clippy_lints/src/missing_const_for_thread_local.rs

use clippy_config::Conf;
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::macros::macro_backtrace;
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::qualify_min_const_fn::is_min_const_fn;
use clippy_utils::source::snippet;
use clippy_utils::{fn_has_unsatisfiable_preds, peel_blocks, sym};
use rustc_errors::Applicability;
use rustc_hir::{Expr, ExprKind, intravisit};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::impl_lint_pass;

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
    /// thread_local! {
    ///     static BUF: String = String::new();
    /// }
    /// ```
    /// Use instead:
    /// ```no_run
    /// thread_local! {
    ///     static BUF: String = const { String::new() };
    /// }
    /// ```
    #[clippy::version = "1.77.0"]
    pub MISSING_CONST_FOR_THREAD_LOCAL,
    perf,
    "suggest using `const` in `thread_local!` macro"
}

impl_lint_pass!(MissingConstForThreadLocal => [MISSING_CONST_FOR_THREAD_LOCAL]);

pub struct MissingConstForThreadLocal {
    msrv: Msrv,
}

impl MissingConstForThreadLocal {
    pub fn new(conf: &'static Conf) -> Self {
        Self { msrv: conf.msrv }
    }
}

#[inline]
fn is_thread_local_initializer(
    cx: &LateContext<'_>,
    fn_kind: intravisit::FnKind<'_>,
    span: rustc_span::Span,
) -> Option<bool> {
    let macro_def_id = span.source_callee()?.macro_def_id?;
    Some(
        cx.tcx.is_diagnostic_item(sym::thread_local_macro, macro_def_id)
            && matches!(fn_kind, intravisit::FnKind::ItemFn(..)),
    )
}

fn is_unreachable(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    if let Some(macro_call) = macro_backtrace(expr.span).next()
        && let Some(diag_name) = cx.tcx.get_diagnostic_name(macro_call.def_id)
    {
        return (matches!(
            diag_name,
            sym::core_panic_macro
                | sym::std_panic_macro
                | sym::core_panic_2015_macro
                | sym::std_panic_2015_macro
                | sym::core_panic_2021_macro
        ) && !cx.tcx.hir_is_inside_const_context(expr.hir_id))
            || matches!(
                diag_name,
                sym::unimplemented_macro | sym::todo_macro | sym::unreachable_macro | sym::unreachable_2015_macro
            );
    }
    false
}

#[inline]
fn initializer_can_be_made_const(cx: &LateContext<'_>, defid: rustc_span::def_id::DefId, msrv: Msrv) -> bool {
    // Building MIR for `fn`s with unsatisfiable preds results in ICE.
    if !fn_has_unsatisfiable_preds(cx, defid)
        && let mir = cx.tcx.optimized_mir(defid)
        && let Ok(()) = is_min_const_fn(cx, mir, msrv)
    {
        return true;
    }
    false
}

impl<'tcx> LateLintPass<'tcx> for MissingConstForThreadLocal {
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        fn_kind: intravisit::FnKind<'tcx>,
        _: &'tcx rustc_hir::FnDecl<'tcx>,
        body: &'tcx rustc_hir::Body<'tcx>,
        span: rustc_span::Span,
        local_defid: rustc_span::def_id::LocalDefId,
    ) {
        let defid = local_defid.to_def_id();
        if is_thread_local_initializer(cx, fn_kind, span).unwrap_or(false)
            // Some implementations of `thread_local!` include an initializer fn.
            // In the case of a const initializer, the init fn is also const,
            // so we can skip the lint in that case. This occurs only on some
            // backends due to conditional compilation:
            // https://doc.rust-lang.org/src/std/sys/common/thread_local/mod.rs.html
            // for details on this issue, see:
            // https://github.com/rust-lang/rust-clippy/pull/12276
            && !cx.tcx.is_const_fn(defid)
            && let ExprKind::Block(block, _) = body.value.kind
            && let Some(unpeeled) = block.expr
            && let ret_expr = peel_blocks(unpeeled)
            // A common pattern around threadlocal! is to make the value unreachable
            // to force an initialization before usage
            // https://github.com/rust-lang/rust-clippy/issues/12637
            // we ensure that this is reachable before we check in mir
            && !is_unreachable(cx, ret_expr)
            && initializer_can_be_made_const(cx, defid, self.msrv)
            // we know that the function is const-qualifiable, so now
            // we need only to get the initializer expression to span-lint it.
            && let initializer_snippet = snippet(cx, ret_expr.span, "thread_local! { ... }")
            && initializer_snippet != "thread_local! { ... }"
            && self.msrv.meets(cx, msrvs::THREAD_LOCAL_CONST_INIT)
        {
            span_lint_and_sugg(
                cx,
                MISSING_CONST_FOR_THREAD_LOCAL,
                unpeeled.span,
                "initializer for `thread_local` value can be made `const`",
                "replace with",
                format!("const {{ {initializer_snippet} }}"),
                Applicability::MachineApplicable,
            );
        }
    }
}