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introduce new fallback algorithm

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We now fallback type variables using the following rules:

* Construct a coercion graph `A -> B` where `A` and `B` are unresolved
  type variables or the `!` type.
* Let D be those variables that are reachable from `!`.
* Let N be those variables that are reachable from a variable not in
D.
* All variables in (D \ N) fallback to `!`.
* All variables in (D & N) fallback to `()`.
This commit is contained in:
Niko Matsakis 2020-11-22 09:58:58 -05:00 • committed by Mark Rousskov
parent e0c38af27c
commit 2ee89144e2
7 changed files with 347 additions and 55 deletions
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279
compiler/rustc_typeck/src/check/fallback.rs
View file

@ -1,4 +1,7 @@
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};
@ -8,22 +11,30 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
pub(super) fn type_inference_fallback(&self) -> bool {
// All type checking constraints were added, try to fallback unsolved variables.
self.select_obligations_where_possible(false, |_| {});
let mut fallback_has_occurred = false;
// Check if we have any unsolved varibales. If not, no need for fallback.
let unsolved_variables = self.unsolved_variables();
if unsolved_variables.is_empty() {
return false;
}
let diverging_fallback = self.calculate_diverging_fallback(&unsolved_variables);
let mut fallback_has_occurred = false;
// We do fallback in two passes, to try to generate
// better error messages.
// The first time, we do *not* replace opaque types.
for ty in &self.unsolved_variables() {
for ty in unsolved_variables {
debug!("unsolved_variable = {:?}", ty);
fallback_has_occurred |= self.fallback_if_possible(ty);
fallback_has_occurred |= self.fallback_if_possible(ty, &diverging_fallback);
}
// We now see if we can make progress. This might
// cause us to unify inference variables for opaque types,
// since we may have unified some other type variables
// during the first phase of fallback.
// This means that we only replace inference variables with their underlying
// opaque types as a last resort.
// We now see if we can make progress. This might cause us to
// unify inference variables for opaque types, since we may
// have unified some other type variables during the first
// phase of fallback. This means that we only replace
// inference variables with their underlying opaque types as a
// last resort.
//
// In code like this:
//
@ -62,36 +73,44 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
//
// - Unconstrained floats are replaced with with `f64`.
//
// - Non-numerics get replaced with `!` when `#![feature(never_type_fallback)]`
// is enabled. Otherwise, they are replaced with `()`.
// - Non-numerics may get replaced with `()` or `!`, depending on
// how they were categorized by `calculate_diverging_fallback`
// (and the setting of `#![feature(never_type_fallback)]`).
//
// Fallback becomes very dubious if we have encountered
// type-checking errors. In that case, fallback to Error.
//
// Fallback becomes very dubious if we have encountered type-checking errors.
// In that case, fallback to Error.
// The return value indicates whether fallback has occurred.
fn fallback_if_possible(&self, ty: Ty<'tcx>) -> bool {
fn fallback_if_possible(
&self,
ty: Ty<'tcx>,
diverging_fallback: &FxHashMap<Ty<'tcx>, Ty<'tcx>>,
) -> bool {
// Careful: we do NOT shallow-resolve `ty`. We know that `ty`
// is an unsolved variable, and we determine its fallback based
// solely on how it was created, not what other type variables
// it may have been unified with since then.
// is an unsolved variable, and we determine its fallback
// based solely on how it was created, not what other type
// variables it may have been unified with since then.
//
// The reason this matters is that other attempts at fallback may
// (in principle) conflict with this fallback, and we wish to generate
// a type error in that case. (However, this actually isn't true right now,
// because we're only using the builtin fallback rules. This would be
// true if we were using user-supplied fallbacks. But it's still useful
// to write the code to detect bugs.)
// The reason this matters is that other attempts at fallback
// may (in principle) conflict with this fallback, and we wish
// to generate a type error in that case. (However, this
// actually isn't true right now, because we're only using the
// builtin fallback rules. This would be true if we were using
// user-supplied fallbacks. But it's still useful to write the
// code to detect bugs.)
//
// (Note though that if we have a general type variable `?T` that is then unified
// with an integer type variable `?I` that ultimately never gets
// resolved to a special integral type, `?T` is not considered unsolved,
// but `?I` is. The same is true for float variables.)
// (Note though that if we have a general type variable `?T`
// that is then unified with an integer type variable `?I`
// that ultimately never gets resolved to a special integral
// type, `?T` is not considered unsolved, but `?I` is. The
// same is true for float variables.)
let fallback = match ty.kind() {
_ if self.is_tainted_by_errors() => self.tcx.ty_error(),
ty::Infer(ty::IntVar(_)) => self.tcx.types.i32,
ty::Infer(ty::FloatVar(_)) => self.tcx.types.f64,
_ => match self.type_var_diverges(ty) {
Diverging::Diverges => self.tcx.mk_diverging_default(),
Diverging::NotDiverging => return false,
_ => match diverging_fallback.get(&ty) {
Some(&fallback_ty) => fallback_ty,
None => return false,
},
};
debug!("fallback_if_possible(ty={:?}): defaulting to `{:?}`", ty, fallback);
@ -105,11 +124,10 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
true
}
/// Second round of fallback: Unconstrained type variables
/// created from the instantiation of an opaque
/// type fall back to the opaque type itself. This is a
/// somewhat incomplete attempt to manage "identity passthrough"
/// for `impl Trait` types.
/// Second round of fallback: Unconstrained type variables created
/// from the instantiation of an opaque type fall back to the
/// opaque type itself. This is a somewhat incomplete attempt to
/// manage "identity passthrough" for `impl Trait` types.
///
/// For example, in this code:
///
@ -158,4 +176,195 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
return false;
}
}
/// The "diverging fallback" system is rather complicated. This is
/// a result of our need to balance 'do the right thing' with
/// backwards compatibility.
///
/// "Diverging" type variables are variables created when we
/// coerce a `!` type into an unbound type variable `?X`. If they
/// never wind up being constrained, the "right and natural" thing
/// is that `?X` should "fallback" to `!`. This means that e.g. an
/// expression like `Some(return)` will ultimately wind up with a
/// type like `Option<!>` (presuming it is not assigned or
/// constrained to have some other type).
///
/// However, the fallback used to be `()` (before the `!` type was
/// added). Moreover, there are cases where the `!` type 'leaks
/// out' from dead code into type variables that affect live
/// code. The most common case is something like this:
///
/// ```rust
/// match foo() {
/// 22 => Default::default(), // call this type `?D`
/// _ => return, // return has type `!`
/// } // call the type of this match `?M`
/// ```
///
/// Here, coercing the type `!` into `?M` will create a diverging
/// type variable `?X` where `?X <: ?M`. We also have that `?D <:
/// ?M`. If `?M` winds up unconstrained, then `?X` will
/// fallback. If it falls back to `!`, then all the type variables
/// will wind up equal to `!` -- this includes the type `?D`
/// (since `!` doesn't implement `Default`, we wind up a "trait
/// not implemented" error in code like this). But since the
/// original fallback was `()`, this code used to compile with `?D
/// = ()`. This is somewhat surprising, since `Default::default()`
/// on its own would give an error because the types are
/// insufficiently constrained.
///
/// Our solution to this dilemma is to modify diverging variables
/// so that they can *either* fallback to `!` (the default) or to
/// `()` (the backwards compatibility case). We decide which
/// fallback to use based on whether there is a coercion pattern
/// like this:
///
/// ```
/// ?Diverging -> ?V
/// ?NonDiverging -> ?V
/// ?V != ?NonDiverging
/// ```
///
/// Here `?Diverging` represents some diverging type variable and
/// `?NonDiverging` represents some non-diverging type
/// variable. `?V` can be any type variable (diverging or not), so
/// long as it is not equal to `?NonDiverging`.
///
/// Intuitively, what we are looking for is a case where a
/// "non-diverging" type variable (like `?M` in our example above)
/// is coerced *into* some variable `?V` that would otherwise
/// fallback to `!`. In that case, we make `?V` fallback to `!`,
/// along with anything that would flow into `?V`.
///
/// The algorithm we use:
/// * Identify all variables that are coerced *into* by a
/// diverging variable. Do this by iterating over each
/// diverging, unsolved variable and finding all variables
/// reachable from there. Call that set `D`.
/// * Walk over all unsolved, non-diverging variables, and find
/// any variable that has an edge into `D`.
fn calculate_diverging_fallback(
&self,
unsolved_variables: &[Ty<'tcx>],
) -> FxHashMap<Ty<'tcx>, Ty<'tcx>> {
debug!("calculate_diverging_fallback({:?})", unsolved_variables);
// Construct a coercion graph where an edge `A -> B` indicates
// a type variable is that is coerced
let coercion_graph = self.create_coercion_graph();
// 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();
// Find all type variables that are reachable from a diverging
// type variable. These will typically default to `!`, unless
// we find later that they are *also* reachable from some
// other type variable outside this set.
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 {
debug!(
"calculate_diverging_fallback: unsolved_vid={:?} diverges={:?}",
unsolved_vid,
self.infcx.ty_vid_diverges(unsolved_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);
}
}
}
debug!(
"calculate_diverging_fallback: roots_reachable_from_diverging={:?}",
roots_reachable_from_diverging,
);
// Find all type variables N0 that are not reachable from a
// diverging variable, and then compute the set reachable from
// N0, which we call N. These are the *non-diverging* type
// variables. (Note that this set consists of "root variables".)
let mut roots_reachable_from_non_diverging = FxHashSet::default();
for &non_diverging_vid in &non_diverging_vids {
let root_vid = self.infcx.root_var(non_diverging_vid);
if roots_reachable_from_diverging.contains(&root_vid) {
continue;
}
roots_reachable_from_non_diverging.extend(coercion_graph.depth_first_search(root_vid));
}
debug!(
"calculate_diverging_fallback: roots_reachable_from_non_diverging={:?}",
roots_reachable_from_non_diverging,
);
// For each diverging variable, figure out whether it can
// reach a member of N. If so, it falls back to `()`. Else
// `!`.
let mut diverging_fallback = FxHashMap::default();
for &diverging_vid in &diverging_vids {
let diverging_ty = self.tcx.mk_ty_var(diverging_vid);
let root_vid = self.infcx.root_var(diverging_vid);
let can_reach_non_diverging = coercion_graph
.depth_first_search(root_vid)
.any(|n| roots_reachable_from_non_diverging.contains(&n));
if can_reach_non_diverging {
debug!("fallback to (): {:?}", diverging_vid);
diverging_fallback.insert(diverging_ty, self.tcx.types.unit);
} else {
debug!("fallback to !: {:?}", diverging_vid);
diverging_fallback.insert(diverging_ty, self.tcx.mk_diverging_default());
}
}
diverging_fallback
}
/// Returns a graph whose nodes are (unresolved) inference variables and where
/// an edge `?A -> ?B` indicates that the variable `?A` is coerced to `?B`.
fn create_coercion_graph(&self) -> VecGraph<ty::TyVid> {
let pending_obligations = self.fulfillment_cx.borrow_mut().pending_obligations();
debug!("create_coercion_graph: pending_obligations={:?}", pending_obligations);
let coercion_edges: Vec<(ty::TyVid, ty::TyVid)> = pending_obligations
.into_iter()
.filter_map(|obligation| {
// The predicates we are looking for look like `Coerce(?A -> ?B)`.
// They will have no bound variables.
obligation.predicate.kind().no_bound_vars()
})
.filter_map(|atom| {
if let ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) = atom {
let a_vid = self.root_vid(a)?;
let b_vid = self.root_vid(b)?;
Some((a_vid, b_vid))
} else {
return None;
};
let a_vid = self.root_vid(a)?;
let b_vid = self.root_vid(b)?;
Some((a_vid, b_vid))
})
.collect();
debug!("create_coercion_graph: coercion_edges={:?}", coercion_edges);
let num_ty_vars = self.infcx.num_ty_vars();
VecGraph::new(num_ty_vars, coercion_edges)
}
/// If `ty` is an unresolved type variable, returns its root vid.
fn root_vid(&self, ty: Ty<'tcx>) -> Option<ty::TyVid> {
Some(self.infcx.root_var(self.infcx.shallow_resolve(ty).ty_vid()?))
}
}