Rollup merge of #5412 - dtolnay:tostring, r=flip1995

Downgrade inefficient_to_string to pedantic

From the [documentation](https://rust-lang.github.io/rust-clippy/master/index.html#inefficient_to_string):

> ```diff
> - ["foo", "bar"].iter().map(|s| s.to_string());
>
> + ["foo", "bar"].iter().map(|&s| s.to_string());
> ```

I feel like saving 10 nanoseconds from the formatting machinery isn't worth asking the programmer to insert extra `&` / `*` noise in the *vast* majority of cases. This is a pedantic lint.

changelog: Remove inefficient_to_string from default set of enabled lints

clippy_lints/src/lib.rs

@@ -1111,6 +1111,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::FILTER_MAP),
         LintId::of(&methods::FILTER_MAP_NEXT),
         LintId::of(&methods::FIND_MAP),
+        LintId::of(&methods::INEFFICIENT_TO_STRING),
         LintId::of(&methods::MAP_FLATTEN),
         LintId::of(&methods::OPTION_MAP_UNWRAP_OR),
         LintId::of(&methods::OPTION_MAP_UNWRAP_OR_ELSE),
@@ -1267,7 +1268,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::EXPECT_FUN_CALL),
         LintId::of(&methods::FILTER_NEXT),
         LintId::of(&methods::FLAT_MAP_IDENTITY),
-        LintId::of(&methods::INEFFICIENT_TO_STRING),
         LintId::of(&methods::INTO_ITER_ON_REF),
         LintId::of(&methods::ITERATOR_STEP_BY_ZERO),
         LintId::of(&methods::ITER_CLONED_COLLECT),
@@ -1657,7 +1657,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&loops::MANUAL_MEMCPY),
         LintId::of(&loops::NEEDLESS_COLLECT),
         LintId::of(&methods::EXPECT_FUN_CALL),
-        LintId::of(&methods::INEFFICIENT_TO_STRING),
         LintId::of(&methods::ITER_NTH),
         LintId::of(&methods::OR_FUN_CALL),
         LintId::of(&methods::SINGLE_CHAR_PATTERN),

clippy_lints/src/methods/mod.rs

@@ -699,7 +699,7 @@
     /// ["foo", "bar"].iter().map(|&s| s.to_string());
     /// ```
     pub INEFFICIENT_TO_STRING,
-    perf,
+    pedantic,
     "using `to_string` on `&&T` where `T: ToString`"
 }
 

src/lintlist/mod.rs

@@ -789,7 +789,7 @@
     },
     Lint {
         name: "inefficient_to_string",
-        group: "perf",
+        group: "pedantic",
         desc: "using `to_string` on `&&T` where `T: ToString`",
         deprecation: None,
         module: "methods",