add lint 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};
```

Replace to:

```rust
use std::{option::{Iter, IterMut, IntoIter}, convert::{TryFrom, TryInto}};
```

compiler/rustc_lint/messages.ftl

@@ -476,6 +476,10 @@ lint_reason_must_be_string_literal = reason must be a string literal
 
 lint_reason_must_come_last = reason in lint attribute must come last
 
+lint_redundant_prelude_imports = unnecessary prelude import {$redundant_crates}
+    .suggestion = remove unused imports
+    .help = change to {$replace}
+
 lint_redundant_semicolons =
     unnecessary trailing {$multiple ->
         [true] semicolons

compiler/rustc_lint/src/lib.rs

@@ -81,6 +81,7 @@ mod opaque_hidden_inferred_bound;
 mod pass_by_value;
 mod passes;
 mod ptr_nulls;
+mod redundant_prelude_imports;
 mod redundant_semicolon;
 mod reference_casting;
 mod traits;
@@ -119,6 +120,7 @@ use noop_method_call::*;
 use opaque_hidden_inferred_bound::*;
 use pass_by_value::*;
 use ptr_nulls::*;
+use redundant_prelude_imports::*;
 use redundant_semicolon::*;
 use reference_casting::*;
 use traits::*;
@@ -178,6 +180,7 @@ early_lint_methods!(
             HiddenUnicodeCodepoints: HiddenUnicodeCodepoints,
             IncompleteInternalFeatures: IncompleteInternalFeatures,
             RedundantSemicolons: RedundantSemicolons,
+            RedundantPreludeImports: RedundantPreludeImports,
             UnusedDocComment: UnusedDocComment,
             UnexpectedCfgs: UnexpectedCfgs,
         ]

compiler/rustc_lint/src/lints.rs

@@ -1360,6 +1360,16 @@ pub struct RedundantSemicolonsDiag {
     pub suggestion: Span,
 }
 
+// redundant_prelude_imports.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_redundant_prelude_imports)]
+pub struct RedundantPreludeImportsDiag {
+    pub redundant_crates: String,
+    #[suggestion(lint_suggestion, code = "{replace}", applicability = "maybe-incorrect")]
+    pub suggestion: Span,
+    pub replace: String,
+}
+
 // traits.rs
 pub struct DropTraitConstraintsDiag<'a> {
     pub predicate: Clause<'a>,

compiler/rustc_lint/src/redundant_prelude_imports.rs

@@ -0,0 +1,254 @@
+use crate::{lints::RedundantPreludeImportsDiag, EarlyContext, EarlyLintPass, LintContext};
+use rustc_ast as ast;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::Span;
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+use std::rc::Rc;
+use std::string::ToString;
+
+declare_lint! {
+    /// The `redundant_prelude_imports` lint detects unnecessary imports in
+    /// std::prelude that can be eliminated.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![allow(unused)]
+    /// # #![warn(redundant_prelude_imports)]
+    /// use std::mem::drop;
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// unnecessary prelude import  'std::mem::drop'
+    pub(super) REDUNDANT_PRELUDE_IMPORTS,
+    Allow,
+    "detects unnecessary imports in std::prelude that can be eliminated"
+}
+
+declare_lint_pass!(RedundantPreludeImports => [REDUNDANT_PRELUDE_IMPORTS]);
+
+#[derive(Debug)]
+struct RedundantImport<'a, 'b> {
+    item: &'a ast::Item,
+    prelude_imports: &'b BTreeMap<&'b str, Symbol>,
+    remove_imports: Vec<(Span, Vec<String>)>,
+    reserve_imports: Vec<(Span, Vec<String>)>,
+}
+
+impl EarlyLintPass for RedundantPreludeImports {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
+        if let ast::ItemKind::Use(ref use_tree) = item.kind {
+            // Use in std::prelude
+            let prelude_imports: BTreeMap<&str, Symbol> = BTreeMap::from([
+                ("std::marker::Copy", sym::rust_2015),
+                ("std::marker::Send", sym::rust_2015),
+                ("std::marker::Sized", sym::rust_2015),
+                ("std::marker::Sync", sym::rust_2015),
+                ("std::marker::Unpin", sym::rust_2015),
+                ("std::ops::Drop", sym::rust_2015),
+                ("std::ops::Fn", sym::rust_2015),
+                ("std::ops::FnMut", sym::rust_2015),
+                ("std::ops::FnOnce", sym::rust_2015),
+                ("std::mem::drop", sym::rust_2015),
+                ("std::boxed::Box", sym::rust_2015),
+                ("std::borrow::ToOwned", sym::rust_2015),
+                ("std::clone::Clone", sym::rust_2015),
+                ("std::cmp::PartialEq", sym::rust_2015),
+                ("std::cmp::PartialOrd", sym::rust_2015),
+                ("std::cmp::Eq", sym::rust_2015),
+                ("std::cmp::Ord", sym::rust_2015),
+                ("std::convert::AsRef", sym::rust_2015),
+                ("std::convert::AsMut", sym::rust_2015),
+                ("std::convert::Into", sym::rust_2015),
+                ("std::convert::From", sym::rust_2015),
+                ("std::default::Default", sym::rust_2015),
+                ("std::iter::Iterator", sym::rust_2015),
+                ("std::iter::Extend", sym::rust_2015),
+                ("std::iter::IntoIterator", sym::rust_2015),
+                ("std::iter::DoubleEndedIterator", sym::rust_2015),
+                ("std::iter::ExactSizeIterator", sym::rust_2015),
+                ("std::option::Option::self", sym::rust_2015),
+                ("std::option::Option::Some", sym::rust_2015),
+                ("std::option::Option::None", sym::rust_2015),
+                ("std::result::Result", sym::rust_2015),
+                ("std::result::Result::Ok", sym::rust_2015),
+                ("std::result::Result::Err", sym::rust_2015),
+                ("std::string::String", sym::rust_2015),
+                ("std::string::ToString", sym::rust_2015),
+                ("std::vec::Vec", sym::rust_2015),
+                ("std::convert::TryFrom", sym::rust_2018),
+                ("std::convert::TryInto", sym::rust_2018),
+                ("std::iter::FromIterator", sym::rust_2018),
+            ]);
+            let mut redundant = RedundantImport {
+                item: item,
+                prelude_imports: &prelude_imports,
+                remove_imports: vec![],
+                reserve_imports: vec![],
+            };
+            let path = vec![];
+            self.check_use_tree(cx, use_tree, &path, &mut redundant);
+            if redundant.remove_imports.len() > 0 {
+                // Delete the usetrees imported by std::prelude.
+                // Use the prefix tree to make suggestion msg for the remaining ones.
+                // For import :
+                // use std::{option::{Iter, IterMut, IntoIter, Option::{self, Some}},
+                // convert::{TryFrom, TryInto}, mem::drop};
+                // Replace to:
+                // use std::{option::{Iter, IterMut, IntoIter}, convert::{TryFrom, TryInto}};
+                let replace = if redundant.reserve_imports.len() == 0 {
+                    "".to_string()
+                } else {
+                    let mut tree =
+                        MakeSuggestionTree::new(MakeSuggestionNode::new("std".to_string()));
+                    redundant.reserve_imports.iter().for_each(|v| {
+                        tree.add_node(&v.1);
+                    });
+                    let mut use_replace = "use ".to_string();
+                    use_replace.push_str(&tree.make_suggestion());
+                    use_replace.push(';');
+                    use_replace
+                };
+                let lint_msg =
+                    &redundant.remove_imports.into_iter().fold("".to_string(), |mut acc, x| {
+                        acc.push_str(" '");
+                        acc.push_str(
+                            &*cx.sess().parse_sess.source_map().span_to_snippet(x.0).unwrap(),
+                        );
+                        acc.push_str("'");
+                        acc
+                    });
+                cx.emit_spanned_lint(
+                    REDUNDANT_PRELUDE_IMPORTS,
+                    item.span,
+                    RedundantPreludeImportsDiag {
+                        redundant_crates: lint_msg.to_string(),
+                        suggestion: item.span,
+                        replace: replace,
+                    },
+                );
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+struct MakeSuggestionNode {
+    prefix: String,
+    child: Rc<RefCell<Vec<MakeSuggestionNode>>>,
+}
+
+impl MakeSuggestionNode {
+    fn new(prefix: String) -> Self {
+        Self { prefix: prefix, child: Rc::new(RefCell::new(vec![])) }
+    }
+
+    fn add_child(&mut self, segment: &Vec<String>) {
+        let len = segment.len();
+        if len <= 1 || segment[0] != self.prefix {
+            return;
+        }
+        let mut find = false;
+        let child_len = self.child.borrow().len();
+        let mut child_segment = segment.clone();
+        child_segment.remove(0);
+        let find_path = child_segment[0].clone();
+        for j in 0..child_len {
+            if self.child.borrow()[j].prefix == find_path {
+                find = true;
+                self.child.borrow_mut()[j].add_child(&child_segment);
+                break;
+            }
+        }
+        if !find {
+            let mut child_node = MakeSuggestionNode::new(find_path);
+            child_node.add_child(&child_segment);
+            self.child.borrow_mut().push(child_node);
+        }
+    }
+
+    fn make_suggestion(&self) -> String {
+        let mut str = self.prefix.clone();
+        if self.child.borrow().len() > 0 {
+            str.push_str("::");
+            let mut child_import = self.child.borrow().iter().fold("".to_string(), |mut acc, x| {
+                let temp = x.make_suggestion();
+                acc.push_str(&*temp);
+                acc.push_str(", ");
+                acc
+            });
+            child_import = child_import[0..child_import.len() - 2].to_string();
+            if self.child.borrow().len() > 1 {
+                str.push_str("{");
+                str.push_str(&*child_import);
+                str.push_str("}");
+            } else {
+                str.push_str(&*child_import);
+            }
+        }
+        str
+    }
+}
+
+#[derive(Debug)]
+struct MakeSuggestionTree {
+    root: MakeSuggestionNode,
+}
+
+impl MakeSuggestionTree {
+    fn new(node: MakeSuggestionNode) -> Self {
+        MakeSuggestionTree { root: node }
+    }
+
+    fn add_node(&mut self, segment: &Vec<String>) {
+        self.root.add_child(segment);
+    }
+
+    fn make_suggestion(&self) -> String {
+        self.root.make_suggestion()
+    }
+}
+
+impl RedundantPreludeImports {
+    fn check_use_tree(
+        &self,
+        cx: &EarlyContext<'_>,
+        use_tree: &ast::UseTree,
+        pre_path: &Vec<String>,
+        redundant: &mut RedundantImport<'_, '_>,
+    ) {
+        let mut pre_path = pre_path.clone();
+        use_tree.prefix.segments.iter().for_each(|p| {
+            pre_path.push(p.ident.to_string());
+        });
+        match use_tree.kind {
+            ast::UseTreeKind::Nested(ref items) => {
+                for (tree, _) in items {
+                    // Recursive process nested usetree.
+                    self.check_use_tree(cx, tree, &pre_path, redundant);
+                }
+            }
+            ast::UseTreeKind::Simple(None) => {
+                let path = pre_path.iter().fold("".to_string(), |mut acc, x| {
+                    acc.push_str(&*x);
+                    acc.push_str("::");
+                    acc
+                });
+                if let Some(val) = redundant.prelude_imports.get(&path[0..path.len()-2])
+                    && (*val == sym::rust_2015
+                    || (*val == sym::rust_2018 && redundant.item.span.at_least_rust_2018())
+                    || (*val == sym::rust_2021 && redundant.item.span.at_least_rust_2021())
+                    || (*val == sym::rust_2024 && redundant.item.span.at_least_rust_2024())) {
+                        redundant.remove_imports.push((use_tree.span, pre_path.clone()));
+                } else {
+                    redundant.reserve_imports.push((use_tree.span, pre_path));
+                }
+            }
+            _ => {}
+        }
+    }
+}

tests/ui/lint/use-redundant/redundant_prelude_imports-rust-2015.fixed

@@ -0,0 +1,13 @@
+// run-rustfix
+// check-pass
+#![allow(unused_imports)]
+#![warn(redundant_prelude_imports)]
+
+
+//~^ WARN unnecessary prelude import  'self' 'Some' 'None'
+
+//~^ WARN unnecessary prelude import  'Ok' 'Err'
+use std::convert::{TryFrom, TryInto};
+
+fn main() {
+}

tests/ui/lint/use-redundant/redundant_prelude_imports-rust-2015.rs

@@ -0,0 +1,13 @@
+// run-rustfix
+// check-pass
+#![allow(unused_imports)]
+#![warn(redundant_prelude_imports)]
+
+use std::option::Option::{self, Some, None};
+//~^ WARN unnecessary prelude import  'self' 'Some' 'None'
+use std::result::Result::{Ok, Err};
+//~^ WARN unnecessary prelude import  'Ok' 'Err'
+use std::convert::{TryFrom, TryInto};
+
+fn main() {
+}

tests/ui/lint/use-redundant/redundant_prelude_imports-rust-2015.stderr

@@ -0,0 +1,20 @@
+warning: unnecessary prelude import  'self' 'Some' 'None'
+  --> $DIR/redundant_prelude_imports-rust-2015.rs:6:1
+   |
+LL | use std::option::Option::{self, Some, None};
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove unused imports
+   |
+note: the lint level is defined here
+  --> $DIR/redundant_prelude_imports-rust-2015.rs:4:9
+   |
+LL | #![warn(redundant_prelude_imports)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unnecessary prelude import  'Ok' 'Err'
+  --> $DIR/redundant_prelude_imports-rust-2015.rs:8:1
+   |
+LL | use std::result::Result::{Ok, Err};
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove unused imports
+
+warning: 2 warnings emitted
+

tests/ui/lint/use-redundant/redundant_prelude_imports-rust-2018.fixed

@@ -0,0 +1,14 @@
+// run-rustfix
+// check-pass
+// edition:2018
+
+#![allow(unused_imports)]
+#![warn(redundant_prelude_imports)]
+
+
+//~^ WARN unnecessary prelude import  'Ok' 'Err'
+
+//~^ WARN unnecessary prelude import  'TryFrom' 'TryInto'
+
+fn main() {
+}

tests/ui/lint/use-redundant/redundant_prelude_imports-rust-2018.rs

@@ -0,0 +1,14 @@
+// run-rustfix
+// check-pass
+// edition:2018
+
+#![allow(unused_imports)]
+#![warn(redundant_prelude_imports)]
+
+use std::result::Result::{Ok, Err};
+//~^ WARN unnecessary prelude import  'Ok' 'Err'
+use std::convert::{TryFrom, TryInto};
+//~^ WARN unnecessary prelude import  'TryFrom' 'TryInto'
+
+fn main() {
+}