rustc_typeck to rustc_hir_analysis

1 files changed, 228 insertions, 0 deletions

diff --git a/compiler/rustc_hir_analysis/src/impl_wf_check.rs b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
new file mode 100644
index 00000000000..c499364056f
--- /dev/null
+++ b/compiler/rustc_hir_analysis/src/impl_wf_check.rs
@@ -0,0 +1,228 @@
+//! This pass enforces various "well-formedness constraints" on impls.
+//! Logically, it is part of wfcheck -- but we do it early so that we
+//! can stop compilation afterwards, since part of the trait matching
+//! infrastructure gets very grumpy if these conditions don't hold. In
+//! particular, if there are type parameters that are not part of the
+//! impl, then coherence will report strange inference ambiguity
+//! errors; if impls have duplicate items, we get misleading
+//! specialization errors. These things can (and probably should) be
+//! fixed, but for the moment it's easier to do these checks early.
+
+use crate::constrained_generic_params as cgp;
+use min_specialization::check_min_specialization;
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_errors::struct_span_err;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
+use rustc_span::{Span, Symbol};
+
+use std::collections::hash_map::Entry::{Occupied, Vacant};
+
+mod min_specialization;
+
+/// Checks that all the type/lifetime parameters on an impl also
+/// appear in the trait ref or self type (or are constrained by a
+/// where-clause). These rules are needed to ensure that, given a
+/// trait ref like `<T as Trait<U>>`, we can derive the values of all
+/// parameters on the impl (which is needed to make specialization
+/// possible).
+///
+/// However, in the case of lifetimes, we only enforce these rules if
+/// the lifetime parameter is used in an associated type. This is a
+/// concession to backwards compatibility; see comment at the end of
+/// the fn for details.
+///
+/// Example:
+///
+/// ```rust,ignore (pseudo-Rust)
+/// impl<T> Trait<Foo> for Bar { ... }
+/// //   ^ T does not appear in `Foo` or `Bar`, error!
+///
+/// impl<T> Trait<Foo<T>> for Bar { ... }
+/// //   ^ T appears in `Foo<T>`, ok.
+///
+/// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
+/// //   ^ T is bound to `<Bar as Iterator>::Item`, ok.
+///
+/// impl<'a> Trait<Foo> for Bar { }
+/// //   ^ 'a is unused, but for back-compat we allow it
+///
+/// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
+/// //   ^ 'a is unused and appears in assoc type, error
+/// ```
+fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
+    let min_specialization = tcx.features().min_specialization;
+    let module = tcx.hir_module_items(module_def_id);
+    for id in module.items() {
+        if matches!(tcx.def_kind(id.def_id), DefKind::Impl) {
+            enforce_impl_params_are_constrained(tcx, id.def_id.def_id);
+            enforce_impl_items_are_distinct(tcx, id.def_id.def_id);
+            if min_specialization {
+                check_min_specialization(tcx, id.def_id.def_id);
+            }
+        }
+    }
+}
+
+pub fn provide(providers: &mut Providers) {
+    *providers = Providers { check_mod_impl_wf, ..*providers };
+}
+
+fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
+    // Every lifetime used in an associated type must be constrained.
+    let impl_self_ty = tcx.type_of(impl_def_id);
+    if impl_self_ty.references_error() {
+        // Don't complain about unconstrained type params when self ty isn't known due to errors.
+        // (#36836)
+        tcx.sess.delay_span_bug(
+            tcx.def_span(impl_def_id),
+            &format!(
+                "potentially unconstrained type parameters weren't evaluated: {:?}",
+                impl_self_ty,
+            ),
+        );
+        return;
+    }
+    let impl_generics = tcx.generics_of(impl_def_id);
+    let impl_predicates = tcx.predicates_of(impl_def_id);
+    let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
+
+    let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
+    cgp::identify_constrained_generic_params(
+        tcx,
+        impl_predicates,
+        impl_trait_ref,
+        &mut input_parameters,
+    );
+
+    // Disallow unconstrained lifetimes, but only if they appear in assoc types.
+    let lifetimes_in_associated_types: FxHashSet<_> = tcx
+        .associated_item_def_ids(impl_def_id)
+        .iter()
+        .flat_map(|def_id| {
+            let item = tcx.associated_item(def_id);
+            match item.kind {
+                ty::AssocKind::Type => {
+                    if item.defaultness(tcx).has_value() {
+                        cgp::parameters_for(&tcx.type_of(def_id), true)
+                    } else {
+                        Vec::new()
+                    }
+                }
+                ty::AssocKind::Fn | ty::AssocKind::Const => Vec::new(),
+            }
+        })
+        .collect();
+
+    for param in &impl_generics.params {
+        match param.kind {
+            // Disallow ANY unconstrained type parameters.
+            ty::GenericParamDefKind::Type { .. } => {
+                let param_ty = ty::ParamTy::for_def(param);
+                if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
+                    report_unused_parameter(tcx, tcx.def_span(param.def_id), "type", param_ty.name);
+                }
+            }
+            ty::GenericParamDefKind::Lifetime => {
+                let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
+                if lifetimes_in_associated_types.contains(&param_lt) && // (*)
+                    !input_parameters.contains(&param_lt)
+                {
+                    report_unused_parameter(
+                        tcx,
+                        tcx.def_span(param.def_id),
+                        "lifetime",
+                        param.name,
+                    );
+                }
+            }
+            ty::GenericParamDefKind::Const { .. } => {
+                let param_ct = ty::ParamConst::for_def(param);
+                if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
+                    report_unused_parameter(
+                        tcx,
+                        tcx.def_span(param.def_id),
+                        "const",
+                        param_ct.name,
+                    );
+                }
+            }
+        }
+    }
+
+    // (*) This is a horrible concession to reality. I think it'd be
+    // better to just ban unconstrained lifetimes outright, but in
+    // practice people do non-hygienic macros like:
+    //
+    // ```
+    // macro_rules! __impl_slice_eq1 {
+    //     ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
+    //         impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
+    //            ....
+    //         }
+    //     }
+    // }
+    // ```
+    //
+    // In a concession to backwards compatibility, we continue to
+    // permit those, so long as the lifetimes aren't used in
+    // associated types. I believe this is sound, because lifetimes
+    // used elsewhere are not projected back out.
+}
+
+fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {
+    let mut err = struct_span_err!(
+        tcx.sess,
+        span,
+        E0207,
+        "the {} parameter `{}` is not constrained by the \
+        impl trait, self type, or predicates",
+        kind,
+        name
+    );
+    err.span_label(span, format!("unconstrained {} parameter", kind));
+    if kind == "const" {
+        err.note(
+            "expressions using a const parameter must map each value to a distinct output value",
+        );
+        err.note(
+            "proving the result of expressions other than the parameter are unique is not supported",
+        );
+    }
+    err.emit();
+}
+
+/// Enforce that we do not have two items in an impl with the same name.
+fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
+    let mut seen_type_items = FxHashMap::default();
+    let mut seen_value_items = FxHashMap::default();
+    for &impl_item_ref in tcx.associated_item_def_ids(impl_def_id) {
+        let impl_item = tcx.associated_item(impl_item_ref);
+        let seen_items = match impl_item.kind {
+            ty::AssocKind::Type => &mut seen_type_items,
+            _ => &mut seen_value_items,
+        };
+        let span = tcx.def_span(impl_item_ref);
+        let ident = impl_item.ident(tcx);
+        match seen_items.entry(ident.normalize_to_macros_2_0()) {
+            Occupied(entry) => {
+                let mut err = struct_span_err!(
+                    tcx.sess,
+                    span,
+                    E0201,
+                    "duplicate definitions with name `{}`:",
+                    ident
+                );
+                err.span_label(*entry.get(), format!("previous definition of `{}` here", ident));
+                err.span_label(span, "duplicate definition");
+                err.emit();
+            }
+            Vacant(entry) => {
+                entry.insert(span);
+            }
+        }
+    }
+}