ci: fail example builds on warnings

Set RUSTFLAGS=-D warnings in the example Build step so a stray lint
(e.g. an unused import) fails CI directly. Without it, an undenied
warning is replayed onto stdout by "cargo size" during the bloat step
and corrupts the size parser, surfacing as an opaque shell error far
from the real cause.

HEAD keeps -D warnings in the bloat size step too so RUSTFLAGS matches
the Build step and cargo reuses the binary; the BASE build keeps the
default lint level so a pre-existing warning on main can't fail an
unrelated PR.

Author: jpds

.github/workflows/ci.yml

@@ -116,10 +116,14 @@ jobs:
       - name: Build
         working-directory: ${{ matrix.dir }}
         run: |
+          # Deny warnings so a stray lint (e.g. an unused import) fails CI here
+          # instead of slipping through. For bloat examples an undenied warning
+          # gets replayed onto stdout by `cargo size` and corrupts the size parser.
+          export RUSTFLAGS="-D warnings"
           # For bloat examples, remap source paths so the absolute workspace
           # prefix doesn't affect binary size (panic messages embed file!() paths).
           if [[ "${{ matrix.bloat }}" == "true" ]]; then
-            export RUSTFLAGS="--remap-path-prefix=$GITHUB_WORKSPACE=/rmk-build"
+            export RUSTFLAGS="$RUSTFLAGS --remap-path-prefix=$GITHUB_WORKSPACE=/rmk-build"
           fi
           cargo build --release
 
@@ -160,7 +164,10 @@ jobs:
           # virtual prefix.  Without this, absolute paths embedded by panic
           # messages (file!() macro) differ in length between $GITHUB_WORKSPACE
           # and the temp worktree, producing false size diffs in .rodata/.text.
-          head_remap="--remap-path-prefix=$GITHUB_WORKSPACE=/rmk-build"
+          # HEAD also keeps `-D warnings` so RUSTFLAGS matches the Build step
+          # above and cargo reuses the binary; BASE uses the default lint level
+          # so a pre-existing warning on main doesn't fail this PR.
+          head_remap="-D warnings --remap-path-prefix=$GITHUB_WORKSPACE=/rmk-build"
           base_remap="--remap-path-prefix=$worktree=/rmk-build"
 
           base_dir="$worktree/${{ matrix.dir }}"