Restrict lint logic and cargo uitest again.

Restrict lint logic to apply on edition2018 only. And update test
reference.

clippy_lints/src/single_component_use_path.rs

@@ -4,6 +4,7 @@ use rustc_session::{declare_lint_pass, declare_tool_lint};
 use syntax::ast::{Item, ItemKind};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
+use rustc_span::edition::Edition;
 
 declare_clippy_lint! {
     /// **What it does:** Checking for imports with single component use path.
@@ -32,6 +33,7 @@ declare_lint_pass!(SingleComponentUsePath => [SINGLE_COMPONENT_USE_PATH]);
 impl EarlyLintPass for SingleComponentUsePath {
     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
         if_chain! {
+            if cx.sess.opts.edition == Edition::Edition2018;
             if let ItemKind::Use(use_tree) = &item.kind;
             if let segments = &use_tree.prefix.segments;
             if segments.len() == 1;

tests/ui/single_component_use_path.fixed

@@ -1,4 +1,5 @@
 // run-rustfix
+// compile-flags: --edition 2018
 #![warn(clippy::single_component_use_path)]
 
 extern crate semver;

tests/ui/single_component_use_path.stderr

@@ -1,5 +1,5 @@
 error[E0254]: the name `semver` is defined multiple times
-  --> $DIR/single_component_use_path.rs:5:5
+  --> $DIR/single_component_use_path.rs:6:5
    |
 LL | extern crate semver;
    | -------------------- previous import of the extern crate `semver` here
@@ -9,7 +9,7 @@ LL | use semver;
    = note: `semver` must be defined only once in the type namespace of this module
 
 error: this import is redundant
-  --> $DIR/single_component_use_path.rs:5:1
+  --> $DIR/single_component_use_path.rs:6:1
    |
 LL | use semver;
    | ^^^^^^^^^^^ help: remove it entirely