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

use rustc_session::lint::builtin::LARGE_ASSIGNMENTS;

use super::*;
use crate::errors::LargeAssignmentsLint;

pub(super) struct MoveCheckState {
    /// Spans for move size lints already emitted. Helps avoid duplicate lints.
    move_size_spans: Vec<Span>,
    /// Set of functions for which it is OK to move large data into.
    skip_move_check_fns: Option<Vec<DefId>>,
}

impl MoveCheckState {
    pub(super) fn new() -> Self {
        MoveCheckState { move_size_spans: vec![], skip_move_check_fns: None }
    }
}

impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
    pub(super) fn check_operand_move_size(
        &mut self,
        operand: &mir::Operand<'tcx>,
        location: Location,
    ) {
        let limit = self.tcx.move_size_limit();
        if limit.0 == 0 {
            return;
        }

        // This function is called by visit_operand() which visits _all_
        // operands, including TerminatorKind::Call operands. But if
        // check_fn_args_move_size() has been called, the operands have already
        // been visited. Do not visit them again.
        if self.visiting_call_terminator {
            return;
        }

        let source_info = self.body.source_info(location);
        debug!(?source_info);

        if let Some(too_large_size) = self.operand_size_if_too_large(limit, operand) {
            self.lint_large_assignment(limit.0, too_large_size, location, source_info.span);
        };
    }

    pub(super) fn check_fn_args_move_size(
        &mut self,
        callee_ty: Ty<'tcx>,
        args: &[Spanned<mir::Operand<'tcx>>],
        fn_span: Span,
        location: Location,
    ) {
        let limit = self.tcx.move_size_limit();
        if limit.0 == 0 {
            return;
        }

        if args.is_empty() {
            return;
        }

        // Allow large moves into container types that themselves are cheap to move
        let ty::FnDef(def_id, _) = *callee_ty.kind() else {
            return;
        };
        if self
            .move_check
            .skip_move_check_fns
            .get_or_insert_with(|| build_skip_move_check_fns(self.tcx))
            .contains(&def_id)
        {
            return;
        }

        debug!(?def_id, ?fn_span);

        for arg in args {
            // Moving args into functions is typically implemented with pointer
            // passing at the llvm-ir level and not by memcpy's. So always allow
            // moving args into functions.
            let operand: &mir::Operand<'tcx> = &arg.node;
            if let mir::Operand::Move(_) = operand {
                continue;
            }

            if let Some(too_large_size) = self.operand_size_if_too_large(limit, operand) {
                self.lint_large_assignment(limit.0, too_large_size, location, arg.span);
            };
        }
    }

    fn operand_size_if_too_large(
        &mut self,
        limit: Limit,
        operand: &mir::Operand<'tcx>,
    ) -> Option<Size> {
        let ty = operand.ty(self.body, self.tcx);
        let ty = self.monomorphize(ty);
        let Ok(layout) = self.tcx.layout_of(ty::ParamEnv::reveal_all().and(ty)) else {
            return None;
        };
        if layout.size.bytes_usize() > limit.0 {
            debug!(?layout);
            Some(layout.size)
        } else {
            None
        }
    }

    fn lint_large_assignment(
        &mut self,
        limit: usize,
        too_large_size: Size,
        location: Location,
        span: Span,
    ) {
        let source_info = self.body.source_info(location);
        debug!(?source_info);
        for reported_span in &self.move_check.move_size_spans {
            if reported_span.overlaps(span) {
                return;
            }
        }
        let lint_root = source_info.scope.lint_root(&self.body.source_scopes);
        debug!(?lint_root);
        let Some(lint_root) = lint_root else {
            // This happens when the issue is in a function from a foreign crate that
            // we monomorphized in the current crate. We can't get a `HirId` for things
            // in other crates.
            // FIXME: Find out where to report the lint on. Maybe simply crate-level lint root
            // but correct span? This would make the lint at least accept crate-level lint attributes.
            return;
        };
        self.tcx.emit_node_span_lint(
            LARGE_ASSIGNMENTS,
            lint_root,
            span,
            LargeAssignmentsLint { span, size: too_large_size.bytes(), limit: limit as u64 },
        );
        self.move_check.move_size_spans.push(span);
    }
}

fn build_skip_move_check_fns(tcx: TyCtxt<'_>) -> Vec<DefId> {
    let fns = [
        (tcx.lang_items().owned_box(), "new"),
        (tcx.get_diagnostic_item(sym::Rc), "new"),
        (tcx.get_diagnostic_item(sym::Arc), "new"),
    ];
    fns.into_iter()
        .filter_map(|(def_id, fn_name)| {
            def_id.and_then(|def_id| assoc_fn_of_type(tcx, def_id, Ident::from_str(fn_name)))
        })
        .collect::<Vec<_>>()
}