Auto merge of rust-lang#6128 - dtolnay:rc_buffer, r=yaahc

Downgrade rc_buffer to restriction

I think Arc\<Vec\<T\>\> and Arc\<String\> and similar are a totally reasonable data structure, as observed by others in the comments on [rust-lang#6044](rust-lang/rust-clippy#6044 (comment)) as well. Doing `Arc::make_mut(&mut self.vec).push(...)` or `Arc::make_mut(&mut self.string).push_str("...")` is a terrific and well performing copy-on-write pattern. Linting this with an enabled-by-default <kbd>performance</kbd> lint strikes me as an unacceptable false positive balance.

As of rust-lang#6090 the documentation of this lint now contains:

> **Known problems:** This pattern can be desirable ...

which should indicate that we shouldn't be linting against correct, reasonable, well-performing patterns with an enabled-by-default lint.

Mentioning rust-lang#6044, rust-lang#6090.
r? `@yaahc,` who reviewed the lint.

---

changelog: Remove rc_buffer from default set of enabled lints

clippy_lints/src/lib.rs

@@ -1172,6 +1172,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&shadow::SHADOW_REUSE),
         LintId::of(&shadow::SHADOW_SAME),
         LintId::of(&strings::STRING_ADD),
+        LintId::of(&types::RC_BUFFER),
         LintId::of(&unwrap_in_result::UNWRAP_IN_RESULT),
         LintId::of(&verbose_file_reads::VERBOSE_FILE_READS),
         LintId::of(&write::PRINT_STDOUT),
@@ -1505,7 +1506,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::CHAR_LIT_AS_U8),
         LintId::of(&types::FN_TO_NUMERIC_CAST),
         LintId::of(&types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
-        LintId::of(&types::RC_BUFFER),
         LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
@@ -1806,7 +1806,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
         LintId::of(&stable_sort_primitive::STABLE_SORT_PRIMITIVE),
         LintId::of(&types::BOX_VEC),
-        LintId::of(&types::RC_BUFFER),
         LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&vec::USELESS_VEC),
     ]);

clippy_lints/src/types.rs

@@ -242,7 +242,7 @@ declare_clippy_lint! {
     /// fn foo(interned: Rc<str>) { ... }
     /// ```
     pub RC_BUFFER,
-    perf,
+    restriction,
     "shared ownership of a buffer type"
 }
 

src/lintlist/mod.rs

@@ -1888,7 +1888,7 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
     },
     Lint {
         name: "rc_buffer",
-        group: "perf",
+        group: "restriction",
         desc: "shared ownership of a buffer type",
         deprecation: None,
         module: "types",