Auto merge of #56645 - pietroalbini:fix-unused-imports, r=estebank

Initial implementation of rustfixable unused_imports lint

This PR adds the initial implementation of rustfixable `unused_imports` lint. The implementation works, but rustfix is not able to apply all the suggestions until #53934 is fixed. It also needs #58296 to hide the suggested note since it's really useless.

cc #47888

<details><summary><code>cargo fix</code> in action on the <code>unused_imports</code> lint</summary>

![screenshot from 2018-12-09 15-49-01](https://user-images.githubusercontent.com/2299951/49698874-3a026080-fbca-11e8-9bf1-24060b6c59c8.png)

</details>

src/librustc/lint/builtin.rs

@@ -473,6 +473,7 @@ pub enum BuiltinLintDiagnostics {
     MacroExpandedMacroExportsAccessedByAbsolutePaths(Span),
     ElidedLifetimesInPaths(usize, Span, bool, Span, String),
     UnknownCrateTypes(Span, String, String),
+    UnusedImports(String, Vec<(Span, String)>),
 }
 
 impl BuiltinLintDiagnostics {
@@ -554,6 +555,15 @@ impl BuiltinLintDiagnostics {
             BuiltinLintDiagnostics::UnknownCrateTypes(span, note, sugg) => {
                 db.span_suggestion(span, &note, sugg, Applicability::MaybeIncorrect);
             }
+            BuiltinLintDiagnostics::UnusedImports(message, replaces) => {
+                if !replaces.is_empty() {
+                    db.multipart_suggestion(
+                        &message,
+                        replaces,
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
         }
     }
 }

src/librustc_resolve/check_unused.rs

@@ -7,23 +7,52 @@
 //
 // Unused trait imports can't be checked until the method resolution. We save
 // candidates here, and do the actual check in librustc_typeck/check_unused.rs.
+//
+// Checking for unused imports is split into three steps:
+//
+//  - `UnusedImportCheckVisitor` walks the AST to find all the unused imports
+//    inside of `UseTree`s, recording their `NodeId`s and grouping them by
+//    the parent `use` item
+//
+//  - `calc_unused_spans` then walks over all the `use` items marked in the
+//    previous step to collect the spans associated with the `NodeId`s and to
+//    calculate the spans that can be removed by rustfix; This is done in a
+//    separate step to be able to collapse the adjacent spans that rustfix
+//    will remove
+//
+//  - `check_crate` finally emits the diagnostics based on the data generated
+//    in the last step
 
 use std::ops::{Deref, DerefMut};
 
 use crate::Resolver;
 use crate::resolve_imports::ImportDirectiveSubclass;
 
-use rustc::{lint, ty};
 use rustc::util::nodemap::NodeMap;
+use rustc::{lint, ty};
+use rustc_data_structures::fx::FxHashSet;
 use syntax::ast;
 use syntax::visit::{self, Visitor};
 use syntax_pos::{Span, MultiSpan, DUMMY_SP};
 
+struct UnusedImport<'a> {
+    use_tree: &'a ast::UseTree,
+    use_tree_id: ast::NodeId,
+    item_span: Span,
+    unused: FxHashSet<ast::NodeId>,
+}
+
+impl<'a> UnusedImport<'a> {
+    fn add(&mut self, id: ast::NodeId) {
+        self.unused.insert(id);
+    }
+}
 
 struct UnusedImportCheckVisitor<'a, 'b: 'a> {
     resolver: &'a mut Resolver<'b>,
     /// All the (so far) unused imports, grouped path list
-    unused_imports: NodeMap<NodeMap<Span>>,
+    unused_imports: NodeMap<UnusedImport<'a>>,
+    base_use_tree: Option<&'a ast::UseTree>,
     base_id: ast::NodeId,
     item_span: Span,
 }
@@ -46,24 +75,39 @@ impl<'a, 'b> DerefMut for UnusedImportCheckVisitor<'a, 'b> {
 impl<'a, 'b> UnusedImportCheckVisitor<'a, 'b> {
     // We have information about whether `use` (import) directives are actually
     // used now. If an import is not used at all, we signal a lint error.
-    fn check_import(&mut self, item_id: ast::NodeId, id: ast::NodeId, span: Span) {
+    fn check_import(&mut self, id: ast::NodeId) {
         let mut used = false;
         self.per_ns(|this, ns| used |= this.used_imports.contains(&(id, ns)));
         if !used {
             if self.maybe_unused_trait_imports.contains(&id) {
                 // Check later.
                 return;
             }
-            self.unused_imports.entry(item_id).or_default().insert(id, span);
+            self.unused_import(self.base_id).add(id);
         } else {
             // This trait import is definitely used, in a way other than
             // method resolution.
             self.maybe_unused_trait_imports.remove(&id);
-            if let Some(i) = self.unused_imports.get_mut(&item_id) {
-                i.remove(&id);
+            if let Some(i) = self.unused_imports.get_mut(&self.base_id) {
+                i.unused.remove(&id);
             }
         }
     }
+
+    fn unused_import(&mut self, id: ast::NodeId) -> &mut UnusedImport<'a> {
+        let use_tree_id = self.base_id;
+        let use_tree = self.base_use_tree.unwrap();
+        let item_span = self.item_span;
+
+        self.unused_imports
+            .entry(id)
+            .or_insert_with(|| UnusedImport {
+                use_tree,
+                use_tree_id,
+                item_span,
+                unused: FxHashSet::default(),
+            })
+    }
 }
 
 impl<'a, 'b> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b> {
@@ -88,31 +132,112 @@ impl<'a, 'b> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b> {
         // This allows the grouping of all the lints in the same item
         if !nested {
             self.base_id = id;
+            self.base_use_tree = Some(use_tree);
         }
 
         if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
-            // If it's the parent group, cover the entire use item
-            let span = if nested {
-                use_tree.span
-            } else {
-                self.item_span
-            };
-
             if items.is_empty() {
-                self.unused_imports
-                    .entry(self.base_id)
-                    .or_default()
-                    .insert(id, span);
+                self.unused_import(self.base_id).add(id);
             }
         } else {
-            let base_id = self.base_id;
-            self.check_import(base_id, id, use_tree.span);
+            self.check_import(id);
         }
 
         visit::walk_use_tree(self, use_tree, id);
     }
 }
 
+enum UnusedSpanResult {
+    Used,
+    FlatUnused(Span, Span),
+    NestedFullUnused(Vec<Span>, Span),
+    NestedPartialUnused(Vec<Span>, Vec<Span>),
+}
+
+fn calc_unused_spans(
+    unused_import: &UnusedImport<'_>,
+    use_tree: &ast::UseTree,
+    use_tree_id: ast::NodeId,
+) -> UnusedSpanResult {
+    // The full span is the whole item's span if this current tree is not nested inside another
+    // This tells rustfix to remove the whole item if all the imports are unused
+    let full_span = if unused_import.use_tree.span == use_tree.span {
+        unused_import.item_span
+    } else {
+        use_tree.span
+    };
+    match use_tree.kind {
+        ast::UseTreeKind::Simple(..) | ast::UseTreeKind::Glob => {
+            if unused_import.unused.contains(&use_tree_id) {
+                UnusedSpanResult::FlatUnused(use_tree.span, full_span)
+            } else {
+                UnusedSpanResult::Used
+            }
+        }
+        ast::UseTreeKind::Nested(ref nested) => {
+            if nested.len() == 0 {
+                return UnusedSpanResult::FlatUnused(use_tree.span, full_span);
+            }
+
+            let mut unused_spans = Vec::new();
+            let mut to_remove = Vec::new();
+            let mut all_nested_unused = true;
+            let mut previous_unused = false;
+            for (pos, (use_tree, use_tree_id)) in nested.iter().enumerate() {
+                let remove = match calc_unused_spans(unused_import, use_tree, *use_tree_id) {
+                    UnusedSpanResult::Used => {
+                        all_nested_unused = false;
+                        None
+                    }
+                    UnusedSpanResult::FlatUnused(span, remove) => {
+                        unused_spans.push(span);
+                        Some(remove)
+                    }
+                    UnusedSpanResult::NestedFullUnused(mut spans, remove) => {
+                        unused_spans.append(&mut spans);
+                        Some(remove)
+                    }
+                    UnusedSpanResult::NestedPartialUnused(mut spans, mut to_remove_extra) => {
+                        all_nested_unused = false;
+                        unused_spans.append(&mut spans);
+                        to_remove.append(&mut to_remove_extra);
+                        None
+                    }
+                };
+                if let Some(remove) = remove {
+                    let remove_span = if nested.len() == 1 {
+                        remove
+                    } else if pos == nested.len() - 1 || !all_nested_unused {
+                        // Delete everything from the end of the last import, to delete the
+                        // previous comma
+                        nested[pos - 1].0.span.shrink_to_hi().to(use_tree.span)
+                    } else {
+                        // Delete everything until the next import, to delete the trailing commas
+                        use_tree.span.to(nested[pos + 1].0.span.shrink_to_lo())
+                    };
+
+                    // Try to collapse adjacent spans into a single one. This prevents all cases of
+                    // overlapping removals, which are not supported by rustfix
+                    if previous_unused && !to_remove.is_empty() {
+                        let previous = to_remove.pop().unwrap();
+                        to_remove.push(previous.to(remove_span));
+                    } else {
+                        to_remove.push(remove_span);
+                    }
+                }
+                previous_unused = remove.is_some();
+            }
+            if unused_spans.is_empty() {
+                UnusedSpanResult::Used
+            } else if all_nested_unused {
+                UnusedSpanResult::NestedFullUnused(unused_spans, full_span)
+            } else {
+                UnusedSpanResult::NestedPartialUnused(unused_spans, to_remove)
+            }
+        }
+    }
+}
+
 pub fn check_crate(resolver: &mut Resolver<'_>, krate: &ast::Crate) {
     for directive in resolver.potentially_unused_imports.iter() {
         match directive.subclass {
@@ -152,14 +277,33 @@ pub fn check_crate(resolver: &mut Resolver<'_>, krate: &ast::Crate) {
     let mut visitor = UnusedImportCheckVisitor {
         resolver,
         unused_imports: Default::default(),
+        base_use_tree: None,
         base_id: ast::DUMMY_NODE_ID,
         item_span: DUMMY_SP,
     };
     visit::walk_crate(&mut visitor, krate);
 
-    for (id, spans) in &visitor.unused_imports {
+    for unused in visitor.unused_imports.values() {
+        let mut fixes = Vec::new();
+        let mut spans = match calc_unused_spans(unused, unused.use_tree, unused.use_tree_id) {
+            UnusedSpanResult::Used => continue,
+            UnusedSpanResult::FlatUnused(span, remove) => {
+                fixes.push((remove, String::new()));
+                vec![span]
+            }
+            UnusedSpanResult::NestedFullUnused(spans, remove) => {
+                fixes.push((remove, String::new()));
+                spans
+            }
+            UnusedSpanResult::NestedPartialUnused(spans, remove) => {
+                for fix in &remove {
+                    fixes.push((*fix, String::new()));
+                }
+                spans
+            }
+        };
+
         let len = spans.len();
-        let mut spans = spans.values().cloned().collect::<Vec<Span>>();
         spans.sort();
         let ms = MultiSpan::from_spans(spans.clone());
         let mut span_snippets = spans.iter()
@@ -177,6 +321,21 @@ pub fn check_crate(resolver: &mut Resolver<'_>, krate: &ast::Crate) {
                           } else {
                               String::new()
                           });
-        visitor.session.buffer_lint(lint::builtin::UNUSED_IMPORTS, *id, ms, &msg);
+
+        let fix_msg = if fixes.len() == 1 && fixes[0].0 == unused.item_span {
+            "remove the whole `use` item"
+        } else if spans.len() > 1 {
+            "remove the unused imports"
+        } else {
+            "remove the unused import"
+        };
+
+        visitor.session.buffer_lint_with_diagnostic(
+            lint::builtin::UNUSED_IMPORTS,
+            unused.use_tree_id,
+            ms,
+            &msg,
+            lint::builtin::BuiltinLintDiagnostics::UnusedImports(fix_msg.into(), fixes),
+        );
     }
 }

src/test/ui/bad/bad-lint-cap2.stderr

@@ -2,7 +2,7 @@ error: unused import: `std::option`
   --> $DIR/bad-lint-cap2.rs:6:5
    |
 LL | use std::option; //~ ERROR
-   |     ^^^^^^^^^^^
+   | ----^^^^^^^^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/bad-lint-cap2.rs:4:9

src/test/ui/bad/bad-lint-cap3.stderr

@@ -2,7 +2,7 @@ warning: unused import: `std::option`
   --> $DIR/bad-lint-cap3.rs:7:5
    |
 LL | use std::option; //~ WARN
-   |     ^^^^^^^^^^^
+   | ----^^^^^^^^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/bad-lint-cap3.rs:4:9

src/test/ui/imports/unused.stderr

@@ -2,7 +2,7 @@ error: unused import: `super::f`
   --> $DIR/unused.rs:7:24
    |
 LL |         pub(super) use super::f; //~ ERROR unused
-   |                        ^^^^^^^^
+   |         ---------------^^^^^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/unused.rs:1:9

src/test/ui/issues/issue-30730.stderr

@@ -2,7 +2,7 @@ error: unused import: `std::thread`
   --> $DIR/issue-30730.rs:3:5
    |
 LL | use std::thread;
-   |     ^^^^^^^^^^^
+   | ----^^^^^^^^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/issue-30730.rs:2:9

src/test/ui/lint/lint-directives-on-use-items-issue-10534.stderr

@@ -2,7 +2,7 @@ error: unused import: `a::x`
   --> $DIR/lint-directives-on-use-items-issue-10534.rs:12:9
    |
 LL |     use a::x; //~ ERROR: unused import
-   |         ^^^^
+   |     ----^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/lint-directives-on-use-items-issue-10534.rs:1:9
@@ -14,7 +14,7 @@ error: unused import: `a::y`
   --> $DIR/lint-directives-on-use-items-issue-10534.rs:21:9
    |
 LL |     use a::y; //~ ERROR: unused import
-   |         ^^^^
+   |     ----^^^^- help: remove the whole `use` item
    |
 note: lint level defined here
   --> $DIR/lint-directives-on-use-items-issue-10534.rs:20:12

src/test/ui/lint/lint-unused-imports.rs

@@ -6,7 +6,7 @@ use bar::c::cc as cal;
 use std::mem::*;            // shouldn't get errors for not using
                             // everything imported
 use std::fmt::{};
-//~^ ERROR unused import: `use std::fmt::{};`
+//~^ ERROR unused import: `std::fmt::{}`
 
 // Should get errors for both 'Some' and 'None'
 use std::option::Option::{Some, None};