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`

Author: surechen

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);