frontend 0.4.1

rustc's frontend with no LLVM and no std: parsing through MIR, as a library
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// `#![no_std]`: these arrive with the standard prelude and name no path, so a `std::`
// search cannot see them - and a `#[derive]` can use them without the name appearing
// in this file at all, which is why they are not trimmed by inspection.
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use alloc::format;
use alloc::string::{String, ToString};
use alloc::vec;
use alloc::vec::Vec;

use crate::rustc_data_structures::unord::{ExtendUnord, UnordSet};
use crate::rustc_errors::{Diag, DiagCtxtHandle, Diagnostic, Level};
use crate::rustc_hir::def::DefKind;
use crate::rustc_hir::def_id::LocalDefId;
use crate::rustc_lint_defs::builtin::UNUSED_IMPORTS;
use crate::rustc_middle::ty::TyCtxt;
use crate::rustc_span::Span;
use tracing::debug;

struct UnusedImport<'tcx> {
    tcx: TyCtxt<'tcx>,
    span: Span,
}

impl<'a, 'tcx> Diagnostic<'a, ()> for UnusedImport<'tcx> {
    fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
        let Self { tcx, span } = self;
        if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span) {
            Diag::new(dcx, level, format!("unused import: `{snippet}`"))
        } else {
            Diag::new(dcx, level, "unused import")
        }
    }
}

pub(super) fn check_unused_traits(tcx: TyCtxt<'_>, (): ()) {
    let mut used_trait_imports = UnordSet::<LocalDefId>::default();

    // FIXME: Use `tcx.hir_par_body_owners()` when we implement creating `DefId`s
    // for anon constants during their parents' typeck.
    // Doing so at current will produce queries cycle errors because it may typeck
    // on anon constants directly.
    for item_def_id in tcx.hir_body_owners() {
        let imports = tcx.used_trait_imports(item_def_id);
        debug!("GatherVisitor: item_def_id={:?} with imports {:#?}", item_def_id, imports);
        used_trait_imports.extend_unord(imports.items().copied());
    }

    for &id in tcx.resolutions(()).maybe_unused_trait_imports.iter() {
        debug_assert_eq!(tcx.def_kind(id), DefKind::Use);
        if tcx.visibility(id).is_public() {
            continue;
        }
        if used_trait_imports.contains(&id) {
            continue;
        }
        let item = tcx.hir_expect_item(id);
        if item.span.is_dummy() {
            continue;
        }
        let (path, _) = item.expect_use();
        tcx.emit_node_span_lint(
            UNUSED_IMPORTS,
            item.hir_id(),
            path.span,
            UnusedImport { tcx, span: path.span },
        );
    }
}