Auto merge of #4683 - HMPerson1:inefficient_to_string, r=Manishearth

Add `inefficient_to_string` lint

Closes #4586

changelog: Add `inefficient_to_string` lint, which checks for calling `to_string` on `&&str`, which would bypass the `str`'s specialization

clippy_lints/src/lib.rs

@@ -808,6 +808,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         methods::EXPECT_FUN_CALL,
         methods::FILTER_NEXT,
         methods::FLAT_MAP_IDENTITY,
+        methods::INEFFICIENT_TO_STRING,
         methods::INTO_ITER_ON_ARRAY,
         methods::INTO_ITER_ON_REF,
         methods::ITER_CLONED_COLLECT,
@@ -1184,6 +1185,7 @@ pub fn register_plugins(reg: &mut rustc_driver::plugin::Registry<'_>, conf: &Con
         loops::MANUAL_MEMCPY,
         loops::NEEDLESS_COLLECT,
         methods::EXPECT_FUN_CALL,
+        methods::INEFFICIENT_TO_STRING,
         methods::ITER_NTH,
         methods::OR_FUN_CALL,
         methods::SINGLE_CHAR_PATTERN,