Custom lifetime error for impl item doesn't conform to trait

src/librustc/infer/error_reporting/nice_region_error/mod.rs

@@ -12,6 +12,7 @@ mod named_anon_conflict;
 mod placeholder_error;
 mod outlives_closure;
 mod static_impl_trait;
+mod trait_impl_difference;
 mod util;
 
 impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
@@ -73,6 +74,7 @@ impl<'cx, 'tcx> NiceRegionError<'cx, 'tcx> {
             .or_else(|| self.try_report_anon_anon_conflict())
             .or_else(|| self.try_report_outlives_closure())
             .or_else(|| self.try_report_static_impl_trait())
+            .or_else(|| self.try_report_impl_not_conforming_to_trait())
     }
 
     pub fn get_regions(&self) -> (Span, ty::Region<'tcx>, ty::Region<'tcx>) {

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

@@ -0,0 +1,53 @@
+//! Error Reporting for `impl` items that do not match the obligations from their `trait`.
+
+use crate::infer::{ValuePairs, Subtype};
+use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::infer::lexical_region_resolve::RegionResolutionError;
+use crate::util::common::ErrorReported;
+
+impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
+    /// Print the error message for lifetime errors when the `impl` doesn't conform to the `trait`.
+    pub(super) fn try_report_impl_not_conforming_to_trait(&self) -> Option<ErrorReported> {
+        if let Some(ref error) = self.error {
+            if let RegionResolutionError::SubSupConflict(
+                _,
+                var_origin,
+                sub_origin,
+                _sub,
+                sup_origin,
+                _sup,
+            ) = error.clone() {
+                match (&sup_origin, &sub_origin) {
+                    (&Subtype(ref sup_trace), &Subtype(ref sub_trace)) => {
+                        if let (
+                            ValuePairs::Types(sub_expected_found),
+                            ValuePairs::Types(sup_expected_found),
+                        ) = (&sub_trace.values, &sup_trace.values) {
+                            if sup_expected_found == sub_expected_found {
+                                let sp = var_origin.span();
+                                let mut err = self.tcx().sess.struct_span_err(
+                                    sp,
+                                    "`impl` item doesn't match `trait` item"
+                                );
+                                err.note(&format!(
+                                    "expected: {:?}\n   found: {:?}",
+                                    sub_expected_found.expected,
+                                    sub_expected_found.found,
+                                ));
+                                err.span_label(sp, &format!(
+                                    "found {:?}",
+                                    sub_expected_found.found,
+                                ));
+                                // FIXME: recover the `FnPtr`'s `HirId`/`Node` to point to it.
+                                err.emit();
+                                return Some(ErrorReported);
+                            }
+                        }
+                    }
+                    _ => {}
+                }
+            }
+        }
+        None
+    }
+}

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

@@ -9,6 +9,6 @@ impl Deref for Struct {
         unimplemented!();
     }
 }
-//~^^^^ ERROR cannot infer an appropriate lifetime for lifetime parameter
+//~^^^^ ERROR `impl` item doesn't match `trait` item
 
 fn main() {}

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

@@ -1,20 +1,11 @@
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements
+error: `impl` item doesn't match `trait` item
   --> $DIR/mismatched_trait_impl-2.rs:8:5
    |
 LL |     fn deref(&self) -> &dyn Trait {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&Struct) -> &dyn Trait
    |
-note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 8:5...
-  --> $DIR/mismatched_trait_impl-2.rs:8:5
-   |
-LL | /     fn deref(&self) -> &dyn Trait {
-LL | |         unimplemented!();
-LL | |     }
-   | |_____^
-   = note: ...but the lifetime must also be valid for the static lifetime...
-   = note: ...so that the method type is compatible with trait:
-           expected fn(&Struct) -> &(dyn Trait + 'static)
-              found fn(&Struct) -> &dyn Trait
+   = note: expected: fn(&Struct) -> &(dyn Trait + 'static)
+              found: fn(&Struct) -> &dyn Trait
 
 error: aborting due to previous error
 

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

@@ -6,7 +6,7 @@ trait Get {
 }
 
 impl Get for i32 {
-    fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 { //~ ERROR cannot infer
+    fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 { //~ ERROR `impl` item doesn't match `trait`
         x //~ ERROR lifetime mismatch
     }
 }

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

@@ -1,24 +1,11 @@
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'a in generic type due to conflicting requirements
+error: `impl` item doesn't match `trait` item
   --> $DIR/mismatched_trait_impl.rs:9:5
    |
 LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &u32, &u32) -> &u32
    |
-note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the method body at 9:5...
-  --> $DIR/mismatched_trait_impl.rs:9:5
-   |
-LL | /     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
-LL | |         x
-LL | |     }
-   | |_____^
-note: ...but the lifetime must also be valid for the lifetime `'a` as defined on the method body at 9:32...
-  --> $DIR/mismatched_trait_impl.rs:9:32
-   |
-LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {
-   |                                ^^
-   = note: ...so that the method type is compatible with trait:
-           expected fn(&i32, &'a u32, &u32) -> &'a u32
-              found fn(&i32, &u32, &u32) -> &u32
+   = note: expected: fn(&i32, &'a u32, &u32) -> &'a u32
+              found: fn(&i32, &u32, &u32) -> &u32
 
 error[E0623]: lifetime mismatch
   --> $DIR/mismatched_trait_impl.rs:10:9

src/test/ui/lifetimes/lifetime-mismatch-between-trait-and-impl.rs

@@ -0,0 +1,12 @@
+trait Foo {
+    fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32;
+}
+
+impl Foo for () {
+    fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
+    //~^ ERROR `impl` item doesn't match `trait` item
+        if x > y { x } else { y }
+    }
+}
+
+fn main() {}

src/test/ui/lifetimes/lifetime-mismatch-between-trait-and-impl.stderr

@@ -0,0 +1,11 @@
+error: `impl` item doesn't match `trait` item
+  --> $DIR/lifetime-mismatch-between-trait-and-impl.rs:6:5
+   |
+LL |     fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&i32, &i32) -> &i32
+   |
+   = note: expected: fn(&i32, &'a i32) -> &'a i32
+              found: fn(&i32, &i32) -> &i32
+
+error: aborting due to previous error
+

src/test/ui/reject-specialized-drops-8142.rs

@@ -52,6 +52,6 @@ impl<One>         Drop for V<One,One>     { fn drop(&mut self) { } } // REJECT
 //~^ ERROR Implementations of Drop cannot be specialized
 
 impl<'lw>         Drop for W<'lw,'lw>     { fn drop(&mut self) { } } // REJECT
-//~^ ERROR cannot infer an appropriate lifetime
+//~^ ERROR `impl` item doesn't match `trait` item
 
 pub fn main() { }

src/test/ui/reject-specialized-drops-8142.stderr

@@ -89,25 +89,14 @@ note: Use same sequence of generic type and region parameters that is on the str
 LL | struct V<Tva, Tvb> { x: *const Tva, y: *const Tvb }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'lw` due to conflicting requirements
+error: `impl` item doesn't match `trait` item
   --> $DIR/reject-specialized-drops-8142.rs:54:1
    |
 LL | impl<'lw>         Drop for W<'lw,'lw>     { fn drop(&mut self) { } } // REJECT
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: first, the lifetime cannot outlive the lifetime `'l1` as defined on the struct at 17:10...
-  --> $DIR/reject-specialized-drops-8142.rs:17:10
-   |
-LL | struct W<'l1, 'l2> { x: &'l1 i8, y: &'l2 u8 }
-   |          ^^^
-note: ...but the lifetime must also be valid for the lifetime `'l2` as defined on the struct at 17:15...
-  --> $DIR/reject-specialized-drops-8142.rs:17:15
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found W<'_, '_>
    |
-LL | struct W<'l1, 'l2> { x: &'l1 i8, y: &'l2 u8 }
-   |               ^^^
-   = note: ...so that the types are compatible:
-           expected W<'l1, 'l2>
-              found W<'_, '_>
+   = note: expected: W<'l1, 'l2>
+              found: W<'_, '_>
 
 error: aborting due to 8 previous errors