Improve compile errors for unimplemented traits (#1436)

* Improve `debug_handler` to use the correct span for specific bounds

This results in better localised error messages, as they now point
directly to the corresponding argument instead of to the macro itself.

* Improve some error messages behind a `nightly-error-messages` feature
flag

This uses the nightly only `rustc_on_unimplemented` attribute to improve
some error messages when users try to use invalid handler functions.
This should be seen as prove of concept, not as full solution for all
potential error cases.

The underlying feature is currently marked as permanently unstable, but
I'm working on getting this specific attribute (or an attribute with
different name, similar functionality) ready to work on a stable compiler.

* Apply suggestions from code review

Co-authored-by: Jonas Platte <[email protected]>

* Enable the `nightly-error-messages` feature unconditionally for nightly compilers

* Use a nightly compiler to run the axum-marcos compile fail tests

* update to newer nightly

* Run axum-macros tests on nightly

* tweak compile error hints a bit

* more tweaks

* update test

Co-authored-by: Jonas Platte <[email protected]>
Co-authored-by: David Pedersen <[email protected]>

.github/workflows/CI.yml

@@ -75,8 +75,6 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        # nightly has an ICE, so ignore it for now
-        # rust: [stable, beta, nightly]
         rust: [stable, beta]
     steps:
     - uses: actions/checkout@master
@@ -92,6 +90,23 @@ jobs:
         command: test
         args: --all --all-features --all-targets
 
+  test-nightly:
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - uses: actions-rs/toolchain@v1
+      with:
+        # same as `axum-macros/rust-toolchain`
+        toolchain: nightly-2022-11-18
+        override: true
+        profile: minimal
+    - uses: Swatinem/rust-cache@v1
+    - name: Run nightly tests
+      working-directory: axum-macros
+      run: |
+        cargo test
+
   # some examples doesn't support our MSRV (such as async-graphql)
   # so we only test axum itself on our MSRV
   test-msrv: