issue better error message when LUB/GLB diverge under new behavior

src/librustc/infer/error_reporting/mod.rs

@@ -762,9 +762,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }
         }
 
-        self.note_error_origin(diag, &cause);
         self.check_and_note_conflicting_crates(diag, terr, span);
         self.tcx.note_and_explain_type_err(diag, terr, span);
+
+        // It reads better to have the error origin as the final
+        // thing.
+        self.note_error_origin(diag, &cause);
     }
 
     pub fn report_and_explain_type_error(&self,
@@ -772,6 +775,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                          terr: &TypeError<'tcx>)
                                          -> DiagnosticBuilder<'tcx>
     {
+        debug!("report_and_explain_type_error(trace={:?}, terr={:?})",
+               trace,
+               terr);
+
         let span = trace.cause.span;
         let failure_str = trace.cause.as_failure_str();
         let mut diag = match trace.cause.code {

src/librustc/infer/glb.rs

@@ -75,6 +75,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
+        debug!("binders(a={:?}, b={:?})", a, b);
         let was_error = self.infcx().probe(|_snapshot| {
             // Subtle: use a fresh combine-fields here because we recover
             // from Err. Doing otherwise could propagate obligations out
@@ -84,6 +85,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
                 .higher_ranked_glb(a, b, self.a_is_expected)
                 .is_err()
         });
+        debug!("binders: was_error={:?}", was_error);
 
         // When higher-ranked types are involved, computing the LUB is
         // very challenging, switch to invariance. This is obviously
@@ -91,6 +93,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
         match self.relate_with_variance(ty::Variance::Invariant, a, b) {
             Ok(_) => Ok(a.clone()),
             Err(err) => {
+                debug!("binders: error occurred, was_error={:?}", was_error);
                 if !was_error {
                     Err(TypeError::OldStyleLUB(Box::new(err)))
                 } else {

src/librustc/infer/lub.rs

@@ -75,6 +75,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
                   -> RelateResult<'tcx, ty::Binder<T>>
         where T: Relate<'tcx>
     {
+        debug!("binders(a={:?}, b={:?})", a, b);
         let was_error = self.infcx().probe(|_snapshot| {
             // Subtle: use a fresh combine-fields here because we recover
             // from Err. Doing otherwise could propagate obligations out
@@ -84,6 +85,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
                 .higher_ranked_lub(a, b, self.a_is_expected)
                 .is_err()
         });
+        debug!("binders: was_error={:?}", was_error);
 
         // When higher-ranked types are involved, computing the LUB is
         // very challenging, switch to invariance. This is obviously
@@ -91,6 +93,7 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
         match self.relate_with_variance(ty::Variance::Invariant, a, b) {
             Ok(_) => Ok(a.clone()),
             Err(err) => {
+                debug!("binders: error occurred, was_error={:?}", was_error);
                 if !was_error {
                     Err(TypeError::OldStyleLUB(Box::new(err)))
                 } else {

src/librustc/ty/error.rs

@@ -298,6 +298,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 db.span_note(found.origin_span,
                              "...that also applies to the same type variable here");
             }
+            OldStyleLUB(err) => {
+                db.note("this was previously accepted by the compiler but has been phased out");
+                db.note("for more information, see https://github.com/rust-lang/rust/issues/45852");
+
+                self.note_and_explain_type_err(db, &err, sp);
+            }
             _ => {}
         }
     }

src/test/ui/lub-glb/old-lub-glb-hr.rs

@@ -0,0 +1,36 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we give a note when the old LUB/GLB algorithm would have
+// succeeded but the new code (which is stricter) gives an error.
+
+fn foo(
+    x: fn(&u8, &u8),
+    y: for<'a> fn(&'a u8, &'a u8),
+) {
+    let z = match 22 {
+        0 => x,
+        _ => y,
+    };
+}
+
+fn bar(
+    x: fn(&u8, &u8),
+    y: for<'a> fn(&'a u8, &'a u8),
+) {
+    let z = match 22 {
+        // No error with an explicit cast:
+        0 => x as for<'a> fn(&'a u8, &'a u8),
+        _ => y,
+    };
+}
+
+fn main() {
+}

src/test/ui/lub-glb/old-lub-glb-hr.stderr

@@ -0,0 +1,22 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/old-lub-glb-hr.rs:18:13
+   |
+18 |       let z = match 22 {
+   |  _____________^
+19 | |         0 => x,
+20 | |         _ => y,
+21 | |     };
+   | |_____^ expected bound lifetime parameter, found concrete lifetime
+   |
+   = note: expected type `for<'r, 's> fn(&'r u8, &'s u8)`
+              found type `for<'a> fn(&'a u8, &'a u8)`
+   = note: this was previously accepted by the compiler but has been phased out
+   = note: for more information, see https://github.com/rust-lang/rust/issues/45852
+note: match arm with an incompatible type
+  --> $DIR/old-lub-glb-hr.rs:20:14
+   |
+20 |         _ => y,
+   |              ^
+
+error: aborting due to previous error
+

src/test/ui/lub-glb/old-lub-glb-object.rs

@@ -0,0 +1,38 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we give a note when the old LUB/GLB algorithm would have
+// succeeded but the new code (which is stricter) gives an error.
+
+trait Foo<T, U> { }
+
+fn foo(
+    x: &for<'a, 'b> Foo<&'a u8, &'b u8>,
+    y: &for<'a> Foo<&'a u8, &'a u8>,
+) {
+    let z = match 22 {
+        0 => x,
+        _ => y,
+    };
+}
+
+fn bar(
+    x: &for<'a, 'b> Foo<&'a u8, &'b u8>,
+    y: &for<'a> Foo<&'a u8, &'a u8>,
+) {
+    // Accepted with explicit case:
+    let z = match 22 {
+        0 => x as &for<'a> Foo<&'a u8, &'a u8>,
+        _ => y,
+    };
+}
+
+fn main() {
+}

src/test/ui/lub-glb/old-lub-glb-object.stderr

@@ -0,0 +1,22 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/old-lub-glb-object.rs:20:13
+   |
+20 |       let z = match 22 {
+   |  _____________^
+21 | |         0 => x,
+22 | |         _ => y,
+23 | |     };
+   | |_____^ expected bound lifetime parameter 'a, found concrete lifetime
+   |
+   = note: expected type `&for<'a, 'b> Foo<&'a u8, &'b u8>`
+              found type `&for<'a> Foo<&'a u8, &'a u8>`
+   = note: this was previously accepted by the compiler but has been phased out
+   = note: for more information, see https://github.com/rust-lang/rust/issues/45852
+note: match arm with an incompatible type
+  --> $DIR/old-lub-glb-object.rs:22:14
+   |
+22 |         _ => y,
+   |              ^
+
+error: aborting due to previous error
+