Name RegionKind::ReVar lifetimes in diagnostics

Author: estebank

src/librustc_infer/infer/error_reporting/mod.rs

@@ -987,13 +987,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         fn push_ty_ref<'tcx>(
-            r: &ty::Region<'tcx>,
+            region: &ty::Region<'tcx>,
             ty: Ty<'tcx>,
             mutbl: hir::Mutability,
             s: &mut DiagnosticStyledString,
         ) {
-            let mut r = r.to_string();
-            if r == "'_" {
+            let mut r = region.to_string();
+            if let ty::RegionKind::ReVar(var) = region {
+                // Show these named, not as `'_` or elide them in "expected/found" notes.
+                r = format!("'z{} ", var.index());
+            } else if r == "'_" {
                 r.clear();
             } else {
                 r.push(' ');

src/librustc_infer/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -1,11 +1,15 @@
 //! Error Reporting for `impl` items that do not match the obligations from their `trait`.
 
+use crate::hir::def_id::DefId;
 use crate::infer::error_reporting::nice_region_error::NiceRegionError;
 use crate::infer::lexical_region_resolve::RegionResolutionError;
 use crate::infer::{Subtype, ValuePairs};
 use crate::traits::ObligationCauseCode::CompareImplMethodObligation;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::ErrorReported;
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::error::ExpectedFound;
+use rustc_middle::ty::fold::TypeFoldable;
+use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 
 impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
@@ -52,9 +56,52 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             .tcx()
             .sess
             .struct_span_err(sp, "`impl` item signature doesn't match `trait` item signature");
-        err.note(&format!("expected `{:?}`\n   found `{:?}`", expected, found));
         err.span_label(sp, &format!("found {:?}", found));
         err.span_label(impl_sp, &format!("expected {:?}", expected));
+
+        struct EarlyBoundRegionHighlighter(FxHashSet<DefId>);
+        impl<'tcx> ty::fold::TypeVisitor<'tcx> for EarlyBoundRegionHighlighter {
+            fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
+                debug!("LateBoundRegionNameCollector visit_region {:?}", r);
+                match *r {
+                    ty::ReFree(free) => {
+                        self.0.insert(free.scope);
+                    }
+
+                    ty::ReEarlyBound(bound) => {
+                        self.0.insert(bound.def_id);
+                    }
+                    _ => {}
+                }
+                r.super_visit_with(self)
+            }
+        }
+
+        let mut visitor = EarlyBoundRegionHighlighter(FxHashSet::default());
+        expected.visit_with(&mut visitor);
+
+        let note = !visitor.0.is_empty();
+
+        if let Some((expected, found)) = self
+            .tcx()
+            .infer_ctxt()
+            .enter(|infcx| infcx.expected_found_str_ty(&ExpectedFound { expected, found }))
+        {
+            err.note_expected_found(&"", expected, &"", found);
+        } else {
+            // This fallback shouldn't be necessary, but let's keep it in just in case.
+            err.note(&format!("expected `{:?}`\n   found `{:?}`", expected, found));
+        }
+        if note {
+            err.note(
+                "the lifetime requirements from the `trait` could not be fulfilled by the \
+                      `impl`",
+            );
+            err.help(
+                "consider adding a named lifetime to the `trait` that constrains the item's \
+                      `self` argument, its inputs and its output with it",
+            );
+        }
         err.emit();
     }
 }

src/test/ui/coercion/coerce-mut.rs

@@ -4,7 +4,7 @@ fn main() {
     let x = 0;
     f(&x);
     //~^ ERROR mismatched types
-    //~| expected mutable reference `&mut i32`
-    //~| found reference `&{integer}`
+    //~| expected mutable reference `&'z1 mut i32`
+    //~| found reference `&'z2 {integer}`
     //~| types differ in mutability
 }

src/test/ui/coercion/coerce-mut.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     f(&x);
    |       ^^ types differ in mutability
    |
-   = note: expected mutable reference `&mut i32`
-                      found reference `&{integer}`
+   = note: expected mutable reference `&'z1 mut i32`
+                      found reference `&'z2 {integer}`
 
 error: aborting due to previous error
 

src/test/ui/compare-method/reordered-type-param.stderr

@@ -11,7 +11,7 @@ LL |   fn b<F:Clone,G>(&self, _x: G) -> G { panic!() }
    |        expected type parameter
    |
    = note: expected fn pointer `fn(&E, F) -> F`
-              found fn pointer `fn(&E, G) -> G`
+              found fn pointer `fn(&'z0 E, G) -> G`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 

src/test/ui/hrtb/hrtb-exists-forall-fn.stderr

@@ -7,7 +7,7 @@ LL |     let _: for<'b> fn(&'b u32) = foo();
    |            expected due to this
    |
    = note: expected fn pointer `for<'b> fn(&'b u32)`
-              found fn pointer `fn(&u32)`
+              found fn pointer `fn(&'z0 u32)`
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/impl-generic-mismatch-ab.stderr

@@ -10,7 +10,7 @@ LL |     fn foo<B: Debug>(&self, a: &impl Debug, b: &B) { }
    |            expected type parameter
    |
    = note: expected fn pointer `fn(&(), &B, &impl Debug)`
-              found fn pointer `fn(&(), &impl Debug, &B)`
+              found fn pointer `fn(&'z0 (), &impl Debug, &B)`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 

src/test/ui/impl-trait/trait_type.stderr

@@ -5,7 +5,7 @@ LL |    fn fmt(&self, x: &str) -> () { }
    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
    |
    = note: expected fn pointer `fn(&MyType, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`
-              found fn pointer `fn(&MyType, &str)`
+              found fn pointer `fn(&'z0 MyType, &str)`
 
 error[E0050]: method `fmt` has 1 parameter but the declaration in trait `std::fmt::Display::fmt` has 2
   --> $DIR/trait_type.rs:12:11

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.stderr

@@ -10,7 +10,9 @@ LL |     fn deref(&self) -> &Self::Target;
    |     --------------------------------- expected fn(&Struct) -> &(dyn Trait + 'static)
    |
    = note: expected `fn(&Struct) -> &(dyn Trait + 'static)`
-              found `fn(&Struct) -> &dyn Trait`
+              found `fn(&'z0 Struct) -> &dyn Trait`
+   = note: the lifetime requirements from the `trait` could not be fulfilled by the `impl`
+   = help: consider adding a named lifetime to the `trait` that constrains the item's `self` argument, its inputs and its output with it
 
 error: aborting due to previous error
 

src/test/ui/in-band-lifetimes/mismatched_trait_impl.stderr

@@ -8,7 +8,9 @@ LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &u32, &u32) -> &u32
    |
    = note: expected `fn(&i32, &'a u32, &u32) -> &'a u32`
-              found `fn(&i32, &u32, &u32) -> &u32`
+              found `fn(&'z0 i32, &'z1 u32, &'z2 u32) -> &'z2 u32`
+   = note: the lifetime requirements from the `trait` could not be fulfilled by the `impl`
+   = help: consider adding a named lifetime to the `trait` that constrains the item's `self` argument, its inputs and its output with it
 
 error[E0623]: lifetime mismatch
   --> $DIR/mismatched_trait_impl.rs:10:9