Better diagnostics when mismatched types due to implict static lifetime

Author: jackh726

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1590,17 +1590,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
         };
         if let Some((expected, found)) = expected_found {
-            let expected_label = match exp_found {
-                Mismatch::Variable(ef) => ef.expected.prefix_string(self.tcx),
-                Mismatch::Fixed(s) => s.into(),
-            };
-            let found_label = match exp_found {
-                Mismatch::Variable(ef) => ef.found.prefix_string(self.tcx),
-                Mismatch::Fixed(s) => s.into(),
-            };
-            let exp_found = match exp_found {
-                Mismatch::Variable(exp_found) => Some(exp_found),
-                Mismatch::Fixed(_) => None,
+            let (expected_label, found_label, exp_found) = match exp_found {
+                Mismatch::Variable(ef) => (
+                    ef.expected.prefix_string(self.tcx),
+                    ef.found.prefix_string(self.tcx),
+                    Some(ef),
+                ),
+                Mismatch::Fixed(s) => (s.into(), s.into(), None),
             };
             match (&terr, expected == found) {
                 (TypeError::Sorts(values), extra) => {

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs

@@ -0,0 +1,84 @@
+//! Error Reporting for when the lifetime for a type doesn't match the `impl` selected for a predicate
+//! to hold.
+
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::error_reporting::note_and_explain_region;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::infer::{SubregionOrigin, TypeTrace};
+use crate::traits::ObligationCauseCode;
+use rustc_errors::{Applicability, ErrorReported};
+use rustc_hir as hir;
+use rustc_hir::intravisit::Visitor;
+use rustc_middle::ty::{self, TypeVisitor};
+use rustc_span::MultiSpan;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    pub(super) fn try_report_mismatched_static_lifetime(&self) -> Option<ErrorReported> {
+        let error = self.error.as_ref()?;
+        debug!("try_report_mismatched_static_lifetime {:?}", error);
+
+        let (origin, sub, sup) = match error.clone() {
+            RegionResolutionError::ConcreteFailure(origin, sub, sup) => (origin, sub, sup),
+            _ => return None,
+        };
+        if *sub != ty::RegionKind::ReStatic {
+            return None;
+        }
+        let cause = match origin {
+            SubregionOrigin::Subtype(box TypeTrace { ref cause, .. }) => cause,
+            _ => return None,
+        };
+        let (parent, impl_def_id) = match &cause.code {
+            ObligationCauseCode::MatchImpl(parent, impl_def_id) => (parent, impl_def_id),
+            _ => return None,
+        };
+        let binding_span = match **parent {
+            ObligationCauseCode::BindingObligation(_def_id, binding_span) => binding_span,
+            _ => return None,
+        };
+        let mut err = self.tcx().sess.struct_span_err(cause.span, "incompatible lifetime on type");
+        // FIXME: we should point at the lifetime
+        let mut multi_span: MultiSpan = vec![binding_span].into();
+        multi_span
+            .push_span_label(binding_span, "introduces a `'static` lifetime requirement".into());
+        err.span_note(multi_span, "because this has an unmet lifetime requirement");
+        note_and_explain_region(self.tcx(), &mut err, "...", sup, "...");
+        if let Some(impl_node) = self.tcx().hir().get_if_local(*impl_def_id) {
+            let ty = self.tcx().type_of(*impl_def_id);
+            let mut v = super::static_impl_trait::TraitObjectVisitor(vec![]);
+            v.visit_ty(ty);
+            let matching_def_ids = v.0;
+
+            let impl_self_ty = match impl_node {
+                hir::Node::Item(hir::Item {
+                    kind: hir::ItemKind::Impl(hir::Impl { self_ty, .. }),
+                    ..
+                }) => self_ty,
+                _ => bug!("Node not an impl."),
+            };
+
+            for matching_def_id in matching_def_ids {
+                let mut hir_v =
+                    super::static_impl_trait::HirTraitObjectVisitor(vec![], matching_def_id);
+                hir_v.visit_ty(&impl_self_ty);
+
+                let mut multi_span: MultiSpan = hir_v.0.clone().into();
+                for span in &hir_v.0 {
+                    multi_span.push_span_label(
+                        *span,
+                        "this has an implicit `'static` lifetime requirement".to_string(),
+                    );
+                    err.span_suggestion_verbose(
+                        span.shrink_to_hi(),
+                        "consider relaxing the implicit `'static` requirement",
+                        " + '_".to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                err.span_note(multi_span, "...does not necessarily outlive the static lifetime introduced by the compatible `impl`");
+            }
+        }
+        err.emit();
+        Some(ErrorReported)
+    }
+}

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mod.rs

@@ -7,6 +7,7 @@ use rustc_span::source_map::Span;
 
 mod different_lifetimes;
 pub mod find_anon_type;
+mod mismatched_static_lifetime;
 mod named_anon_conflict;
 mod placeholder_error;
 mod static_impl_trait;
@@ -58,6 +59,7 @@ impl<'cx, 'tcx> NiceRegionError<'cx, 'tcx> {
             .or_else(|| self.try_report_impl_not_conforming_to_trait())
             .or_else(|| self.try_report_anon_anon_conflict())
             .or_else(|| self.try_report_static_impl_trait())
+            .or_else(|| self.try_report_mismatched_static_lifetime())
     }
 
     pub fn regions(&self) -> Option<(Span, ty::Region<'tcx>, ty::Region<'tcx>)> {

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -185,7 +185,11 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             }
         }
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sub_origin {
-            if let ObligationCauseCode::ItemObligation(item_def_id) = cause.code {
+            let code = match &cause.code {
+                ObligationCauseCode::MatchImpl(parent, ..) => &**parent,
+                _ => &cause.code,
+            };
+            if let ObligationCauseCode::ItemObligation(item_def_id) = *code {
                 // Same case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a `'static`
                 // lifetime as above, but called using a fully-qualified path to the method:
                 // `Foo::qux(bar)`.
@@ -468,7 +472,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 }
 
 /// Collect all the trait objects in a type that could have received an implicit `'static` lifetime.
-struct TraitObjectVisitor(Vec<DefId>);
+pub(super) struct TraitObjectVisitor(pub(super) Vec<DefId>);
 
 impl TypeVisitor<'_> for TraitObjectVisitor {
     fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
@@ -485,7 +489,7 @@ impl TypeVisitor<'_> for TraitObjectVisitor {
 }
 
 /// Collect all `hir::Ty<'_>` `Span`s for trait objects with an implicit lifetime.
-struct HirTraitObjectVisitor(Vec<Span>, DefId);
+pub(super) struct HirTraitObjectVisitor(pub(super) Vec<Span>, pub(super) DefId);
 
 impl<'tcx> Visitor<'tcx> for HirTraitObjectVisitor {
     type Map = ErasedMap<'tcx>;

compiler/rustc_middle/src/traits/mod.rs

@@ -333,6 +333,9 @@ pub enum ObligationCauseCode<'tcx> {
     /// This is purely for diagnostic purposes - it is always
     /// correct to use `MiscObligation` instead
     WellFormed(Option<hir::HirId>),
+
+    /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
+    MatchImpl(Lrc<ObligationCauseCode<'tcx>>, DefId),
 }
 
 impl ObligationCauseCode<'_> {

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1903,7 +1903,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             | ObligationCauseCode::UnifyReceiver(..)
             | ObligationCauseCode::OpaqueType
             | ObligationCauseCode::MiscObligation
-            | ObligationCauseCode::WellFormed(..) => {}
+            | ObligationCauseCode::WellFormed(..)
+            | ObligationCauseCode::MatchImpl(..) => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1903,9 +1903,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
 
+        let cause = ObligationCause::new(
+            obligation.cause.span,
+            obligation.cause.body_id,
+            ObligationCauseCode::MatchImpl(Lrc::new(obligation.cause.code.clone()), impl_def_id),
+        );
+
         let InferOk { obligations, .. } = self
             .infcx
-            .at(&obligation.cause, obligation.param_env)
+            .at(&cause, obligation.param_env)
             .eq(placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
         nested_obligations.extend(obligations);

src/test/ui/generic-associated-types/issue-78113-lifetime-mismatch-dyn-trait-box.rs

@@ -0,0 +1,19 @@
+// Test for diagnostics when we have mismatched lifetime due to implict 'static lifetime in GATs
+
+// check-fail
+
+#![feature(generic_associated_types)]
+
+pub trait A {}
+impl A for &dyn A {}
+impl A for Box<dyn A> {}
+
+pub trait B {
+    type T<'a>: A;
+}
+
+impl B for () {
+    type T<'a> = Box<dyn A + 'a>; //~ incompatible lifetime on type
+}
+
+fn main() {}

src/test/ui/generic-associated-types/issue-78113-lifetime-mismatch-dyn-trait-box.stderr

@@ -0,0 +1,28 @@
+error: incompatible lifetime on type
+  --> $DIR/issue-78113-lifetime-mismatch-dyn-trait-box.rs:16:5
+   |
+LL |     type T<'a> = Box<dyn A + 'a>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: because this has an unmet lifetime requirement
+  --> $DIR/issue-78113-lifetime-mismatch-dyn-trait-box.rs:12:17
+   |
+LL |     type T<'a>: A;
+   |                 ^ introduces a `'static` lifetime requirement
+note: ...the lifetime `'a` as defined on the associated item at 16:12...
+  --> $DIR/issue-78113-lifetime-mismatch-dyn-trait-box.rs:16:12
+   |
+LL |     type T<'a> = Box<dyn A + 'a>;
+   |            ^^
+note: ...does not necessarily outlive the static lifetime introduced by the compatible `impl`
+  --> $DIR/issue-78113-lifetime-mismatch-dyn-trait-box.rs:9:20
+   |
+LL | impl A for Box<dyn A> {}
+   |                    ^ this has an implicit `'static` lifetime requirement
+help: consider relaxing the implicit `'static` requirement
+   |
+LL | impl A for Box<dyn A + '_> {}
+   |                      ^^^^
+
+error: aborting due to previous error
+

src/test/ui/wf/wf-in-foreign-fn-decls-issue-80468.rs

@@ -13,5 +13,5 @@ pub struct Ref<'a>(&'a u8);
 impl Trait for Ref {} //~ ERROR:  implicit elided lifetime not allowed here
 
 extern "C" {
-    pub fn repro(_: Wrapper<Ref>); //~ ERROR: mismatched types
+    pub fn repro(_: Wrapper<Ref>); //~ ERROR: incompatible lifetime on type
 }