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::{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
37pub(crate) struct ImpossiblePredicates;
38
39fn has_impossible_predicates(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
40    let predicates = tcx.predicates_of(def_id).instantiate_identity(tcx);
41    tracing::trace!(?predicates);
42    let predicates =
43        predicates.predicates.into_iter().map(Unnormalized::skip_norm_wip).filter(|p| {
44            !p.has_type_flags(
45                // Only consider global clauses to simplify.
46                TypeFlags::HAS_FREE_LOCAL_NAMES
47                // Clauses that refer to unevaluated constants as they cause cycles.
48                | TypeFlags::HAS_CT_PROJECTION,
49            )
50        });
51    let predicates: Vec<_> = traits::elaborate(tcx, predicates).collect();
52    tracing::trace!(?predicates);
53    predicates.references_error() || traits::impossible_predicates(tcx, predicates)
54}
55
56impl<'tcx> MirPass<'tcx> for ImpossiblePredicates {
57    #[tracing::instrument(level = "trace", skip(self, tcx, body))]
58    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
59        tracing::trace!(def_id = ?body.source.def_id());
60        let impossible = body.tainted_by_errors.is_some()
61            || has_impossible_predicates(tcx, body.source.def_id());
62        if impossible {
63            trace!("found unsatisfiable predicates");
64            // Clear the body to only contain a single `unreachable` statement.
65            let bbs = body.basic_blocks.as_mut();
66            bbs.raw.truncate(1);
67            bbs[START_BLOCK].statements.clear();
68            bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
69            body.var_debug_info.clear();
70            body.local_decls.raw.truncate(body.arg_count + 1);
71        }
72    }
73
74    fn is_required(&self) -> bool {
75        true
76    }
77}
rustc_mir_transform/impossible_predicates.rs

rustc_mir_transform/
impossible_predicates.rs
