clippy_lints/src/std_instead_of_core.rs

use clippy_config::Conf;
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::is_from_proc_macro;
use clippy_utils::msrvs::Msrv;
use rustc_attr_data_structures::{StabilityLevel, StableSince};
use rustc_errors::Applicability;
use rustc_hir::def::Res;
use rustc_hir::def_id::DefId;
use rustc_hir::{Block, Body, HirId, Path, PathSegment};
use rustc_lint::{LateContext, LateLintPass, Lint, LintContext};
use rustc_session::impl_lint_pass;
use rustc_span::symbol::kw;
use rustc_span::{Span, sym};

declare_clippy_lint! {
    /// ### What it does
    /// Finds items imported through `std` when available through `core`.
    ///
    /// ### Why restrict this?
    /// Crates which have `no_std` compatibility may wish to ensure types are imported from core to ensure
    /// disabling `std` does not cause the crate to fail to compile. This lint is also useful for crates
    /// migrating to become `no_std` compatible.
    ///
    /// ### Example
    /// ```no_run
    /// use std::hash::Hasher;
    /// ```
    /// Use instead:
    /// ```no_run
    /// use core::hash::Hasher;
    /// ```
    #[clippy::version = "1.64.0"]
    pub STD_INSTEAD_OF_CORE,
    restriction,
    "type is imported from std when available in core"
}

declare_clippy_lint! {
    /// ### What it does
    /// Finds items imported through `std` when available through `alloc`.
    ///
    /// ### Why restrict this?
    /// Crates which have `no_std` compatibility and require alloc may wish to ensure types are imported from
    /// alloc to ensure disabling `std` does not cause the crate to fail to compile. This lint is also useful
    /// for crates migrating to become `no_std` compatible.
    ///
    /// ### Example
    /// ```no_run
    /// use std::vec::Vec;
    /// ```
    /// Use instead:
    /// ```no_run
    /// # extern crate alloc;
    /// use alloc::vec::Vec;
    /// ```
    #[clippy::version = "1.64.0"]
    pub STD_INSTEAD_OF_ALLOC,
    restriction,
    "type is imported from std when available in alloc"
}

declare_clippy_lint! {
    /// ### What it does
    /// Finds items imported through `alloc` when available through `core`.
    ///
    /// ### Why restrict this?
    /// Crates which have `no_std` compatibility and may optionally require alloc may wish to ensure types are
    /// imported from core to ensure disabling `alloc` does not cause the crate to fail to compile. This lint
    /// is also useful for crates migrating to become `no_std` compatible.
    ///
    /// ### Known problems
    /// The lint is only partially aware of the required MSRV for items that were originally in `std` but moved
    /// to `core`.
    ///
    /// ### Example
    /// ```no_run
    /// # extern crate alloc;
    /// use alloc::slice::from_ref;
    /// ```
    /// Use instead:
    /// ```no_run
    /// use core::slice::from_ref;
    /// ```
    #[clippy::version = "1.64.0"]
    pub ALLOC_INSTEAD_OF_CORE,
    restriction,
    "type is imported from alloc when available in core"
}

pub struct StdReexports {
    lint_point: (Span, Option<LintPoint>),
    msrv: Msrv,
}

impl StdReexports {
    pub fn new(conf: &'static Conf) -> Self {
        Self {
            lint_point: Default::default(),
            msrv: conf.msrv,
        }
    }

    fn lint_if_finish(&mut self, cx: &LateContext<'_>, (span, item): (Span, Option<LintPoint>)) {
        if span.source_equal(self.lint_point.0) {
            return;
        }

        if !self.lint_point.0.is_dummy() {
            emit_lints(cx, &self.lint_point);
        }

        self.lint_point = (span, item);
    }
}

impl_lint_pass!(StdReexports => [STD_INSTEAD_OF_CORE, STD_INSTEAD_OF_ALLOC, ALLOC_INSTEAD_OF_CORE]);

type LintPoint = (&'static Lint, &'static str, &'static str);

impl<'tcx> LateLintPass<'tcx> for StdReexports {
    fn check_path(&mut self, cx: &LateContext<'tcx>, path: &Path<'tcx>, _: HirId) {
        if let Res::Def(_, def_id) = path.res
            && let Some(first_segment) = get_first_segment(path)
            && is_stable(cx, def_id, self.msrv)
            && !path.span.in_external_macro(cx.sess().source_map())
            && !is_from_proc_macro(cx, &first_segment.ident)
        {
            let (lint, used_mod, replace_with) = match first_segment.ident.name {
                sym::std => match cx.tcx.crate_name(def_id.krate) {
                    sym::core => (STD_INSTEAD_OF_CORE, "std", "core"),
                    sym::alloc => (STD_INSTEAD_OF_ALLOC, "std", "alloc"),
                    _ => {
                        self.lint_if_finish(cx, (first_segment.ident.span, None));
                        return;
                    },
                },
                sym::alloc => {
                    if cx.tcx.crate_name(def_id.krate) == sym::core {
                        (ALLOC_INSTEAD_OF_CORE, "alloc", "core")
                    } else {
                        self.lint_if_finish(cx, (first_segment.ident.span, None));
                        return;
                    }
                },
                _ => return,
            };

            self.lint_if_finish(cx, (first_segment.ident.span, Some((lint, used_mod, replace_with))));
        }
    }

    fn check_block_post(&mut self, cx: &LateContext<'tcx>, _: &Block<'tcx>) {
        self.lint_if_finish(cx, Default::default());
    }

    fn check_body_post(&mut self, cx: &LateContext<'tcx>, _: &Body<'tcx>) {
        self.lint_if_finish(cx, Default::default());
    }

    fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
        self.lint_if_finish(cx, Default::default());
    }
}

fn emit_lints(cx: &LateContext<'_>, (span, item): &(Span, Option<LintPoint>)) {
    let Some((lint, used_mod, replace_with)) = item else {
        return;
    };

    span_lint_and_sugg(
        cx,
        lint,
        *span,
        format!("used import from `{used_mod}` instead of `{replace_with}`"),
        format!("consider importing the item from `{replace_with}`"),
        (*replace_with).to_string(),
        Applicability::MachineApplicable,
    );
}

/// Returns the first named segment of a [`Path`].
///
/// If this is a global path (such as `::std::fmt::Debug`), then the segment after [`kw::PathRoot`]
/// is returned.
fn get_first_segment<'tcx>(path: &Path<'tcx>) -> Option<&'tcx PathSegment<'tcx>> {
    match path.segments {
        // A global path will have PathRoot as the first segment. In this case, return the segment after.
        [x, y, ..] if x.ident.name == kw::PathRoot => Some(y),
        [x, ..] => Some(x),
        _ => None,
    }
}

/// Checks if all ancestors of `def_id` meet `msrv` to avoid linting [unstable moves](https://github.com/rust-lang/rust/pull/95956)
/// or now stable moves that were once unstable.
///
/// Does not catch individually moved items
fn is_stable(cx: &LateContext<'_>, mut def_id: DefId, msrv: Msrv) -> bool {
    loop {
        if let Some(stability) = cx.tcx.lookup_stability(def_id)
            && let StabilityLevel::Stable {
                since,
                allowed_through_unstable_modules: None,
            } = stability.level
        {
            let stable = match since {
                StableSince::Version(v) => msrv.meets(cx, v),
                StableSince::Current => msrv.current(cx).is_none(),
                StableSince::Err => false,
            };

            if !stable {
                return false;
            }
        }

        match cx.tcx.opt_parent(def_id) {
            Some(parent) => def_id = parent,
            None => return true,
        }
    }
}