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remove reliance on "diverging" type variables

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Instead, we now record those type variables that are the target of a
`NeverToAny` adjustment and consider those to be the "diverging" type
variables. This allows us to remove the special case logic that
creates a type variable for `!` in coercion.
This commit is contained in:
Niko Matsakis 2020-11-23 07:43:33 -05:00 committed by Mark Rousskov
parent ebb8ff9edf
commit 1c004f2d43
4 changed files with 53 additions and 39 deletions
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19
compiler/rustc_typeck/src/check/coercion.rs
View file

@ -159,24 +159,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
// Coercing from `!` to any type is allowed:
if a.is_never() {
// Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
// type variable, we want `?T` to fallback to `!` if not
// otherwise constrained. An example where this arises:
//
// let _: Option<?T> = Some({ return; });
//
// here, we would coerce from `!` to `?T`.
return if b.is_ty_var() {
// Micro-optimization: no need for this if `b` is
// already resolved in some way.
let diverging_ty = self.next_diverging_ty_var(TypeVariableOrigin {
kind: TypeVariableOriginKind::AdjustmentType,
span: self.cause.span,
});
self.coerce_from_inference_variable(diverging_ty, b, simple(Adjust::NeverToAny))
} else {
success(simple(Adjust::NeverToAny)(b), b, vec![])
};
return success(simple(Adjust::NeverToAny)(b), b, vec![]);
}
// Coercing *from* an unresolved inference variable means that

57
compiler/rustc_typeck/src/check/fallback.rs
View file

@ -2,7 +2,6 @@ use crate::check::FnCtxt;
use rustc_data_structures::{
fx::FxHashMap, graph::vec_graph::VecGraph, graph::WithSuccessors, stable_set::FxHashSet,
};
use rustc_infer::infer::type_variable::Diverging;
use rustc_middle::ty::{self, Ty};
impl<'tcx> FnCtxt<'_, 'tcx> {
@ -255,8 +254,27 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
// Extract the unsolved type inference variable vids; note that some
// unsolved variables are integer/float variables and are excluded.
let unsolved_vids: Vec<_> =
unsolved_variables.iter().filter_map(|ty| ty.ty_vid()).collect();
let unsolved_vids = unsolved_variables.iter().filter_map(|ty| ty.ty_vid());
// Compute the diverging root vids D -- that is, the root vid of
// those type variables that (a) are the target of a coercion from
// a `!` type and (b) have not yet been solved.
//
// These variables are the ones that are targets for fallback to
// either `!` or `()`.
let diverging_roots: FxHashSet<ty::TyVid> = self
.diverging_type_vars
.borrow()
.iter()
.map(|&ty| self.infcx.shallow_resolve(ty))
.filter_map(|ty| ty.ty_vid())
.map(|vid| self.infcx.root_var(vid))
.collect();
debug!(
"calculate_diverging_fallback: diverging_type_vars={:?}",
self.diverging_type_vars.borrow()
);
debug!("calculate_diverging_fallback: diverging_roots={:?}", diverging_roots);
// Find all type variables that are reachable from a diverging
// type variable. These will typically default to `!`, unless
@ -265,27 +283,24 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
let mut roots_reachable_from_diverging = FxHashSet::default();
let mut diverging_vids = vec![];
let mut non_diverging_vids = vec![];
for &unsolved_vid in &unsolved_vids {
for unsolved_vid in unsolved_vids {
let root_vid = self.infcx.root_var(unsolved_vid);
debug!(
"calculate_diverging_fallback: unsolved_vid={:?} diverges={:?}",
"calculate_diverging_fallback: unsolved_vid={:?} root_vid={:?} diverges={:?}",
unsolved_vid,
self.infcx.ty_vid_diverges(unsolved_vid)
root_vid,
diverging_roots.contains(&root_vid),
);
match self.infcx.ty_vid_diverges(unsolved_vid) {
Diverging::Diverges => {
diverging_vids.push(unsolved_vid);
let root_vid = self.infcx.root_var(unsolved_vid);
debug!(
"calculate_diverging_fallback: root_vid={:?} reaches {:?}",
root_vid,
coercion_graph.depth_first_search(root_vid).collect::<Vec<_>>()
);
roots_reachable_from_diverging
.extend(coercion_graph.depth_first_search(root_vid));
}
Diverging::NotDiverging => {
non_diverging_vids.push(unsolved_vid);
}
if diverging_roots.contains(&root_vid) {
diverging_vids.push(unsolved_vid);
debug!(
"calculate_diverging_fallback: root_vid={:?} reaches {:?}",
root_vid,
coercion_graph.depth_first_search(root_vid).collect::<Vec<_>>()
);
roots_reachable_from_diverging.extend(coercion_graph.depth_first_search(root_vid));
} else {
non_diverging_vids.push(unsolved_vid);
}
}
debug!(

9
compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
View file

@ -286,6 +286,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
return;
}
for a in &adj {
if let Adjust::NeverToAny = a.kind {
if a.target.is_ty_var() {
self.diverging_type_vars.borrow_mut().insert(a.target);
debug!("apply_adjustments: adding `{:?}` as diverging type var", a.target);
}
}
}
let autoborrow_mut = adj.iter().any(|adj| {
matches!(
adj,

7
compiler/rustc_typeck/src/check/inherited.rs
View file

@ -1,6 +1,7 @@
use super::callee::DeferredCallResolution;
use super::MaybeInProgressTables;
use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_hir::def_id::{DefIdMap, LocalDefId};
use rustc_hir::HirIdMap;
@ -56,6 +57,11 @@ pub struct Inherited<'a, 'tcx> {
pub(super) constness: hir::Constness,
pub(super) body_id: Option<hir::BodyId>,
/// Whenever we introduce an adjustment from `!` into a type variable,
/// we record that type variable here. This is later used to inform
/// fallback. See the `fallback` module for details.
pub(super) diverging_type_vars: RefCell<FxHashSet<Ty<'tcx>>>,
}
impl<'a, 'tcx> Deref for Inherited<'a, 'tcx> {
@ -121,6 +127,7 @@ impl Inherited<'a, 'tcx> {
deferred_call_resolutions: RefCell::new(Default::default()),
deferred_cast_checks: RefCell::new(Vec::new()),
deferred_generator_interiors: RefCell::new(Vec::new()),
diverging_type_vars: RefCell::new(Default::default()),
constness,
body_id,
}