1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
use crate::errors::{ExternCrateNotIdiomatic, UnusedExternCrate};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_middle::ty::TyCtxt;
use rustc_session::lint;
use rustc_span::{Span, Symbol};

pub fn check_crate(tcx: TyCtxt<'_>) {
    let mut used_trait_imports: FxHashSet<LocalDefId> = FxHashSet::default();

    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(imports.iter());
    }

    for &id in tcx.maybe_unused_trait_imports(()) {
        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 hir::ItemKind::Use(path, _) = item.kind else { unreachable!() };
        tcx.struct_span_lint_hir(lint::builtin::UNUSED_IMPORTS, item.hir_id(), path.span, |lint| {
            let msg = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(path.span) {
                format!("unused import: `{}`", snippet)
            } else {
                "unused import".to_owned()
            };
            lint.build(&msg).emit();
        });
    }

    unused_crates_lint(tcx);
}

fn unused_crates_lint(tcx: TyCtxt<'_>) {
    let lint = lint::builtin::UNUSED_EXTERN_CRATES;

    // Collect first the crates that are completely unused.  These we
    // can always suggest removing (no matter which edition we are
    // in).
    let unused_extern_crates: FxHashMap<LocalDefId, Span> = tcx
        .maybe_unused_extern_crates(())
        .iter()
        .filter(|&&(def_id, _)| {
            // The `def_id` here actually was calculated during resolution (at least
            // at the time of this writing) and is being shipped to us via a side
            // channel of the tcx. There may have been extra expansion phases,
            // however, which ended up removing the `def_id` *after* expansion.
            //
            // As a result we need to verify that `def_id` is indeed still valid for
            // our AST and actually present in the HIR map. If it's not there then
            // there's safely nothing to warn about, and otherwise we carry on with
            // our execution.
            //
            // Note that if we carry through to the `extern_mod_stmt_cnum` query
            // below it'll cause a panic because `def_id` is actually bogus at this
            // point in time otherwise.
            if tcx.hir().find(tcx.hir().local_def_id_to_hir_id(def_id)).is_none() {
                return false;
            }
            true
        })
        .filter(|&&(def_id, _)| {
            tcx.extern_mod_stmt_cnum(def_id).map_or(true, |cnum| {
                !tcx.is_compiler_builtins(cnum)
                    && !tcx.is_panic_runtime(cnum)
                    && !tcx.has_global_allocator(cnum)
                    && !tcx.has_panic_handler(cnum)
            })
        })
        .cloned()
        .collect();

    // Collect all the extern crates (in a reliable order).
    let mut crates_to_lint = vec![];

    for id in tcx.hir().items() {
        if matches!(tcx.def_kind(id.def_id), DefKind::ExternCrate) {
            let item = tcx.hir().item(id);
            if let hir::ItemKind::ExternCrate(orig_name) = item.kind {
                crates_to_lint.push(ExternCrateToLint {
                    def_id: item.def_id.to_def_id(),
                    span: item.span,
                    orig_name,
                    warn_if_unused: !item.ident.as_str().starts_with('_'),
                });
            }
        }
    }

    let extern_prelude = &tcx.resolutions(()).extern_prelude;

    for extern_crate in &crates_to_lint {
        let def_id = extern_crate.def_id.expect_local();
        let item = tcx.hir().expect_item(def_id);

        // If the crate is fully unused, we suggest removing it altogether.
        // We do this in any edition.
        if extern_crate.warn_if_unused {
            if let Some(&span) = unused_extern_crates.get(&def_id) {
                // Removal suggestion span needs to include attributes (Issue #54400)
                let id = tcx.hir().local_def_id_to_hir_id(def_id);
                let span_with_attrs = tcx
                    .hir()
                    .attrs(id)
                    .iter()
                    .map(|attr| attr.span)
                    .fold(span, |acc, attr_span| acc.to(attr_span));

                tcx.emit_spanned_lint(lint, id, span, UnusedExternCrate { span: span_with_attrs });
                continue;
            }
        }

        // If we are not in Rust 2018 edition, then we don't make any further
        // suggestions.
        if !tcx.sess.rust_2018() {
            continue;
        }

        // If the extern crate isn't in the extern prelude,
        // there is no way it can be written as a `use`.
        let orig_name = extern_crate.orig_name.unwrap_or(item.ident.name);
        if !extern_prelude.get(&orig_name).map_or(false, |from_item| !from_item) {
            continue;
        }

        // If the extern crate is renamed, then we cannot suggest replacing it with a use as this
        // would not insert the new name into the prelude, where other imports in the crate may be
        // expecting it.
        if extern_crate.orig_name.is_some() {
            continue;
        }

        let id = tcx.hir().local_def_id_to_hir_id(def_id);
        // If the extern crate has any attributes, they may have funky
        // semantics we can't faithfully represent using `use` (most
        // notably `#[macro_use]`). Ignore it.
        if !tcx.hir().attrs(id).is_empty() {
            continue;
        }

        let base_replacement = match extern_crate.orig_name {
            Some(orig_name) => format!("use {} as {};", orig_name, item.ident.name),
            None => format!("use {};", item.ident.name),
        };
        let vis = tcx.sess.source_map().span_to_snippet(item.vis_span).unwrap_or_default();
        let add_vis = |to| if vis.is_empty() { to } else { format!("{} {}", vis, to) };
        tcx.emit_spanned_lint(
            lint,
            id,
            extern_crate.span,
            ExternCrateNotIdiomatic {
                span: extern_crate.span,
                msg_code: add_vis("use".to_string()),
                suggestion_code: add_vis(base_replacement),
            },
        );
    }
}

struct ExternCrateToLint {
    /// `DefId` of the extern crate
    def_id: DefId,

    /// span from the item
    span: Span,

    /// if `Some`, then this is renamed (`extern crate orig_name as
    /// crate_name`), and -- perhaps surprisingly -- this stores the
    /// *original* name (`item.name` will contain the new name)
    orig_name: Option<Symbol>,

    /// if `false`, the original name started with `_`, so we shouldn't lint
    /// about it going unused (but we should still emit idiom lints).
    warn_if_unused: bool,
}