Add files to expose the currently used external types. (#1303)

To get ready for release candidate, we should keep track of these as it
affects how stable we can consider being.

Add check to CI.

.github/workflows/ci.yml

@@ -48,6 +48,23 @@ jobs:
         args: --all -- --check
     - name: Lint
       run: ./scripts/lint.sh
+  external-types:
+    strategy:
+      matrix:
+        example: [opentelemetry, opentelemetry-sdk]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          components: rustfmt
+          override: true
+      - name: external-type-check
+        run: |
+          cargo install cargo-check-external-types
+          cd ${{ matrix.example }}
+          cargo check-external-types --config allowed-external-types.toml
   non-default-examples:
     strategy:
       matrix:

opentelemetry-sdk/allowed-external-types.toml

@@ -0,0 +1,18 @@
+# Copyright The OpenTelemetry Authors
+# SPDX-License-Identifier: Apache-2.0
+# This is used with cargo-check-external-types to reduce the surface area of downstream crates from
+# the public API. Ideally this can have a few exceptions as possible.
+allowed_external_types = [
+    "opentelemetry::*",
+    "async_channel::Receiver",
+    "async_channel::Sender",
+    "async_std::stream::interval::Interval",
+    "futures_channel::oneshot::Sender", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "futures_core::future::BoxFuture", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "futures_core::stream::Stream", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "opentelemetry_http::HttpClient", # TODO: We probably shouldn't be depending on another SDK not in the API from the SDK.
+    "tokio_stream::wrappers::interval::IntervalStream", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "tokio_stream::wrappers::mpsc_bounded::ReceiverStream", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "tokio::sync::mpsc::bounded::Sender",
+    "tokio::time::sleep::Sleep",
+]

opentelemetry/allowed-external-types.toml

@@ -0,0 +1,19 @@
+# Copyright The OpenTelemetry Authors
+# SPDX-License-Identifier: Apache-2.0
+# This is used with cargo-check-external-types to reduce the surface area of downstream crates from
+# the public API. Ideally this can have a few exceptions as possible.
+allowed_external_types = [
+    "indexmap::map::core::Entry",
+    "indexmap::map::iter::Drain",
+    "indexmap::map::iter::IterMut",
+    "indexmap::map::iter::Iter",
+    "indexmap::map::iter::IntoIter",
+    "indexmap::map::iter::IntoKeys",
+    "indexmap::map::iter::Keys",
+    "indexmap::map::iter::IntoValues",
+    "indexmap::map::iter::Values",
+    "indexmap::map::iter::ValuesMut",
+    "equivalent::Equivalent",
+    "futures_sink::Sink", # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+    "futures_core::stream::Stream",  # TODO: This is a pre-1.0 crate, we can't easily stabilize with this in the public API
+]