree);
+            // decorate each of the UseTree with a `use` keyword (and any attributes inherited)
+            let new_uses = new_use_trees.into_iter().map(|x| {
+                let mut new = item_use.clone();
+                new.tree = x;
+                syn::Item::Use(new)
+            });
+
+            let step = new_uses.len();
+            // replace the old use with the new uses
+            items.splice(pos..pos + 1, new_uses);
+            pos += step; // do not process freshly inserted items
+        }
+    }
+}
+
+impl VisitMut for Visitor<'_> {
+    fn visit_item_mod_mut(&mut self, item_mod: &mut syn::ItemMod) {
+        self.current_path.push(item_mod.ident.to_string());
+        if let Some((_, items)) = &mut item_mod.content {
+            self.visit_item_list(items);
+        }
+        syn::visit_mut::visit_item_mod_mut(self, item_mod);
+        self.current_path.pop();
+    }
+    fn visit_file_mut(&mut self, file: &mut syn::File) {
+        self.visit_item_list(&mut file.items);
+        syn::visit_mut::visit_file_mut(self, file);
+    }
+
+    tracking!(visit_item_fn_mut);
+    tracking!(visit_impl_item_method_mut);
+    tracking!(visit_item_impl_mut);
+    tracking!(visit_field_mut);
+    tracking!(visit_item_struct_mut);
+    tracking!(visit_item_trait_mut);
+}
+
+#[derive(Default)]
+pub struct SplitUse {}
+
+impl Pass for SplitUse {
+    fn process_file(
+        &mut self,
+        krate: &mut syn::File,
+        _: &SourceFile,
+        checker: &mut PassController,
+    ) -> ProcessState {
+        let mut visitor = Visitor::new(checker);
+        visitor.visit_file_mut(krate);
+        visitor.process_state
+    }
+
+    fn name(&self) -> &'static str {
+        "split-use"
+    }
+}