rustc_mir_transform/
impossible_predicates.rs

1//! Check if it's even possible to satisfy the 'where' clauses
2//! for this item.
3//!
4//! It's possible to `#!feature(trivial_bounds)]` to write
5//! a function with impossible to satisfy clauses, e.g.:
6//! `fn foo() where String: Copy {}`.
7//!
8//! We don't usually need to worry about this kind of case,
9//! since we would get a compilation error if the user tried
10//! to call it. However, since we optimize even without any
11//! calls to the function, we need to make sure that it even
12//! makes sense to try to evaluate the body.
13//!
14//! If there are unsatisfiable where clauses, then all bets are
15//! off, and we just give up.
16//!
17//! We manually filter the predicates, skipping anything that's not
18//! "global". We are in a potentially generic context
19//! (e.g. we are evaluating a function without instantiating generic
20//! parameters, so this filtering serves two purposes:
21//!
22//! 1. We skip evaluating any predicates that we would
23//!    never be able prove are unsatisfiable (e.g. `<T as Foo>`
24//! 2. We avoid trying to normalize predicates involving generic
25//!    parameters (e.g. `<T as Foo>::MyItem`). This can confuse
26//!    the normalization code (leading to cycle errors), since
27//!    it's usually never invoked in this way.
28
29use rustc_middle::mir::{Body, START_BLOCK, TerminatorKind};
30use rustc_middle::ty::{self, Ty, TyCtxt, TypeFlags, TypeVisitableExt, Unnormalized};
31use rustc_span::def_id::DefId;
32use rustc_trait_selection::traits;
33use tracing::trace;
34
35use crate::pass_manager::MirPass;
36
37fn is_structurally_unsized<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
38    match ty.kind() {
39        ty::Str | ty::Slice(_) | ty::Dynamic(_, _) | ty::Foreign(_) => true,
40        ty::Tuple(tys) => tys.last().is_some_and(|ty| is_structurally_unsized(tcx, *ty)),
41        ty::Adt(def, args) => {
42            def.sizedness_constraint(tcx, ty::SizedTraitKind::Sized).is_some_and(|ty| {
43                is_structurally_unsized(tcx, ty.instantiate(tcx, args).skip_norm_wip())
44            })
45        }
46        _ => false,
47    }
48}
49
50fn has_structurally_impossible_sized_predicate<'tcx>(
51    tcx: TyCtxt<'tcx>,
52    sized_trait: DefId,
53    predicate: ty::Clause<'tcx>,
54) -> bool {
55    let Some(trait_predicate) = predicate.as_trait_clause() else {
56        return false;
57    };
58    let trait_predicate = trait_predicate.skip_binder();
59
60    trait_predicate.polarity == ty::PredicatePolarity::Positive
61        && trait_predicate.def_id() == sized_trait
62        && is_structurally_unsized(tcx, trait_predicate.self_ty())
63}
64
65pub(crate) struct ImpossiblePredicates;
66
67pub(crate) fn has_impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
68    let predicates = tcx.predicates_of(def_id).instantiate_identity(tcx);
69    tracing::trace!(?predicates);
70
71    // Some `Sized` predicates that mention local generics are still impossible
72    // for every instantiation, e.g. `dyn Trait<T>: Sized`.
73    if let Some(sized_trait) = tcx.lang_items().sized_trait() {
74        if predicates.predicates.iter().copied().map(Unnormalized::skip_norm_wip).any(|predicate| {
75            has_structurally_impossible_sized_predicate(tcx, sized_trait, predicate)
76        }) {
77            return true;
78        }
79    }
80
81    let predicates =
82        predicates.predicates.into_iter().map(Unnormalized::skip_norm_wip).filter(|p| {
83            !p.has_type_flags(
84                // Only consider global clauses to simplify.
85                TypeFlags::HAS_FREE_LOCAL_NAMES
86                // Clauses that refer to alias constants as they cause cycles.
87                | TypeFlags::HAS_CONST_ALIAS,
88            )
89        });
90    let predicates: Vec<_> = traits::elaborate(tcx, predicates).collect();
91    tracing::trace!(?predicates);
92    predicates.references_error() || traits::impossible_predicates(tcx, predicates)
93}
94
95impl<'tcx> MirPass<'tcx> for ImpossiblePredicates {
96    #[tracing::instrument(level = "trace", skip(self, tcx, body))]
97    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
98        tracing::trace!(def_id = ?body.source.def_id());
99        let impossible = body.tainted_by_errors.is_some()
100            || has_impossible_predicates(tcx, body.source.def_id());
101        if impossible {
102            trace!("found unsatisfiable predicates");
103            // Clear the body to only contain a single `unreachable` statement.
104            let bbs = body.basic_blocks.as_mut();
105            bbs.raw.truncate(1);
106            bbs[START_BLOCK].statements.clear();
107            bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
108            body.var_debug_info.clear();
109            body.local_decls.raw.truncate(body.arg_count + 1);
110        }
111    }
112
113    fn is_required(&self) -> bool {
114        true
115    }
116}
rustc_mir_transform/impossible_predicates.rs

rustc_mir_transform/
impossible_predicates.rs
