check redundant prelude imports

detects unnecessary imports in std::prelude that can be eliminated.

For example import:

```rust
use std::{option::{Iter, IterMut, IntoIter, Option::{self, Some}}, convert::{TryFrom, TryInto}, mem::drop};
```

delete : `Option::{self, Some}`  and `mem::drop`

compiler/rustc_macros/src/hash_stable.rs

@@ -1,6 +1,6 @@
-use proc_macro2::{self, Ident};
+use proc_macro2::Ident;
 use quote::quote;
-use syn::{self, parse_quote};
+use syn::parse_quote;
 
 struct Attributes {
     ignore: bool,

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -12,7 +12,7 @@ use crate::Namespace::{self, MacroNS, TypeNS, ValueNS};
 use crate::{errors, BindingKey, MacroData, NameBindingData};
 use crate::{Determinacy, ExternPreludeEntry, Finalize, Module, ModuleKind, ModuleOrUniformRoot};
 use crate::{NameBinding, NameBindingKind, ParentScope, PathResult, PerNS, ResolutionError};
-use crate::{Resolver, ResolverArenas, Segment, ToNameBinding, VisResolutionError};
+use crate::{Resolver, ResolverArenas, Segment, ToNameBinding, Used, VisResolutionError};
 
 use rustc_ast::visit::{self, AssocCtxt, Visitor};
 use rustc_ast::{self as ast, AssocItem, AssocItemKind, MetaItemKind, StmtKind};
@@ -358,7 +358,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
             root_span,
             root_id,
             vis: Cell::new(Some(vis)),
-            used: Cell::new(false),
+            used: Default::default(),
         });
 
         self.r.indeterminate_imports.push(import);
@@ -852,7 +852,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
             span: item.span,
             module_path: Vec::new(),
             vis: Cell::new(Some(vis)),
-            used: Cell::new(used),
+            used: Cell::new(used.then_some(Used::Other)),
         });
         self.r.potentially_unused_imports.push(import);
         let imported_binding = self.r.import(binding, import);
@@ -1061,7 +1061,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
                 span,
                 module_path: Vec::new(),
                 vis: Cell::new(Some(ty::Visibility::Restricted(CRATE_DEF_ID))),
-                used: Cell::new(false),
+                used: Default::default(),
             })
         };
 
@@ -1225,7 +1225,7 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
                     span,
                     module_path: Vec::new(),
                     vis: Cell::new(Some(vis)),
-                    used: Cell::new(true),
+                    used: Cell::new(Some(Used::Other)),
                 });
                 let import_binding = self.r.import(binding, import);
                 self.r.define(self.r.graph_root, ident, MacroNS, import_binding);

compiler/rustc_resolve/src/check_unused.rs

@@ -182,6 +182,26 @@ impl<'a, 'b, 'tcx> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b, 'tcx> {
             }
         } else {
             self.check_import(id);
+            let import = self.r.redundant_imports.remove(&id);
+            if let Some(import) = import {
+                if let ImportKind::Single {
+                    source,
+                    target,
+                    ref source_bindings,
+                    ref target_bindings,
+                    ..
+                } = import.kind
+                    && !self.unused_imports.contains_key(&self.base_id)
+                {
+                    self.r.check_for_redundant_imports(
+                        source,
+                        import,
+                        source_bindings,
+                        target_bindings,
+                        target,
+                    );
+                }
+            }
         }
 
         visit::walk_use_tree(self, use_tree, id);
@@ -286,7 +306,7 @@ impl Resolver<'_, '_> {
 
         for import in self.potentially_unused_imports.iter() {
             match import.kind {
-                _ if import.used.get()
+                _ if import.used.get().is_some()
                     || import.expect_vis().is_public()
                     || import.span.is_dummy() =>
                 {

compiler/rustc_resolve/src/diagnostics.rs

@@ -33,8 +33,8 @@ use crate::errors::{
 };
 use crate::imports::{Import, ImportKind};
 use crate::late::{PatternSource, Rib};
-use crate::path_names_to_string;
 use crate::{errors as errs, BindingKey};
+use crate::{path_names_to_string, Used};
 use crate::{AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, BindingError, Finalize};
 use crate::{HasGenericParams, MacroRulesScope, Module, ModuleKind, ModuleOrUniformRoot};
 use crate::{LexicalScopeBinding, NameBinding, NameBindingKind, PrivacyError, VisResolutionError};
@@ -1482,7 +1482,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                         );
                         // Silence the 'unused import' warning we might get,
                         // since this diagnostic already covers that import.
-                        self.record_use(ident, binding, false);
+                        self.record_use(ident, binding, Some(Used::Other));
                         return;
                     }
                 }

compiler/rustc_resolve/src/ident.rs

@@ -14,8 +14,8 @@ use crate::late::{
     ConstantHasGenerics, HasGenericParams, NoConstantGenericsReason, PathSource, Rib, RibKind,
 };
 use crate::macros::{sub_namespace_match, MacroRulesScope};
-use crate::BindingKey;
 use crate::{errors, AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, Determinacy, Finalize};
+use crate::{BindingKey, Used};
 use crate::{ImportKind, LexicalScopeBinding, Module, ModuleKind, ModuleOrUniformRoot};
 use crate::{NameBinding, NameBindingKind, ParentScope, PathResult, PrivacyError, Res};
 use crate::{ResolutionError, Resolver, Scope, ScopeSet, Segment, ToNameBinding, Weak};
@@ -335,14 +335,14 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 ModuleKind::Block => {} // We can see through blocks
                 _ => break,
             }
-
             let item = self.resolve_ident_in_module_unadjusted(
                 ModuleOrUniformRoot::Module(module),
                 ident,
                 ns,
                 parent_scope,
                 finalize,
                 ignore_binding,
+                None,
             );
             if let Ok(binding) = item {
                 // The ident resolves to an item.
@@ -509,6 +509,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                             !matches!(scope_set, ScopeSet::Late(..)),
                             finalize,
                             ignore_binding,
+                            Some(Used::Scope),
                         );
                         match binding {
                             Ok(binding) => {
@@ -579,6 +580,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                 parent_scope,
                                 None,
                                 ignore_binding,
+                                Some(Used::Other),
                             ) {
                                 if use_prelude || this.is_builtin_macro(binding.res()) {
                                     result = Ok((binding, Flags::MISC_FROM_PRELUDE));
@@ -754,6 +756,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             false,
             finalize,
             ignore_binding,
+            Some(Used::Other),
         )
     }
 
@@ -766,6 +769,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         parent_scope: &ParentScope<'a>,
         finalize: Option<Finalize>,
         ignore_binding: Option<NameBinding<'a>>,
+        used: Option<Used>,
     ) -> Result<NameBinding<'a>, Determinacy> {
         self.resolve_ident_in_module_unadjusted_ext(
             module,
@@ -775,6 +779,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             false,
             finalize,
             ignore_binding,
+            used,
         )
         .map_err(|(determinacy, _)| determinacy)
     }
@@ -793,6 +798,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         // This binding should be ignored during in-module resolution, so that we don't get
         // "self-confirming" import resolutions during import validation and checking.
         ignore_binding: Option<NameBinding<'a>>,
+        used: Option<Used>,
     ) -> Result<NameBinding<'a>, (Determinacy, Weak)> {
         let module = match module {
             ModuleOrUniformRoot::Module(module) => module,
@@ -849,15 +855,13 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         let key = BindingKey::new(ident, ns);
         let resolution =
             self.resolution(module, key).try_borrow_mut().map_err(|_| (Determined, Weak::No))?; // This happens when there is a cycle of imports.
-
         // If the primary binding is unusable, search further and return the shadowed glob
         // binding if it exists. What we really want here is having two separate scopes in
         // a module - one for non-globs and one for globs, but until that's done use this
         // hack to avoid inconsistent resolution ICEs during import validation.
         let binding = [resolution.binding, resolution.shadowed_glob]
             .into_iter()
             .find_map(|binding| if binding == ignore_binding { None } else { binding });
-
         if let Some(Finalize { path_span, report_private, .. }) = finalize {
             let Some(binding) = binding else {
                 return Err((Determined, Weak::No));
@@ -901,8 +905,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     self.macro_expanded_macro_export_errors.insert((path_span, binding.span));
                 }
             }
-
-            self.record_use(ident, binding, restricted_shadowing);
+            self.record_use(ident, binding, used);
             return Ok(binding);
         }
 
@@ -923,6 +926,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         // Check if one of single imports can still define the name,
         // if it can then our result is not determined and can be invalidated.
         for single_import in &resolution.single_imports {
+            debug!("single_import:{:?}", single_import);
             let Some(import_vis) = single_import.vis.get() else {
                 continue;
             };
@@ -1029,6 +1033,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 adjusted_parent_scope,
                 None,
                 ignore_binding,
+                Some(Used::Other),
             );
 
             match result {

compiler/rustc_resolve/src/imports.rs

@@ -10,9 +10,9 @@ use crate::errors::{
 use crate::Determinacy::{self, *};
 use crate::{fluent_generated as fluent, Namespace::*};
 use crate::{module_to_string, names_to_string, ImportSuggestion};
-use crate::{AmbiguityKind, BindingKey, ModuleKind, ResolutionError, Resolver, Segment};
+use crate::{AmbiguityKind, BindingKey, ResolutionError, Resolver, Segment};
 use crate::{Finalize, Module, ModuleOrUniformRoot, ParentScope, PerNS, ScopeSet};
-use crate::{NameBinding, NameBindingData, NameBindingKind, PathResult};
+use crate::{NameBinding, NameBindingData, NameBindingKind, PathResult, Used};
 
 use rustc_ast::NodeId;
 use rustc_data_structures::fx::FxHashSet;
@@ -169,7 +169,7 @@ pub(crate) struct ImportData<'a> {
     /// The resolution of `module_path`.
     pub imported_module: Cell<Option<ModuleOrUniformRoot<'a>>>,
     pub vis: Cell<Option<ty::Visibility>>,
-    pub used: Cell<bool>,
+    pub used: Cell<Option<Used>>,
 }
 
 /// All imports are unique and allocated on a same arena,
@@ -282,7 +282,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
 
         self.arenas.alloc_name_binding(NameBindingData {
-            kind: NameBindingKind::Import { binding, import, used: Cell::new(false) },
+            kind: NameBindingKind::Import { binding, import },
             ambiguity: None,
             warn_ambiguity: false,
             span: import.span,
@@ -478,9 +478,9 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 let key = BindingKey::new(target, ns);
                 let _ = this.try_define(import.parent_scope.module, key, dummy_binding, false);
             });
-            self.record_use(target, dummy_binding, false);
+            self.record_use(target, dummy_binding, Some(Used::Other));
         } else if import.imported_module.get().is_none() {
-            import.used.set(true);
+            import.used.set(Some(Used::Other));
             if let Some(id) = import.id() {
                 self.used_imports.insert(id);
             }
@@ -1040,11 +1040,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                     && initial_binding.is_extern_crate()
                                     && !initial_binding.is_import()
                                 {
-                                    this.record_use(
-                                        ident,
-                                        target_binding,
-                                        import.module_path.is_empty(),
-                                    );
+                                    let used = if import.module_path.is_empty() {
+                                        Used::Scope
+                                    } else {
+                                        Used::Other
+                                    };
+                                    this.record_use(ident, target_binding, Some(used));
                                 }
                             }
                             initial_binding.res()
@@ -1291,14 +1292,16 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 this.import_res_map.entry(import_id).or_default()[ns] = Some(binding.res());
             }
         });
-
-        self.check_for_redundant_imports(ident, import, source_bindings, target_bindings, target);
+        if let ImportKind::Single { id, .. } = import.kind {
+            debug!("redundant_imports insert id:{:?} import:{:?}", id, import);
+            self.redundant_imports.insert(id, import);
+        }
 
         debug!("(resolving single import) successfully resolved import");
         None
     }
 
-    fn check_for_redundant_imports(
+    pub(crate) fn check_for_redundant_imports(
         &mut self,
         ident: Ident,
         import: Import<'a>,
@@ -1313,13 +1316,15 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         if import.parent_scope.expansion != LocalExpnId::ROOT {
             return;
         }
-
         // Skip if we are inside a named module (in contrast to an anonymous
         // module defined by a block).
-        if let ModuleKind::Def(..) = import.parent_scope.module.kind {
+        // Skip if the import is public or was used through non scope-based resolution,
+        // e.g. through a module-relative path.
+        if self.effective_visibilities.is_exported(self.local_def_id(import.root_id))
+            || import.used.get() == Some(Used::Other)
+        {
             return;
         }
-
         let mut is_redundant = PerNS { value_ns: None, type_ns: None, macro_ns: None };
 
         let mut redundant_span = PerNS { value_ns: None, type_ns: None, macro_ns: None };
@@ -1348,7 +1353,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 }
             }
         });
-
         if !is_redundant.is_empty() && is_redundant.present_items().all(|is_redundant| is_redundant)
         {
             let mut redundant_spans: Vec<_> = redundant_span.present_items().collect();
@@ -1372,7 +1376,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             self.tcx.sess.create_err(CannotGlobImportAllCrates { span: import.span }).emit();
             return;
         };
-
         if module.is_trait() {
             self.tcx.sess.create_err(ItemsInTraitsAreNotImportable { span: import.span }).emit();
             return;

compiler/rustc_resolve/src/late.rs

@@ -7,8 +7,8 @@
 //! `build_reduced_graph.rs`, `macros.rs` and `imports.rs`.
 
 use crate::errors::ImportsCannotReferTo;
-use crate::BindingKey;
 use crate::{path_names_to_string, rustdoc, BindingError, Finalize, LexicalScopeBinding};
+use crate::{BindingKey, Used};
 use crate::{Module, ModuleOrUniformRoot, NameBinding, ParentScope, PathResult};
 use crate::{ResolutionError, Resolver, Segment, UseError};
 
@@ -3482,7 +3482,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 // whether they can be shadowed by fresh bindings or not, so force an error.
                 // issues/33118#issuecomment-233962221 (see below) still applies here,
                 // but we have to ignore it for backward compatibility.
-                self.r.record_use(ident, binding, false);
+                self.r.record_use(ident, binding, Some(Used::Other));
                 return None;
             }
             LexicalScopeBinding::Item(binding) => (binding.res(), Some(binding)),
@@ -3497,7 +3497,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             ) if is_syntactic_ambiguity => {
                 // Disambiguate in favor of a unit struct/variant or constant pattern.
                 if let Some(binding) = binding {
-                    self.r.record_use(ident, binding, false);
+                    self.r.record_use(ident, binding, Some(Used::Other));
                 }
                 Some(res)
             }