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};
31use rustc_trait_selection::traits;
32use tracing::trace;
33
34use crate::pass_manager::MirPass;
35
36pub(crate) struct ImpossiblePredicates;
37
38impl<'tcx> MirPass<'tcx> for ImpossiblePredicates {
39    #[tracing::instrument(level = "trace", skip(self, tcx, body))]
40    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
41        tracing::trace!(def_id = ?body.source.def_id());
42        let predicates = tcx.predicates_of(body.source.def_id()).instantiate_identity(tcx);
43        tracing::trace!(?predicates);
44        let predicates = predicates.predicates.into_iter().filter(|p| {
45            !p.has_type_flags(
46                // Only consider global clauses to simplify.
47                TypeFlags::HAS_FREE_LOCAL_NAMES
48                // Clauses that refer to unevaluated constants as they cause cycles.
49                | TypeFlags::HAS_CT_PROJECTION,
50            )
51        });
52        let predicates: Vec<_> = traits::elaborate(tcx, predicates).collect();
53        tracing::trace!(?predicates);
54        if predicates.references_error() || traits::impossible_predicates(tcx, predicates) {
55            trace!("found unsatisfiable predicates");
56            // Clear the body to only contain a single `unreachable` statement.
57            let bbs = body.basic_blocks.as_mut();
58            bbs.raw.truncate(1);
59            bbs[START_BLOCK].statements.clear();
60            bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
61            body.var_debug_info.clear();
62            body.local_decls.raw.truncate(body.arg_count + 1);
63        }
64    }
65
66    fn is_required(&self) -> bool {
67        true
68    }
69}
rustc_mir_transform/impossible_predicates.rs

rustc_mir_transform/
impossible_predicates.rs
