Add test

tests/ui-internal/slow_symbol_comparisons.fixed

@@ -0,0 +1,24 @@
+#![feature(rustc_private)]
+#![warn(clippy::slow_symbol_comparisons)]
+
+extern crate rustc_span;
+
+use clippy_utils::sym;
+use rustc_span::Symbol;
+
+fn main() {
+    let symbol = sym!(example);
+    let other_symbol = sym!(other_example);
+
+    // Should lint
+    let slow_comparison = symbol.as_str() == "example";
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+    let slow_comparison_macro = symbol.as_str() == "example";
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+    let slow_comparison_backwards = symbol.as_str() == "example";
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+
+    // Should not lint
+    let faster_comparison = symbol.as_str() == "other_example";
+    let preinterned_comparison = symbol == other_symbol;
+}

tests/ui-internal/slow_symbol_comparisons.rs

@@ -0,0 +1,24 @@
+#![feature(rustc_private)]
+#![warn(clippy::slow_symbol_comparisons)]
+
+extern crate rustc_span;
+
+use clippy_utils::sym;
+use rustc_span::Symbol;
+
+fn main() {
+    let symbol = sym!(example);
+    let other_symbol = sym!(other_example);
+
+    // Should lint
+    let slow_comparison = symbol == Symbol::intern("example");
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+    let slow_comparison_macro = symbol == sym!(example);
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+    let slow_comparison_backwards = sym!(example) == symbol;
+    //~^ error: comparing `Symbol` via `Symbol::intern`
+
+    // Should not lint
+    let faster_comparison = symbol.as_str() == "other_example";
+    let preinterned_comparison = symbol == other_symbol;
+}

tests/ui-internal/slow_symbol_comparisons.stderr

@@ -0,0 +1,23 @@
+error: comparing `Symbol` via `Symbol::intern`
+  --> tests/ui-internal/slow_symbol_comparisons.rs:14:27
+   |
+LL |     let slow_comparison = symbol == Symbol::intern("example");
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `Symbol::as_str` and check the string instead: `symbol.as_str() == "example"`
+   |
+   = note: `-D clippy::slow-symbol-comparisons` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::slow_symbol_comparisons)]`
+
+error: comparing `Symbol` via `Symbol::intern`
+  --> tests/ui-internal/slow_symbol_comparisons.rs:16:33
+   |
+LL |     let slow_comparison_macro = symbol == sym!(example);
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^ help: use `Symbol::as_str` and check the string instead: `symbol.as_str() == "example"`
+
+error: comparing `Symbol` via `Symbol::intern`
+  --> tests/ui-internal/slow_symbol_comparisons.rs:18:37
+   |
+LL |     let slow_comparison_backwards = sym!(example) == symbol;
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^ help: use `Symbol::as_str` and check the string instead: `symbol.as_str() == "example"`
+
+error: aborting due to 3 previous errors
+