Report cycle error using 'deepest' obligation in the cycle

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

@@ -224,7 +224,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 
-        self.report_overflow_error(&cycle[0], false);
+        // The 'deepest' obligation is most likely to have a useful
+        // cause 'backtrace'
+        self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
     }
 
     fn report_selection_error(

src/test/incremental/const-generics/hash-tyvid-regression-1.rs

@@ -9,7 +9,8 @@ where
     use std::convert::TryFrom;
     <[T; N.get()]>::try_from(())
     //~^ error: the trait bound
-    //~^^ error: mismatched types
+    //~| error: the trait bound
+    //~| error: mismatched types
 }
 
 fn main() {}

src/test/ui/traits/cycle-cache-err-60010.rs

@@ -25,7 +25,7 @@ struct Runtime<DB: Database> {
 }
 struct SalsaStorage {
     _parse: <ParseQuery as Query<RootDatabase>>::Data,
-    //~^ ERROR overflow evaluating the requirement `RootDatabase: SourceDatabase`
+    //~^ ERROR overflow
 }
 
 impl Database for RootDatabase {

src/test/ui/traits/cycle-cache-err-60010.stderr

@@ -1,9 +1,27 @@
-error[E0275]: overflow evaluating the requirement `RootDatabase: SourceDatabase`
+error[E0275]: overflow evaluating the requirement `SalsaStorage: RefUnwindSafe`
   --> $DIR/cycle-cache-err-60010.rs:27:13
    |
 LL |     _parse: <ParseQuery as Query<RootDatabase>>::Data,
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: required because it appears within the type `*const SalsaStorage`
+   = note: required because it appears within the type `Unique<SalsaStorage>`
+   = note: required because it appears within the type `Box<SalsaStorage>`
+note: required because it appears within the type `Runtime<RootDatabase>`
+  --> $DIR/cycle-cache-err-60010.rs:23:8
+   |
+LL | struct Runtime<DB: Database> {
+   |        ^^^^^^^
+note: required because it appears within the type `RootDatabase`
+  --> $DIR/cycle-cache-err-60010.rs:20:8
+   |
+LL | struct RootDatabase {
+   |        ^^^^^^^^^^^^
+note: required because of the requirements on the impl of `SourceDatabase` for `RootDatabase`
+  --> $DIR/cycle-cache-err-60010.rs:44:9
+   |
+LL | impl<T> SourceDatabase for T
+   |         ^^^^^^^^^^^^^^     ^
 note: required because of the requirements on the impl of `Query<RootDatabase>` for `ParseQuery`
   --> $DIR/cycle-cache-err-60010.rs:37:10
    |

src/test/ui/traits/inductive-overflow/lifetime.rs

@@ -15,8 +15,8 @@ impl<'a> Y for C<'a> {
 struct C<'a>(&'a ());
 struct X<T: Y>(T::P);
 
-impl<T: NotAuto> NotAuto for Box<T> {}
-impl<T: Y> NotAuto for X<T> where T::P: NotAuto {} //~ NOTE: required
+impl<T: NotAuto> NotAuto for Box<T> {} //~ NOTE: required
+impl<T: Y> NotAuto for X<T> where T::P: NotAuto {}
 impl<'a> NotAuto for C<'a> {}
 
 fn is_send<S: NotAuto>() {}
@@ -26,6 +26,6 @@ fn main() {
     // Should only be a few notes.
     is_send::<X<C<'static>>>();
     //~^ ERROR overflow evaluating
-    //~| 2 redundant requirements hidden
+    //~| 3 redundant requirements hidden
     //~| required because of
 }

src/test/ui/traits/inductive-overflow/lifetime.stderr

@@ -1,15 +1,15 @@
-error[E0275]: overflow evaluating the requirement `Box<X<C<'_>>>: NotAuto`
+error[E0275]: overflow evaluating the requirement `X<C<'_>>: NotAuto`
   --> $DIR/lifetime.rs:27:5
    |
 LL |     is_send::<X<C<'static>>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^
    |
-note: required because of the requirements on the impl of `NotAuto` for `X<C<'_>>`
-  --> $DIR/lifetime.rs:19:12
+note: required because of the requirements on the impl of `NotAuto` for `Box<X<C<'_>>>`
+  --> $DIR/lifetime.rs:18:18
    |
-LL | impl<T: Y> NotAuto for X<T> where T::P: NotAuto {}
-   |            ^^^^^^^     ^^^^
-   = note: 2 redundant requirements hidden
+LL | impl<T: NotAuto> NotAuto for Box<T> {}
+   |                  ^^^^^^^     ^^^^^^
+   = note: 3 redundant requirements hidden
    = note: required because of the requirements on the impl of `NotAuto` for `X<C<'static>>`
 note: required by a bound in `is_send`
   --> $DIR/lifetime.rs:22:15

src/test/ui/traits/inductive-overflow/simultaneous.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `{integer}: Tweedledee`
+error[E0275]: overflow evaluating the requirement `{integer}: Tweedledum`
   --> $DIR/simultaneous.rs:18:5
    |
 LL |     is_ee(4);

src/test/ui/type-alias-impl-trait/auto-trait-leakage3.stderr

@@ -5,10 +5,10 @@ LL |     type Foo = impl std::fmt::Debug;
    |                ^^^^^^^^^^^^^^^^^^^^
    |
 note: ...which requires type-checking `m::bar`...
-  --> $DIR/auto-trait-leakage3.rs:14:5
+  --> $DIR/auto-trait-leakage3.rs:15:9
    |
-LL |     pub fn bar() {
-   |     ^^^^^^^^^^^^
+LL |         is_send(foo());
+   |         ^^^^^^^
    = note: ...which requires evaluating trait selection obligation `impl std::fmt::Debug: std::marker::Send`...
    = note: ...which again requires computing type of `m::Foo::{opaque#0}`, completing the cycle
 note: cycle used when checking item types in module `m`

src/test/ui/type-alias-impl-trait/inference-cycle.stderr

@@ -5,10 +5,10 @@ LL |     type Foo = impl std::fmt::Debug;
    |                ^^^^^^^^^^^^^^^^^^^^
    |
 note: ...which requires type-checking `m::bar`...
-  --> $DIR/inference-cycle.rs:14:5
+  --> $DIR/inference-cycle.rs:15:9
    |
-LL |     pub fn bar() {
-   |     ^^^^^^^^^^^^
+LL |         is_send(foo()); // Today: error
+   |         ^^^^^^^
    = note: ...which requires evaluating trait selection obligation `impl std::fmt::Debug: std::marker::Send`...
    = note: ...which again requires computing type of `m::Foo::{opaque#0}`, completing the cycle
 note: cycle used when checking item types in module `m`