Auto merge of #5058 - xiongmao86:issue4903, r=flip1995

Closes Issue4903

fixes #4903.

Check list:
- [x] Followed [lint naming conventions][lint_naming]
- [x] Added passing UI tests (including committed `.stderr` file)
- [x] `cargo test` passes locally
- [x] Executed `./util/dev update_lints`
- [x] Added lint documentation
- [x] Run `./util/dev fmt`

[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints
---

changelog: implement lint that warn about single component path imports(`use ident;`).

10 files changed, 115 insertions, 7 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 78020c2dac6..d5dbbabb3c5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1284,6 +1284,7 @@ Released 2018-09-13
 [`should_implement_trait`]: https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait
 [`similar_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#similar_names
 [`single_char_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern
+[`single_component_path_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_component_path_imports
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next
diff --git a/README.md b/README.md
index 8f65ff94ca8..0cb14eda6a8 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 349 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 350 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 91240362445..7008c6d6839 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -284,6 +284,7 @@ pub mod replace_consts;
 pub mod returns;
 pub mod serde_api;
 pub mod shadow;
+pub mod single_component_path_imports;
 pub mod slow_vector_initialization;
 pub mod strings;
 pub mod suspicious_trait_impl;
@@ -741,6 +742,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &shadow::SHADOW_REUSE,
         &shadow::SHADOW_SAME,
         &shadow::SHADOW_UNRELATED,
+        &single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS,
         &slow_vector_initialization::SLOW_VECTOR_INITIALIZATION,
         &strings::STRING_ADD,
         &strings::STRING_ADD_ASSIGN,
@@ -993,6 +995,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| box utils::internal_lints::ProduceIce);
     store.register_late_pass(|| box let_underscore::LetUnderscore);
     store.register_late_pass(|| box atomic_ordering::AtomicOrdering);
+    store.register_early_pass(|| box single_component_path_imports::SingleComponentPathImports);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1296,6 +1299,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&returns::NEEDLESS_RETURN),
         LintId::of(&returns::UNUSED_UNIT),
         LintId::of(&serde_api::SERDE_API_MISUSE),
+        LintId::of(&single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS),
         LintId::of(&slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
         LintId::of(&strings::STRING_LIT_AS_BYTES),
         LintId::of(&suspicious_trait_impl::SUSPICIOUS_ARITHMETIC_IMPL),
@@ -1431,6 +1435,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&returns::LET_AND_RETURN),
         LintId::of(&returns::NEEDLESS_RETURN),
         LintId::of(&returns::UNUSED_UNIT),
+        LintId::of(&single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS),
         LintId::of(&strings::STRING_LIT_AS_BYTES),
         LintId::of(&tabs_in_doc_comments::TABS_IN_DOC_COMMENTS),
         LintId::of(&to_digit_is_some::TO_DIGIT_IS_SOME),
diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index d00d8dfbd3e..6e5015eb844 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -2310,7 +2310,7 @@ fn lint_unwrap(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>, unwrap_args: &[hi
             &format!("used `unwrap()` on `{}` value", kind,),
             &format!(
                 "if you don't want to handle the `{}` case gracefully, consider \
-                using `expect()` to provide a better panic message",
+                 using `expect()` to provide a better panic message",
                 none_value,
             ),
         );
@@ -2679,7 +2679,7 @@ fn lint_filter_flat_map<'a, 'tcx>(
     if match_trait_method(cx, expr, &paths::ITERATOR) {
         let msg = "called `filter(p).flat_map(q)` on an `Iterator`";
         let hint = "this is more succinctly expressed by calling `.flat_map(..)` \
-            and filtering by returning `iter::empty()`";
+                    and filtering by returning `iter::empty()`";
         span_lint_and_help(cx, FILTER_MAP, expr.span, msg, hint);
     }
 }
@@ -2695,7 +2695,7 @@ fn lint_filter_map_flat_map<'a, 'tcx>(
     if match_trait_method(cx, expr, &paths::ITERATOR) {
         let msg = "called `filter_map(p).flat_map(q)` on an `Iterator`";
         let hint = "this is more succinctly expressed by calling `.flat_map(..)` \
-            and filtering by returning `iter::empty()`";
+                    and filtering by returning `iter::empty()`";
         span_lint_and_help(cx, FILTER_MAP, expr.span, msg, hint);
     }
 }
@@ -3148,7 +3148,7 @@ fn lint_option_as_ref_deref<'a, 'tcx>(
 
         let msg = format!(
             "called `{0}` (or with one of deref aliases) on an Option value. \
-                        This can be done more directly by calling `{1}` instead",
+             This can be done more directly by calling `{1}` instead",
             current_method, hint
         );
         span_lint_and_sugg(
diff --git a/clippy_lints/src/single_component_path_imports.rs b/clippy_lints/src/single_component_path_imports.rs
new file mode 100644
index 00000000000..eb3261bebe3
--- /dev/null
+++ b/clippy_lints/src/single_component_path_imports.rs
@@ -0,0 +1,61 @@
+use crate::utils::span_lint_and_sugg;
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::edition::Edition;
+use syntax::ast::{Item, ItemKind, UseTreeKind};
+
+declare_clippy_lint! {
+    /// **What it does:** Checking for imports with single component use path.
+    ///
+    /// **Why is this bad?** Import with single component use path such as `use cratename;`
+    /// is not necessary, and thus should be removed.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust, ignore
+    /// use regex;
+    ///
+    /// fn main() {
+    ///     regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+    /// }
+    /// ```
+    /// Better as
+    /// ```rust, ignore
+    /// fn main() {
+    ///     regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+    /// }
+    /// ```
+    pub SINGLE_COMPONENT_PATH_IMPORTS,
+    style,
+    "imports with single component path are redundant"
+}
+
+declare_lint_pass!(SingleComponentPathImports => [SINGLE_COMPONENT_PATH_IMPORTS]);
+
+impl EarlyLintPass for SingleComponentPathImports {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
+        if_chain! {
+            if cx.sess.opts.edition == Edition::Edition2018;
+            if !item.vis.node.is_pub();
+            if let ItemKind::Use(use_tree) = &item.kind;
+            if let segments = &use_tree.prefix.segments;
+            if segments.len() == 1;
+            if let UseTreeKind::Simple(None, _, _) = use_tree.kind;
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    SINGLE_COMPONENT_PATH_IMPORTS,
+                    item.span,
+                    "this import is redundant",
+                    "remove it entirely",
+                    String::new(),
+                    Applicability::MachineApplicable
+                );
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/types.rs b/clippy_lints/src/types.rs
index 4fcf39d2142..e47f2480a54 100644
--- a/clippy_lints/src/types.rs
+++ b/clippy_lints/src/types.rs
@@ -316,7 +316,7 @@ impl Types {
                                 OPTION_OPTION,
                                 hir_ty.span,
                                 "consider using `Option<T>` instead of `Option<Option<T>>` or a custom \
-                                enum if you need to distinguish all 3 cases",
+                                 enum if you need to distinguish all 3 cases",
                             );
                             return; // don't recurse into the type
                         }
diff --git a/src/lintlist/mod.rs b/src/lintlist/mod.rs
index 5dbcb4483f2..b762c8d1395 100644
--- a/src/lintlist/mod.rs
+++ b/src/lintlist/mod.rs
@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 349] = [
+pub const ALL_LINTS: [Lint; 350] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -1856,6 +1856,13 @@ pub const ALL_LINTS: [Lint; 349] = [
         module: "methods",
     },
     Lint {
+        name: "single_component_path_imports",
+        group: "style",
+        desc: "imports with single component path are redundant",
+        deprecation: None,
+        module: "single_component_path_imports",
+    },
+    Lint {
         name: "single_match",
         group: "style",
         desc: "a `match` statement with a single nontrivial arm (i.e., where the other arm is `_ => {}`) instead of `if let`",
diff --git a/tests/ui/single_component_path_imports.fixed b/tests/ui/single_component_path_imports.fixed
new file mode 100644
index 00000000000..7a882efc4d1
--- /dev/null
+++ b/tests/ui/single_component_path_imports.fixed
@@ -0,0 +1,12 @@
+// run-rustfix
+// compile-flags: --edition 2018
+#![warn(clippy::single_component_path_imports)]
+#![allow(unused_imports)]
+
+
+use serde as edres;
+pub use serde;
+
+fn main() {
+    regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+}
diff --git a/tests/ui/single_component_path_imports.rs b/tests/ui/single_component_path_imports.rs
new file mode 100644
index 00000000000..d084425cd70
--- /dev/null
+++ b/tests/ui/single_component_path_imports.rs
@@ -0,0 +1,12 @@
+// run-rustfix
+// compile-flags: --edition 2018
+#![warn(clippy::single_component_path_imports)]
+#![allow(unused_imports)]
+
+use regex;
+use serde as edres;
+pub use serde;
+
+fn main() {
+    regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
+}
diff --git a/tests/ui/single_component_path_imports.stderr b/tests/ui/single_component_path_imports.stderr
new file mode 100644
index 00000000000..519ada0169a
--- /dev/null
+++ b/tests/ui/single_component_path_imports.stderr
@@ -0,0 +1,10 @@
+error: this import is redundant
+  --> $DIR/single_component_path_imports.rs:6:1
+   |
+LL | use regex;
+   | ^^^^^^^^^^ help: remove it entirely
+   |
+   = note: `-D clippy::single-component-path-imports` implied by `-D warnings`
+
+error: aborting due to previous error
+