Rollup merge of #87088 - FabianWolff:issue-87060, r=estebank

Fix stray notes when the source code is not available

Fixes #87060. To reproduce it with a local build of rustc, you have to copy the compiler (e.g. `build/x86_64-unknown-linux-gnu/stage1/`) somewhere and then rename the compiler source directory (maybe there is a smarter way as well). Then, rustc won't find the standard library sources and report stray notes such as
```
note: deref defined here
```
with no location for "here". Another example I've found is this:
```rust
use std::ops::Add;

fn foo<T: Add<Output=()>>(x: T) {
    x + x;
}

fn main() {}
```
```
error[E0382]: use of moved value: `x`
  --> binop.rs:4:9
   |
3  | fn foo<T: Add<Output=()>>(x: T) {
   |                           - move occurs because `x` has type `T`, which does not implement the `Copy` trait
4  |     x + x;
   |     ----^
   |     |   |
   |     |   value used here after move
   |     `x` moved due to usage in operator
   |
note: calling this operator moves the left-hand side
help: consider further restricting this bound
   |
3  | fn foo<T: Add<Output=()> + Copy>(x: T) {
   |                          ^^^^^^

error: aborting due to previous error
```
where, again, the note is supposed to point somewhere but doesn't. I have fixed this by checking whether the corresponding source code is actually available before emitting the note.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -218,7 +218,19 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                             );
                             if self.fn_self_span_reported.insert(fn_span) {
                                 err.span_note(
-                                    self_arg.span,
+                                    // Check whether the source is accessible
+                                    if self
+                                        .infcx
+                                        .tcx
+                                        .sess
+                                        .source_map()
+                                        .span_to_snippet(self_arg.span)
+                                        .is_ok()
+                                    {
+                                        self_arg.span
+                                    } else {
+                                        fn_call_span
+                                    },
                                     "calling this operator moves the left-hand side",
                                 );
                             }
@@ -429,7 +441,10 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     deref_target_ty
                 ));
 
-                err.span_note(deref_target, "deref defined here");
+                // Check first whether the source is accessible (issue #87060)
+                if self.infcx.tcx.sess.source_map().span_to_snippet(deref_target).is_ok() {
+                    err.span_note(deref_target, "deref defined here");
+                }
             }
 
             if let Some((_, mut old_err)) =