rustc_lint/
foreign_modules.rs

1use rustc_abi::FIRST_VARIANT;
2use rustc_data_structures::stack::ensure_sufficient_stack;
3use rustc_data_structures::unord::{UnordMap, UnordSet};
4use rustc_hir as hir;
5use rustc_hir::attrs::AttributeKind;
6use rustc_hir::def::DefKind;
7use rustc_hir::find_attr;
8use rustc_middle::query::Providers;
9use rustc_middle::ty::{self, AdtDef, Instance, Ty, TyCtxt};
10use rustc_session::declare_lint;
11use rustc_span::{Span, Symbol};
12use tracing::{debug, instrument};
13
14use crate::lints::{BuiltinClashingExtern, BuiltinClashingExternSub};
15use crate::{LintVec, types};
16
17pub(crate) fn provide(providers: &mut Providers) {
18    *providers = Providers { clashing_extern_declarations, ..*providers };
19}
20
21pub(crate) fn get_lints() -> LintVec {
22    vec![CLASHING_EXTERN_DECLARATIONS]
23}
24
25fn clashing_extern_declarations(tcx: TyCtxt<'_>, (): ()) {
26    let mut lint = ClashingExternDeclarations::new();
27    for id in tcx.hir_crate_items(()).foreign_items() {
28        lint.check_foreign_item(tcx, id);
29    }
30}
31
32declare_lint! {
33    /// The `clashing_extern_declarations` lint detects when an `extern fn`
34    /// has been declared with the same name but different types.
35    ///
36    /// ### Example
37    ///
38    /// ```rust
39    /// mod m {
40    ///     unsafe extern "C" {
41    ///         fn foo();
42    ///     }
43    /// }
44    ///
45    /// unsafe extern "C" {
46    ///     fn foo(_: u32);
47    /// }
48    /// ```
49    ///
50    /// {{produces}}
51    ///
52    /// ### Explanation
53    ///
54    /// Because two symbols of the same name cannot be resolved to two
55    /// different functions at link time, and one function cannot possibly
56    /// have two types, a clashing extern declaration is almost certainly a
57    /// mistake. Check to make sure that the `extern` definitions are correct
58    /// and equivalent, and possibly consider unifying them in one location.
59    ///
60    /// This lint does not run between crates because a project may have
61    /// dependencies which both rely on the same extern function, but declare
62    /// it in a different (but valid) way. For example, they may both declare
63    /// an opaque type for one or more of the arguments (which would end up
64    /// distinct types), or use types that are valid conversions in the
65    /// language the `extern fn` is defined in. In these cases, the compiler
66    /// can't say that the clashing declaration is incorrect.
67    pub CLASHING_EXTERN_DECLARATIONS,
68    Warn,
69    "detects when an extern fn has been declared with the same name but different types"
70}
71
72struct ClashingExternDeclarations {
73    /// Map of function symbol name to the first-seen hir id for that symbol name.. If seen_decls
74    /// contains an entry for key K, it means a symbol with name K has been seen by this lint and
75    /// the symbol should be reported as a clashing declaration.
76    // FIXME: Technically, we could just store a &'tcx str here without issue; however, the
77    // `impl_lint_pass` macro doesn't currently support lints parametric over a lifetime.
78    seen_decls: UnordMap<Symbol, hir::OwnerId>,
79}
80
81/// Differentiate between whether the name for an extern decl came from the link_name attribute or
82/// just from declaration itself. This is important because we don't want to report clashes on
83/// symbol name if they don't actually clash because one or the other links against a symbol with a
84/// different name.
85enum SymbolName {
86    /// The name of the symbol + the span of the annotation which introduced the link name.
87    Link(Symbol, Span),
88    /// No link name, so just the name of the symbol.
89    Normal(Symbol),
90}
91
92impl SymbolName {
93    fn get_name(&self) -> Symbol {
94        match self {
95            SymbolName::Link(s, _) | SymbolName::Normal(s) => *s,
96        }
97    }
98}
99
100impl ClashingExternDeclarations {
101    pub(crate) fn new() -> Self {
102        ClashingExternDeclarations { seen_decls: Default::default() }
103    }
104
105    /// Insert a new foreign item into the seen set. If a symbol with the same name already exists
106    /// for the item, return its HirId without updating the set.
107    fn insert(&mut self, tcx: TyCtxt<'_>, fi: hir::ForeignItemId) -> Option<hir::OwnerId> {
108        let did = fi.owner_id.to_def_id();
109        let instance = Instance::new_raw(did, ty::List::identity_for_item(tcx, did));
110        let name = Symbol::intern(tcx.symbol_name(instance).name);
111        if let Some(&existing_id) = self.seen_decls.get(&name) {
112            // Avoid updating the map with the new entry when we do find a collision. We want to
113            // make sure we're always pointing to the first definition as the previous declaration.
114            // This lets us avoid emitting "knock-on" diagnostics.
115            Some(existing_id)
116        } else {
117            self.seen_decls.insert(name, fi.owner_id)
118        }
119    }
120
121    #[instrument(level = "trace", skip(self, tcx))]
122    fn check_foreign_item<'tcx>(&mut self, tcx: TyCtxt<'tcx>, this_fi: hir::ForeignItemId) {
123        let DefKind::Fn = tcx.def_kind(this_fi.owner_id) else { return };
124        let Some(existing_did) = self.insert(tcx, this_fi) else { return };
125
126        let existing_decl_ty = tcx.type_of(existing_did).skip_binder();
127        let this_decl_ty = tcx.type_of(this_fi.owner_id).instantiate_identity();
128        debug!(
129            "ClashingExternDeclarations: Comparing existing {:?}: {:?} to this {:?}: {:?}",
130            existing_did, existing_decl_ty, this_fi.owner_id, this_decl_ty
131        );
132
133        // Check that the declarations match.
134        if !structurally_same_type(
135            tcx,
136            ty::TypingEnv::non_body_analysis(tcx, this_fi.owner_id),
137            existing_decl_ty,
138            this_decl_ty,
139        ) {
140            let orig = name_of_extern_decl(tcx, existing_did);
141
142            // Finally, emit the diagnostic.
143            let this = tcx.item_name(this_fi.owner_id.to_def_id());
144            let orig = orig.get_name();
145            let previous_decl_label = get_relevant_span(tcx, existing_did);
146            let mismatch_label = get_relevant_span(tcx, this_fi.owner_id);
147            let sub =
148                BuiltinClashingExternSub { tcx, expected: existing_decl_ty, found: this_decl_ty };
149            let decorator = if orig == this {
150                BuiltinClashingExtern::SameName {
151                    this,
152                    orig,
153                    previous_decl_label,
154                    mismatch_label,
155                    sub,
156                }
157            } else {
158                BuiltinClashingExtern::DiffName {
159                    this,
160                    orig,
161                    previous_decl_label,
162                    mismatch_label,
163                    sub,
164                }
165            };
166            tcx.emit_node_span_lint(
167                CLASHING_EXTERN_DECLARATIONS,
168                this_fi.hir_id(),
169                mismatch_label,
170                decorator,
171            );
172        }
173    }
174}
175
176/// Get the name of the symbol that's linked against for a given extern declaration. That is,
177/// the name specified in a #[link_name = ...] attribute if one was specified, else, just the
178/// symbol's name.
179fn name_of_extern_decl(tcx: TyCtxt<'_>, fi: hir::OwnerId) -> SymbolName {
180    if let Some((overridden_link_name, overridden_link_name_span)) =
181        tcx.codegen_fn_attrs(fi).symbol_name.map(|overridden_link_name| {
182            // FIXME: Instead of searching through the attributes again to get span
183            // information, we could have codegen_fn_attrs also give span information back for
184            // where the attribute was defined. However, until this is found to be a
185            // bottleneck, this does just fine.
186            (
187                overridden_link_name,
188                find_attr!(tcx.get_all_attrs(fi), AttributeKind::LinkName {span, ..} => *span)
189                    .unwrap(),
190            )
191        })
192    {
193        SymbolName::Link(overridden_link_name, overridden_link_name_span)
194    } else {
195        SymbolName::Normal(tcx.item_name(fi.to_def_id()))
196    }
197}
198
199/// We want to ensure that we use spans for both decls that include where the
200/// name was defined, whether that was from the link_name attribute or not.
201fn get_relevant_span(tcx: TyCtxt<'_>, fi: hir::OwnerId) -> Span {
202    match name_of_extern_decl(tcx, fi) {
203        SymbolName::Normal(_) => tcx.def_span(fi),
204        SymbolName::Link(_, annot_span) => annot_span,
205    }
206}
207
208/// Checks whether two types are structurally the same enough that the declarations shouldn't
209/// clash. We need this so we don't emit a lint when two modules both declare an extern struct,
210/// with the same members (as the declarations shouldn't clash).
211fn structurally_same_type<'tcx>(
212    tcx: TyCtxt<'tcx>,
213    typing_env: ty::TypingEnv<'tcx>,
214    a: Ty<'tcx>,
215    b: Ty<'tcx>,
216) -> bool {
217    let mut seen_types = UnordSet::default();
218    let result = structurally_same_type_impl(&mut seen_types, tcx, typing_env, a, b);
219    if cfg!(debug_assertions) && result {
220        // Sanity-check: must have same ABI, size and alignment.
221        // `extern` blocks cannot be generic, so we'll always get a layout here.
222        let a_layout = tcx.layout_of(typing_env.as_query_input(a)).unwrap();
223        let b_layout = tcx.layout_of(typing_env.as_query_input(b)).unwrap();
224        assert_eq!(a_layout.backend_repr, b_layout.backend_repr);
225        assert_eq!(a_layout.size, b_layout.size);
226        assert_eq!(a_layout.align, b_layout.align);
227    }
228    result
229}
230
231fn structurally_same_type_impl<'tcx>(
232    seen_types: &mut UnordSet<(Ty<'tcx>, Ty<'tcx>)>,
233    tcx: TyCtxt<'tcx>,
234    typing_env: ty::TypingEnv<'tcx>,
235    a: Ty<'tcx>,
236    b: Ty<'tcx>,
237) -> bool {
238    debug!("structurally_same_type_impl(tcx, a = {:?}, b = {:?})", a, b);
239
240    // Given a transparent newtype, reach through and grab the inner
241    // type unless the newtype makes the type non-null.
242    let non_transparent_ty = |mut ty: Ty<'tcx>| -> Ty<'tcx> {
243        loop {
244            if let ty::Adt(def, args) = *ty.kind() {
245                let is_transparent = def.repr().transparent();
246                let is_non_null = types::nonnull_optimization_guaranteed(tcx, def);
247                debug!(?ty, is_transparent, is_non_null);
248                if is_transparent && !is_non_null {
249                    debug_assert_eq!(def.variants().len(), 1);
250                    let v = &def.variant(FIRST_VARIANT);
251                    // continue with `ty`'s non-ZST field,
252                    // otherwise `ty` is a ZST and we can return
253                    if let Some(field) = types::transparent_newtype_field(tcx, v) {
254                        ty = field.ty(tcx, args);
255                        continue;
256                    }
257                }
258            }
259            debug!("non_transparent_ty -> {:?}", ty);
260            return ty;
261        }
262    };
263
264    let a = non_transparent_ty(a);
265    let b = non_transparent_ty(b);
266
267    if !seen_types.insert((a, b)) {
268        // We've encountered a cycle. There's no point going any further -- the types are
269        // structurally the same.
270        true
271    } else if a == b {
272        // All nominally-same types are structurally same, too.
273        true
274    } else {
275        // Do a full, depth-first comparison between the two.
276        let is_primitive_or_pointer =
277            |ty: Ty<'tcx>| ty.is_primitive() || matches!(ty.kind(), ty::RawPtr(..) | ty::Ref(..));
278
279        ensure_sufficient_stack(|| {
280            match (a.kind(), b.kind()) {
281                (&ty::Adt(a_def, a_gen_args), &ty::Adt(b_def, b_gen_args)) => {
282                    // Only `repr(C)` types can be compared structurally.
283                    if !(a_def.repr().c() && b_def.repr().c()) {
284                        return false;
285                    }
286                    // If the types differ in their packed-ness, align, or simd-ness they conflict.
287                    let repr_characteristica =
288                        |def: AdtDef<'tcx>| (def.repr().pack, def.repr().align, def.repr().simd());
289                    if repr_characteristica(a_def) != repr_characteristica(b_def) {
290                        return false;
291                    }
292
293                    // Grab a flattened representation of all fields.
294                    let a_fields = a_def.variants().iter().flat_map(|v| v.fields.iter());
295                    let b_fields = b_def.variants().iter().flat_map(|v| v.fields.iter());
296
297                    // Perform a structural comparison for each field.
298                    a_fields.eq_by(
299                        b_fields,
300                        |&ty::FieldDef { did: a_did, .. }, &ty::FieldDef { did: b_did, .. }| {
301                            structurally_same_type_impl(
302                                seen_types,
303                                tcx,
304                                typing_env,
305                                tcx.type_of(a_did).instantiate(tcx, a_gen_args),
306                                tcx.type_of(b_did).instantiate(tcx, b_gen_args),
307                            )
308                        },
309                    )
310                }
311                (ty::Array(a_ty, a_len), ty::Array(b_ty, b_len)) => {
312                    // For arrays, we also check the length.
313                    a_len == b_len
314                        && structurally_same_type_impl(seen_types, tcx, typing_env, *a_ty, *b_ty)
315                }
316                (ty::Slice(a_ty), ty::Slice(b_ty)) => {
317                    structurally_same_type_impl(seen_types, tcx, typing_env, *a_ty, *b_ty)
318                }
319                (ty::RawPtr(a_ty, a_mutbl), ty::RawPtr(b_ty, b_mutbl)) => {
320                    a_mutbl == b_mutbl
321                        && structurally_same_type_impl(seen_types, tcx, typing_env, *a_ty, *b_ty)
322                }
323                (ty::Ref(_a_region, a_ty, a_mut), ty::Ref(_b_region, b_ty, b_mut)) => {
324                    // For structural sameness, we don't need the region to be same.
325                    a_mut == b_mut
326                        && structurally_same_type_impl(seen_types, tcx, typing_env, *a_ty, *b_ty)
327                }
328                (ty::FnDef(..), ty::FnDef(..)) => {
329                    let a_poly_sig = a.fn_sig(tcx);
330                    let b_poly_sig = b.fn_sig(tcx);
331
332                    // We don't compare regions, but leaving bound regions around ICEs, so
333                    // we erase them.
334                    let a_sig = tcx.instantiate_bound_regions_with_erased(a_poly_sig);
335                    let b_sig = tcx.instantiate_bound_regions_with_erased(b_poly_sig);
336
337                    (a_sig.abi, a_sig.safety, a_sig.c_variadic)
338                        == (b_sig.abi, b_sig.safety, b_sig.c_variadic)
339                        && a_sig.inputs().iter().eq_by(b_sig.inputs().iter(), |a, b| {
340                            structurally_same_type_impl(seen_types, tcx, typing_env, *a, *b)
341                        })
342                        && structurally_same_type_impl(
343                            seen_types,
344                            tcx,
345                            typing_env,
346                            a_sig.output(),
347                            b_sig.output(),
348                        )
349                }
350                (ty::Tuple(..), ty::Tuple(..)) => {
351                    // Tuples are not `repr(C)` so these cannot be compared structurally.
352                    false
353                }
354                // For these, it's not quite as easy to define structural-sameness quite so easily.
355                // For the purposes of this lint, take the conservative approach and mark them as
356                // not structurally same.
357                (ty::Dynamic(..), ty::Dynamic(..))
358                | (ty::Error(..), ty::Error(..))
359                | (ty::Closure(..), ty::Closure(..))
360                | (ty::Coroutine(..), ty::Coroutine(..))
361                | (ty::CoroutineWitness(..), ty::CoroutineWitness(..))
362                | (ty::Alias(ty::Projection, ..), ty::Alias(ty::Projection, ..))
363                | (ty::Alias(ty::Inherent, ..), ty::Alias(ty::Inherent, ..))
364                | (ty::Alias(ty::Opaque, ..), ty::Alias(ty::Opaque, ..)) => false,
365
366                // These definitely should have been caught above.
367                (ty::Bool, ty::Bool)
368                | (ty::Char, ty::Char)
369                | (ty::Never, ty::Never)
370                | (ty::Str, ty::Str) => unreachable!(),
371
372                // An Adt and a primitive or pointer type. This can be FFI-safe if non-null
373                // enum layout optimisation is being applied.
374                (ty::Adt(..) | ty::Pat(..), _) if is_primitive_or_pointer(b) => {
375                    if let Some(a_inner) = types::repr_nullable_ptr(tcx, typing_env, a) {
376                        a_inner == b
377                    } else {
378                        false
379                    }
380                }
381                (_, ty::Adt(..) | ty::Pat(..)) if is_primitive_or_pointer(a) => {
382                    if let Some(b_inner) = types::repr_nullable_ptr(tcx, typing_env, b) {
383                        b_inner == a
384                    } else {
385                        false
386                    }
387                }
388
389                _ => false,
390            }
391        })
392    }
393}