Make doc CI use nightly (#16147)

# Objective

Fixes #15427
Follow-up to #15428

## Solution

Use nightly and add the environment variables as suggested here:
https://github.com/bevyengine/bevy/pull/15428#pullrequestreview-2331294421

## Testing

CI should run

.github/workflows/ci.yml

@@ -127,7 +127,7 @@ jobs:
       - name: Check Compile
         # See tools/ci/src/main.rs for the commands this runs
         run: cargo run -p ci -- compile
-  
+
   check-compiles-no-std:
     runs-on: ubuntu-latest
     timeout-minutes: 30
@@ -301,7 +301,7 @@ jobs:
         with:
           name: example-run-macos
           path: example-run/
-        
+
   check-doc:
     runs-on: ubuntu-latest
     timeout-minutes: 30
@@ -316,7 +316,9 @@ jobs:
             ~/.cargo/git/db/
             target/
           key: ${{ runner.os }}-check-doc-${{ hashFiles('**/Cargo.toml') }}
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ env.NIGHTLY_TOOLCHAIN }}
       - name: Install Linux dependencies
         uses: ./.github/actions/install-linux-deps
         with:
@@ -327,7 +329,7 @@ jobs:
         run: cargo run -p ci -- doc
         env:
           CARGO_INCREMENTAL: 0
-          RUSTFLAGS: "-C debuginfo=0"
+          RUSTFLAGS: "-C debuginfo=0 --cfg docsrs_dep"
       # This currently report a lot of false positives
       # Enable it again once it's fixed - https://github.com/bevyengine/bevy/issues/1983
       # - name: Installs cargo-deadlinks

.github/workflows/weekly.yml

@@ -75,7 +75,7 @@ jobs:
         run: cargo run -p ci -- doc
         env:
           CARGO_INCREMENTAL: 0
-          RUSTFLAGS: "-C debuginfo=0"
+          RUSTFLAGS: "-C debuginfo=0 --cfg docsrs_dep"
 
   open-issue:
     name: Warn that weekly CI fails
@@ -83,7 +83,7 @@ jobs:
     needs: [test, lint, check-compiles, check-doc]
     permissions:
       issues: write
-    # Use always() so the job doesn't get canceled if any other jobs fail 
+    # Use always() so the job doesn't get canceled if any other jobs fail
     if: ${{ always() && contains(needs.*.result, 'failure') }}
     steps:
       - name: Create issue
@@ -94,7 +94,7 @@ jobs:
             --jq '.[0].number')
           if [[ -n $previous_issue_number ]]; then
             gh issue comment $previous_issue_number \
-              --body "Weekly pipeline still fails: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}" 
+              --body "Weekly pipeline still fails: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}"
           else
             gh issue create \
               --title "$TITLE" \
@@ -106,6 +106,6 @@ jobs:
           GH_REPO: ${{ github.repository }}
           TITLE: Main branch fails to compile on Rust beta.
           LABELS: C-Bug,S-Needs-Triage
-          BODY: | 
+          BODY: |
             ## Weekly CI run has failed.
             [The offending run.](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }})

tools/ci/src/commands/doc_check.rs

@@ -16,6 +16,6 @@ impl Prepare for DocCheckCommand {
             ),
             "Please fix doc warnings in output above.",
         )
-        .with_env_var("RUSTDOCFLAGS", "-D warnings")]
+        .with_env_var("RUSTDOCFLAGS", "-D warnings --cfg=docsrs")]
     }
 }