Rollup merge of rust-lang#98022 - compiler-errors:erroneous-borrowck-span, r=oli-obk

Fix erroneous span for borrowck error

I am not confident that this is the correct fix, but it does the job. Open to suggestions for a real fix instead.

Fixes rust-lang#97997

The issue is that we pass a [dummy location](https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_middle/mir/visit.rs.html#302) when type-checking the ["required consts"](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Body.html#structfield.required_consts) that are needed by the MIR body during borrowck. This means that when we fail to evaluate the constant, we use the span of `bb0[0]`, instead of the actual span of the constant.

There are quite a few other places that use `START_BLOCK.start_location()`, `Location::START`, etc. when calling for a random/unspecified `Location` value. This is because, unlike (for example) `Span`, we don't have a dummy/miscellaneous value to use instead. I would appreciate guidance (either in this PR, or a follow-up) on what needs to be done to clean this up in general.

Author: JohnTitor

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -357,12 +357,20 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                 .add_element(live_region_vid, location);
         });
 
+        // HACK(compiler-errors): Constants that are gathered into Body.required_consts
+        // have their locations erased...
+        let locations = if location != Location::START {
+            location.to_locations()
+        } else {
+            Locations::All(constant.span)
+        };
+
         if let Some(annotation_index) = constant.user_ty {
             if let Err(terr) = self.cx.relate_type_and_user_type(
                 constant.literal.ty(),
                 ty::Variance::Invariant,
                 &UserTypeProjection { base: annotation_index, projs: vec![] },
-                location.to_locations(),
+                locations,
                 ConstraintCategory::Boring,
             ) {
                 let annotation = &self.cx.user_type_annotations[annotation_index];
@@ -390,12 +398,9 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                                      promoted: &Body<'tcx>,
                                      ty,
                                      san_ty| {
-                        if let Err(terr) = verifier.cx.eq_types(
-                            ty,
-                            san_ty,
-                            location.to_locations(),
-                            ConstraintCategory::Boring,
-                        ) {
+                        if let Err(terr) =
+                            verifier.cx.eq_types(ty, san_ty, locations, ConstraintCategory::Boring)
+                        {
                             span_mirbug!(
                                 verifier,
                                 promoted,
@@ -416,7 +421,7 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                     }
                 } else {
                     if let Err(terr) = self.cx.fully_perform_op(
-                        location.to_locations(),
+                        locations,
                         ConstraintCategory::Boring,
                         self.cx.param_env.and(type_op::ascribe_user_type::AscribeUserType::new(
                             constant.literal.ty(),
@@ -435,7 +440,6 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                 }
             } else if let Some(static_def_id) = constant.check_static_ptr(tcx) {
                 let unnormalized_ty = tcx.type_of(static_def_id);
-                let locations = location.to_locations();
                 let normalized_ty = self.cx.normalize(unnormalized_ty, locations);
                 let literal_ty = constant.literal.ty().builtin_deref(true).unwrap().ty;
 
@@ -454,7 +458,7 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                 self.cx.normalize_and_prove_instantiated_predicates(
                     def_id,
                     instantiated_predicates,
-                    location.to_locations(),
+                    locations,
                 );
             }
         }

src/test/ui/hrtb/hrtb-just-for-static.stderr

@@ -2,7 +2,7 @@ error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-just-for-static.rs:24:5
    |
 LL |     want_hrtb::<StaticInt>()
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |     ^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
    |
    = note: `StaticInt` must implement `Foo<&'0 isize>`, for any lifetime `'0`...
    = note: ...but it actually implements `Foo<&'static isize>`

src/test/ui/nll/issue-97997.rs

@@ -0,0 +1,16 @@
+trait Foo {
+    const ASSOC: bool = true;
+}
+impl<T> Foo for fn(T) {}
+
+fn foo(_x: i32) {}
+
+fn impls_foo<T: Foo>(_x: T) {}
+
+fn main() {
+    impls_foo(foo as fn(i32));
+
+    <fn(&u8) as Foo>::ASSOC;
+    //~^ ERROR implementation of `Foo` is not general enough
+    //~| ERROR implementation of `Foo` is not general enough
+}

src/test/ui/nll/issue-97997.stderr

@@ -0,0 +1,20 @@
+error: implementation of `Foo` is not general enough
+  --> $DIR/issue-97997.rs:13:5
+   |
+LL |     <fn(&u8) as Foo>::ASSOC;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r u8)`
+   = note: ...but `Foo` is actually implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`
+
+error: implementation of `Foo` is not general enough
+  --> $DIR/issue-97997.rs:13:5
+   |
+LL |     <fn(&u8) as Foo>::ASSOC;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
+   |
+   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r u8)`
+   = note: ...but `Foo` is actually implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`
+
+error: aborting due to 2 previous errors
+