New Lint: [`thread_local_initializer_can_be_made_const`]

Adds a new lint to suggest using `const` on `thread_local!`
initializers that can be evaluated at compile time.

Impl details:

The lint relies on the expansion of `thread_local!`. For non
const-labelled initializers, `thread_local!` produces a function
called `__init` that lazily initializes the value. We check the function
and decide whether the body can be const. The body of the function is
exactly the initializer. If so, we lint the body.

changelog: new lint [`thread_local_initializer_can_be_made_const`]

CHANGELOG.md

@@ -5616,6 +5616,7 @@ Released 2018-09-13
 [`temporary_cstring_as_ptr`]: https://rust-lang.github.io/rust-clippy/master/index.html#temporary_cstring_as_ptr
 [`test_attr_in_doctest`]: https://rust-lang.github.io/rust-clippy/master/index.html#test_attr_in_doctest
 [`tests_outside_test_module`]: https://rust-lang.github.io/rust-clippy/master/index.html#tests_outside_test_module
+[`thread_local_initializer_can_be_made_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#thread_local_initializer_can_be_made_const
 [`to_digit_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_digit_is_some
 [`to_string_in_display`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_display
 [`to_string_in_format_args`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_format_args

clippy_lints/src/declared_lints.rs

@@ -652,6 +652,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::tabs_in_doc_comments::TABS_IN_DOC_COMMENTS_INFO,
     crate::temporary_assignment::TEMPORARY_ASSIGNMENT_INFO,
     crate::tests_outside_test_module::TESTS_OUTSIDE_TEST_MODULE_INFO,
+    crate::thread_local_initializer_can_be_made_const::THREAD_LOCAL_INITIALIZER_CAN_BE_MADE_CONST_INFO,
     crate::to_digit_is_some::TO_DIGIT_IS_SOME_INFO,
     crate::trailing_empty_array::TRAILING_EMPTY_ARRAY_INFO,
     crate::trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS_INFO,

clippy_lints/src/lib.rs

@@ -323,6 +323,7 @@ mod swap_ptr_to_ref;
 mod tabs_in_doc_comments;
 mod temporary_assignment;
 mod tests_outside_test_module;
+mod thread_local_initializer_can_be_made_const;
 mod to_digit_is_some;
 mod trailing_empty_array;
 mod trait_bounds;
@@ -1088,6 +1089,9 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
             behavior: pub_underscore_fields_behavior,
         })
     });
+    store.register_late_pass(move |_| {
+        Box::new(thread_local_initializer_can_be_made_const::ThreadLocalInitializerCanBeMadeConst::new(msrv()))
+    });
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/thread_local_initializer_can_be_made_const.rs

@@ -0,0 +1,102 @@
+use clippy_config::msrvs::Msrv;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::fn_has_unsatisfiable_preds;
+use clippy_utils::qualify_min_const_fn::is_min_const_fn;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::{intravisit, ExprKind};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::impl_lint_pass;
+use rustc_span::sym::thread_local_macro;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Suggests to use `const` in `thread_local!` macro if possible.
+    /// ### Why is this bad?
+    ///
+    /// The `thread_local!` macro wraps static declarations and makes them thread-local.
+    /// It supports using a `const` keyword that may be used for declarations that can
+    /// be evaluated as a constant expression. This can enable a more efficient thread
+    /// local implementation that can avoid lazy initialization. For types that do not
+    /// need to be dropped, this can enable an even more efficient implementation that
+    /// does not need to track any additional state.
+    ///
+    /// https://doc.rust-lang.org/std/macro.thread_local.html
+    ///
+    /// ### Example
+    /// ```no_run
+    /// // example code where clippy issues a warning
+    /// thread_local! {
+    ///     static BUF: String = String::new();
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// // example code which does not raise clippy warning
+    /// thread_local! {
+    ///     static BUF: String = const { String::new() };
+    /// }
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub THREAD_LOCAL_INITIALIZER_CAN_BE_MADE_CONST,
+    perf,
+    "suggest using `const` in `thread_local!` macro"
+}
+
+pub struct ThreadLocalInitializerCanBeMadeConst {
+    msrv: Msrv,
+}
+
+impl ThreadLocalInitializerCanBeMadeConst {
+    #[must_use]
+    pub fn new(msrv: Msrv) -> Self {
+        Self { msrv }
+    }
+}
+
+impl_lint_pass!(ThreadLocalInitializerCanBeMadeConst => [THREAD_LOCAL_INITIALIZER_CAN_BE_MADE_CONST]);
+
+impl<'tcx> LateLintPass<'tcx> for ThreadLocalInitializerCanBeMadeConst {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        fn_kind: rustc_hir::intravisit::FnKind<'tcx>,
+        _: &'tcx rustc_hir::FnDecl<'tcx>,
+        body: &'tcx rustc_hir::Body<'tcx>,
+        span: rustc_span::Span,
+        defid: rustc_span::def_id::LocalDefId,
+    ) {
+        if in_external_macro(cx.sess(), span)
+            && let Some(callee) = span.source_callee()
+            && let Some(macro_def_id) = callee.macro_def_id
+            && cx.tcx.is_diagnostic_item(thread_local_macro, macro_def_id)
+            && let intravisit::FnKind::ItemFn(..) = fn_kind
+            // Building MIR for `fn`s with unsatisfiable preds results in ICE.
+            && !fn_has_unsatisfiable_preds(cx, defid.to_def_id())
+            && let mir = cx.tcx.optimized_mir(defid.to_def_id())
+            && let Ok(()) = is_min_const_fn(cx.tcx, mir, &self.msrv)
+            // this is the `__init` function emitted by the `thread_local!` macro
+            // when the `const` keyword is not used. We avoid checking the `__init` directly
+            // as that is not a public API.
+            // we know that the function is const-qualifiable, so now we need only to get the
+            // initializer expression to span-lint it.
+            && let ExprKind::Block(block, _) = body.value.kind
+            && let Some(ret_expr) = block.expr
+            && let initializer_snippet = snippet(cx, ret_expr.span, "thread_local! { ... }")
+            && initializer_snippet != "thread_local! { ... }"
+        {
+            span_lint_and_sugg(
+                cx,
+                THREAD_LOCAL_INITIALIZER_CAN_BE_MADE_CONST,
+                ret_expr.span,
+                "initializer for `thread_local` value can be made `const`",
+                "replace with",
+                format!("const {{ {initializer_snippet} }}"),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+
+    extract_msrv_attr!(LateContext);
+}

tests/ui/thread_local_initializer_can_be_made_const.fixed

@@ -0,0 +1,30 @@
+#![warn(clippy::thread_local_initializer_can_be_made_const)]
+
+use std::cell::RefCell;
+
+fn main() {
+    // lint and suggest const
+    thread_local! {
+        static BUF_1: RefCell<String> = const { RefCell::new(String::new()) };
+    }
+    //~^^ ERROR: initializer for `thread_local` value can be made `const`
+
+    // don't lint
+    thread_local! {
+        static BUF_2: RefCell<String> = const { RefCell::new(String::new()) };
+    }
+
+    thread_local! {
+        static SIMPLE:i32 = const { 1 };
+    }
+    //~^^ ERROR: initializer for `thread_local` value can be made `const`
+
+    // lint and suggest const for all non const items
+    thread_local! {
+        static BUF_3_CAN_BE_MADE_CONST: RefCell<String> = const { RefCell::new(String::new()) };
+        static CONST_MIXED_WITH:i32 = const { 1 };
+        static BUF_4_CAN_BE_MADE_CONST: RefCell<String> = const { RefCell::new(String::new()) };
+    }
+    //~^^^^ ERROR: initializer for `thread_local` value can be made `const`
+    //~^^^ ERROR: initializer for `thread_local` value can be made `const`
+}

tests/ui/thread_local_initializer_can_be_made_const.rs

@@ -0,0 +1,30 @@
+#![warn(clippy::thread_local_initializer_can_be_made_const)]
+
+use std::cell::RefCell;
+
+fn main() {
+    // lint and suggest const
+    thread_local! {
+        static BUF_1: RefCell<String> = RefCell::new(String::new());
+    }
+    //~^^ ERROR: initializer for `thread_local` value can be made `const`
+
+    // don't lint
+    thread_local! {
+        static BUF_2: RefCell<String> = const { RefCell::new(String::new()) };
+    }
+
+    thread_local! {
+        static SIMPLE:i32 = 1;
+    }
+    //~^^ ERROR: initializer for `thread_local` value can be made `const`
+
+    // lint and suggest const for all non const items
+    thread_local! {
+        static BUF_3_CAN_BE_MADE_CONST: RefCell<String> = RefCell::new(String::new());
+        static CONST_MIXED_WITH:i32 = const { 1 };
+        static BUF_4_CAN_BE_MADE_CONST: RefCell<String> = RefCell::new(String::new());
+    }
+    //~^^^^ ERROR: initializer for `thread_local` value can be made `const`
+    //~^^^ ERROR: initializer for `thread_local` value can be made `const`
+}

tests/ui/thread_local_initializer_can_be_made_const.stderr

@@ -0,0 +1,29 @@
+error: initializer for `thread_local` value can be made `const`
+  --> $DIR/thread_local_initializer_can_be_made_const.rs:8:41
+   |
+LL |         static BUF_1: RefCell<String> = RefCell::new(String::new());
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `const { RefCell::new(String::new()) }`
+   |
+   = note: `-D clippy::thread-local-initializer-can-be-made-const` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::thread_local_initializer_can_be_made_const)]`
+
+error: initializer for `thread_local` value can be made `const`
+  --> $DIR/thread_local_initializer_can_be_made_const.rs:18:29
+   |
+LL |         static SIMPLE:i32 = 1;
+   |                             ^ help: replace with: `const { 1 }`
+
+error: initializer for `thread_local` value can be made `const`
+  --> $DIR/thread_local_initializer_can_be_made_const.rs:24:59
+   |
+LL |         static BUF_3_CAN_BE_MADE_CONST: RefCell<String> = RefCell::new(String::new());
+   |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `const { RefCell::new(String::new()) }`
+
+error: initializer for `thread_local` value can be made `const`
+  --> $DIR/thread_local_initializer_can_be_made_const.rs:26:59
+   |
+LL |         static BUF_4_CAN_BE_MADE_CONST: RefCell<String> = RefCell::new(String::new());
+   |                                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `const { RefCell::new(String::new()) }`
+
+error: aborting due to 4 previous errors
+