Don't anonymize bound region names during typeck

compiler/rustc_typeck/src/check/writeback.rs

@@ -732,14 +732,40 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
     }
 }
 
+struct EraseEarlyRegions<'tcx> {
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'tcx> TypeFolder<'tcx> for EraseEarlyRegions<'tcx> {
+    fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if ty.has_type_flags(ty::TypeFlags::HAS_POTENTIAL_FREE_REGIONS) {
+            ty.super_fold_with(self)
+        } else {
+            ty
+        }
+    }
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        if let ty::ReLateBound(..) = r { r } else { self.tcx.lifetimes.re_erased }
+    }
+}
+
 impl<'cx, 'tcx> TypeFolder<'tcx> for Resolver<'cx, 'tcx> {
     fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
         self.tcx
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         match self.infcx.fully_resolve(t) {
-            Ok(t) => self.infcx.tcx.erase_regions(t),
+            Ok(t) => {
+                // Do not anonymize late-bound regions
+                // (e.g. keep `for<'a>` named `for<'a>`).
+                // This allows NLL to generate error messages that
+                // refer to the higher-ranked lifetime names written by the user.
+                EraseEarlyRegions { tcx: self.infcx.tcx }.fold_ty(t)
+            }
             Err(_) => {
                 debug!("Resolver::fold_ty: input type `{:?}` not fully resolvable", t);
                 self.report_type_error(t);

src/test/ui/hr-subtype/hr-subtype.bound_a_b_ret_a_vs_bound_a_ret_a.nll.stderr

@@ -8,8 +8,8 @@ LL | / check! { bound_a_b_ret_a_vs_bound_a_ret_a: (for<'a,'b> fn(&'a u32, &'b u3
 LL | | for<'a>    fn(&'a u32, &'a u32) -> &'a u32) }
    | |_____________________________________________- in this macro invocation
    |
-   = note: expected enum `Option<for<'r, 's> fn(&'r u32, &'s u32) -> &'r u32>`
-              found enum `Option<for<'r> fn(&'r u32, &'r u32) -> &'r u32>`
+   = note: expected enum `Option<for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32>`
+              found enum `Option<for<'a> fn(&'a u32, &'a u32) -> &'a u32>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error

src/test/ui/hr-subtype/hr-subtype.bound_a_vs_free_x.nll.stderr

@@ -8,7 +8,7 @@ LL | / check! { bound_a_vs_free_x: (for<'a> fn(&'a u32),
 LL | | fn(&'x u32)) }
    | |______________- in this macro invocation
    |
-   = note: expected enum `Option<for<'r> fn(&'r u32)>`
+   = note: expected enum `Option<for<'a> fn(&'a u32)>`
               found enum `Option<fn(&u32)>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 

src/test/ui/hr-subtype/hr-subtype.bound_inv_a_b_vs_bound_inv_a.nll.stderr

@@ -8,8 +8,8 @@ LL | / check! { bound_inv_a_b_vs_bound_inv_a: (for<'a,'b> fn(Inv<'a>, Inv<'b>),
 LL | | for<'a>    fn(Inv<'a>, Inv<'a>)) }
    | |__________________________________- in this macro invocation
    |
-   = note: expected enum `Option<for<'r, 's> fn(Inv<'r>, Inv<'s>)>`
-              found enum `Option<for<'r> fn(Inv<'r>, Inv<'r>)>`
+   = note: expected enum `Option<for<'a, 'b> fn(Inv<'a>, Inv<'b>)>`
+              found enum `Option<for<'a> fn(Inv<'a>, Inv<'a>)>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
@@ -22,8 +22,8 @@ LL | / check! { bound_inv_a_b_vs_bound_inv_a: (for<'a,'b> fn(Inv<'a>, Inv<'b>),
 LL | | for<'a>    fn(Inv<'a>, Inv<'a>)) }
    | |__________________________________- in this macro invocation
    |
-   = note: expected enum `Option<for<'r, 's> fn(Inv<'r>, Inv<'s>)>`
-              found enum `Option<for<'r> fn(Inv<'r>, Inv<'r>)>`
+   = note: expected enum `Option<for<'a, 'b> fn(Inv<'a>, Inv<'b>)>`
+              found enum `Option<for<'a> fn(Inv<'a>, Inv<'a>)>`
    = note: this error originates in the macro `check` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

src/test/ui/lub-glb/old-lub-glb-hr-noteq1.nll.stderr

@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |         _ => y,
    |              ^ one type is more general than the other
    |
-   = note: expected fn pointer `for<'r, 's> fn(&'r u8, &'s u8) -> &'r u8`
-              found fn pointer `for<'r> fn(&'r u8, &'r u8) -> &'r u8`
+   = note: expected fn pointer `for<'a, 'b> fn(&'a u8, &'b u8) -> &'a u8`
+              found fn pointer `for<'a> fn(&'a u8, &'a u8) -> &'a u8`
 
 error: aborting due to previous error
 

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

@@ -4,17 +4,17 @@ error[E0308]: mismatched types
 LL |         _ => y,
    |              ^ one type is more general than the other
    |
-   = note: expected trait object `dyn for<'r, 's> Foo<&'r u8, &'s u8>`
-              found trait object `dyn for<'r> Foo<&'r u8, &'r u8>`
+   = note: expected trait object `dyn for<'a, 'b> Foo<&'a u8, &'b u8>`
+              found trait object `dyn for<'a> Foo<&'a u8, &'a u8>`
 
 error[E0308]: mismatched types
   --> $DIR/old-lub-glb-object.rs:10:14
    |
 LL |         _ => y,
    |              ^ one type is more general than the other
    |
-   = note: expected trait object `dyn for<'r, 's> Foo<&'r u8, &'s u8>`
-              found trait object `dyn for<'r> Foo<&'r u8, &'r u8>`
+   = note: expected trait object `dyn for<'a, 'b> Foo<&'a u8, &'b u8>`
+              found trait object `dyn for<'a> Foo<&'a u8, &'a u8>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/nll/relate_tys/fn-subtype.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL |     let y: for<'a> fn(&'a ()) = x;
    |                                 ^ one type is more general than the other
    |
-   = note: expected fn pointer `for<'r> fn(&'r ())`
+   = note: expected fn pointer `for<'a> fn(&'a ())`
               found fn pointer `fn(&())`
 
 error: aborting due to previous error

src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL |     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
    |                                                          ^^^^^^^^^ one type is more general than the other
    |
-   = note: expected fn pointer `for<'r, 's> fn(&'r u32, &'s u32) -> &'r u32`
+   = note: expected fn pointer `for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32`
               found fn pointer `for<'a> fn(&'a u32, &'a u32) -> &'a u32`
 
 error[E0308]: mismatched types
@@ -14,7 +14,7 @@ LL |     let _: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
    = note: expected fn pointer `for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32`
-              found fn pointer `for<'r> fn(&'r u32, &'r u32) -> &'r u32`
+              found fn pointer `for<'a> fn(&'a u32, &'a u32) -> &'a u32`
 
 error: aborting due to 2 previous errors
 

src/test/ui/nll/relate_tys/trait-hrtb.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL |     let y: Box<dyn for<'a> Foo<'a>> = x;
    |                                       ^ one type is more general than the other
    |
-   = note: expected trait object `dyn for<'r> Foo<'r>`
+   = note: expected trait object `dyn for<'a> Foo<'a>`
               found trait object `dyn Foo<'_>`
 
 error: aborting due to previous error

src/test/ui/rfc1623.nll.stderr

@@ -31,7 +31,7 @@ LL | |
 LL | | };
    | |_^ one type is more general than the other
    |
-   = note: expected type `for<'r, 's> Fn<(&'r Foo<'s>,)>`
+   = note: expected type `for<'a, 'b> Fn<(&'a Foo<'b>,)>`
               found type `Fn<(&Foo<'_>,)>`
 
 error[E0308]: mismatched types
@@ -46,7 +46,7 @@ LL | |
 LL | | };
    | |_^ one type is more general than the other
    |
-   = note: expected type `for<'r, 's> Fn<(&'r Foo<'s>,)>`
+   = note: expected type `for<'a, 'b> Fn<(&'a Foo<'b>,)>`
               found type `Fn<(&Foo<'_>,)>`
 
 error: implementation of `FnOnce` is not general enough
@@ -61,7 +61,7 @@ LL | |
 LL | | };
    | |_^ implementation of `FnOnce` is not general enough
    |
-   = note: `fn(&'2 Foo<'_>) -> &'2 Foo<'_> {id::<&'2 Foo<'_>>}` must implement `FnOnce<(&'1 Foo<'_>,)>`, for any lifetime `'1`...
+   = note: `fn(&'2 Foo<'_>) -> &'2 Foo<'_> {id::<&'2 Foo<'_>>}` must implement `FnOnce<(&'1 Foo<'b>,)>`, for any lifetime `'1`...
    = note: ...but it actually implements `FnOnce<(&'2 Foo<'_>,)>`, for some specific lifetime `'2`
 
 error: implementation of `FnOnce` is not general enough
@@ -76,7 +76,7 @@ LL | |
 LL | | };
    | |_^ implementation of `FnOnce` is not general enough
    |
-   = note: `fn(&Foo<'2>) -> &Foo<'2> {id::<&Foo<'2>>}` must implement `FnOnce<(&Foo<'1>,)>`, for any lifetime `'1`...
+   = note: `fn(&Foo<'2>) -> &Foo<'2> {id::<&Foo<'2>>}` must implement `FnOnce<(&'a Foo<'1>,)>`, for any lifetime `'1`...
    = note: ...but it actually implements `FnOnce<(&Foo<'2>,)>`, for some specific lifetime `'2`
 
 error: aborting due to 5 previous errors