Rollup merge of rust-lang#58805 - fabric-and-ink:redundant_import, r=petrochenkov

Lint for redundant imports

This is an attempt at solving rust-lang#10178. The changes are suggested by `@petrochenkov`. I need some feedback on how to continue, since I get a lot of false positives in macro invocations in `libcore`. I suspect that the test

```
...
if directive.parent_scope.expansion == Mark::root() {
    return;
}
...
```

is wrong. But I don't know how to check correctly if I am at a macro expansion root – given that this is the reason for the errors in the first place.

Todos:

- [x] Solve problem with false positives
- [x] Turn hard error into warning
- [x] Add unit tests

Closes rust-lang#10178

Author: Centril

src/liballoc/borrow.rs

@@ -135,7 +135,7 @@ impl<T> ToOwned for T
 /// Another example showing how to keep `Cow` in a struct:
 ///
 /// ```
-/// use std::borrow::{Cow, ToOwned};
+/// use std::borrow::Cow;
 ///
 /// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
 ///     values: Cow<'a, [X]>,

src/libcore/cell.rs

@@ -1423,7 +1423,6 @@ impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
 ///
 /// ```
 /// use std::cell::UnsafeCell;
-/// use std::marker::Sync;
 ///
 /// # #[allow(dead_code)]
 /// struct NotThreadSafe<T> {

src/librustc/lint/builtin.rs

@@ -482,6 +482,7 @@ pub enum BuiltinLintDiagnostics {
     UnknownCrateTypes(Span, String, String),
     UnusedImports(String, Vec<(Span, String)>),
     NestedImplTrait { outer_impl_trait_span: Span, inner_impl_trait_span: Span },
+    RedundantImport(Vec<(Span, bool)>, ast::Ident),
 }
 
 impl BuiltinLintDiagnostics {
@@ -578,6 +579,15 @@ impl BuiltinLintDiagnostics {
                 db.span_label(outer_impl_trait_span, "outer `impl Trait`");
                 db.span_label(inner_impl_trait_span, "nested `impl Trait` here");
             }
+            BuiltinLintDiagnostics::RedundantImport(spans, ident) => {
+                for (span, is_imported) in spans {
+                    let introduced = if is_imported { "imported" } else { "defined" };
+                    db.span_label(
+                        span,
+                        format!("the item `{}` is already {} here", ident, introduced)
+                    );
+                }
+            }
         }
     }
 }

src/librustc/ty/query/on_disk_cache.rs

@@ -778,7 +778,6 @@ impl<'enc, 'a, 'tcx, E> CacheEncoder<'enc, 'a, 'tcx, E>
                                                  value: &V)
                                                  -> Result<(), E::Error>
     {
-        use crate::ty::codec::TyEncoder;
         let start_pos = self.position();
 
         tag.encode(self)?;

src/librustc_codegen_llvm/context.rs

@@ -377,7 +377,6 @@ impl MiscMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     // Returns a Value of the "eh_unwind_resume" lang item if one is defined,
     // otherwise declares it as an external function.
     fn eh_unwind_resume(&self) -> &'ll Value {
-        use crate::attributes;
         let unwresume = &self.eh_unwind_resume;
         if let Some(llfn) = unwresume.get() {
             return llfn;

src/librustc_codegen_ssa/mir/rvalue.rs

@@ -750,7 +750,6 @@ fn cast_int_to_float<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>>(
         // All inputs greater or equal to (f32::MAX + 0.5 ULP) are rounded to infinity,
         // and for everything else LLVM's uitofp works just fine.
         use rustc_apfloat::ieee::Single;
-        use rustc_apfloat::Float;
         const MAX_F32_PLUS_HALF_ULP: u128 = ((1 << (Single::PRECISION + 1)) - 1)
                                             << (Single::MAX_EXP - Single::PRECISION as i16);
         let max = bx.cx().const_uint_big(int_ty, MAX_F32_PLUS_HALF_ULP);

src/librustc_errors/lib.rs

@@ -155,7 +155,7 @@ impl CodeSuggestion {
     /// Returns the assembled code suggestions and whether they should be shown with an underline.
     pub fn splice_lines(&self, cm: &SourceMapperDyn)
                         -> Vec<(String, Vec<SubstitutionPart>)> {
-        use syntax_pos::{CharPos, Loc, Pos};
+        use syntax_pos::{CharPos, Pos};
 
         fn push_trailing(buf: &mut String,
                          line_opt: Option<&Cow<'_, str>>,

src/librustc_interface/profile/mod.rs

@@ -61,7 +61,6 @@ fn total_duration(traces: &[trace::Rec]) -> Duration {
 fn profile_queries_thread(r: Receiver<ProfileQueriesMsg>) {
     use self::trace::*;
     use std::fs::File;
-    use std::time::{Instant};
 
     let mut profq_msgs: Vec<ProfileQueriesMsg> = vec![];
     let mut frame: StackFrame = StackFrame { parse_st: ParseState::Clear, traces: vec![] };

src/librustc_mir/hair/pattern/mod.rs

@@ -427,7 +427,6 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 
                 let mut kind = match (lo, hi) {
                     (PatternKind::Constant { value: lo }, PatternKind::Constant { value: hi }) => {
-                        use std::cmp::Ordering;
                         let cmp = compare_const_vals(
                             self.tcx,
                             lo,

src/librustc_mir/interpret/operator.rs

@@ -331,8 +331,6 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
         val: ImmTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx, Scalar<M::PointerTag>> {
         use rustc::mir::UnOp::*;
-        use rustc_apfloat::ieee::{Single, Double};
-        use rustc_apfloat::Float;
 
         let layout = val.layout;
         let val = val.to_scalar()?;