Define a dedicated error type for HandleOrNull and HandleOrInvalid.

[sunfishcode]

Define a NotHandle type, that implements std::error::Error, and use
it as the error type in HandleOrNull and HandleOrInvalid.

This addresses this concern.

r? @joshtriplett

[joshtriplett]

@bors r+

[bors]

📌 Commit 733ef08 has been approved by joshtriplett

[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

37 error: aborting due to 2 previous errors


The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-issue-49593-box-never.nofallback/coerce-issue-49593-box-never.nofallback.stderr
To only update this specific test, also pass `--test-args coercion/coerce-issue-49593-box-never.rs`


error in revision `nofallback`: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/coercion/coerce-issue-49593-box-never.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--cfg" "nofallback" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-issue-49593-box-never.nofallback" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/coerce-issue-49593-box-never.nofallback/auxiliary"
stdout: none
--- stderr -------------------------------
error[E0277]: the trait bound `(): std::error::Error` is not satisfied
   |
   |
LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
   = help: the following other types implement trait `std::error::Error`:
             !
             &'a T
             AccessError
---

error[E0277]: the trait bound `(): std::error::Error` is not satisfied
  --> /checkout/src/test/ui/coercion/coerce-issue-49593-box-never.rs:22:49
   |
LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
   = help: the following other types implement trait `std::error::Error`:
             !
             &'a T
             AccessError

[sunfishcode]

The rollup PR failed because of the diff

-	           and 43 others
+	           and 45 others

I added a commit to fix this, but now this PR is failing with the diff

-	           and 45 others
+	           and 43 others

For now, I'll close this PR and wait until whatever other PR it's racing against settles in, and then re-open it.

[sunfishcode]

After rebasing this, github doesn't apparently allow me to reopen this PR, so I created a new one: #96195.