travis: add a rustls build

... that doesn't run any tests (yet)

Closes #6750

.travis.yml

@@ -124,6 +124,16 @@ jobs:
         - libpsl-dev
         - libbrotli-dev
         - libzstd-dev
+  - env:
+    - T=debug RUSTLS="yes" C="--with-rustls=$HOME/crust --without-ssl" NOTESTS=1
+    addons:
+      apt:
+        <<: *common_apt
+        packages:
+        - *common_packages
+        - libpsl-dev
+        - libbrotli-dev
+        - libzstd-dev
   - env:
     - T=novalgrind LIBRESSL=yes C="--with-ssl=$HOME/libressl" LD_LIBRARY_PATH=/home/travis/libressl/lib:/usr/local/lib
     - OVERRIDE_CC="CC=gcc-8" OVERRIDE_CXX="CXX=g++-8"

scripts/travis/before_script.sh

@@ -126,6 +126,17 @@ if [ "$TRAVIS_OS_NAME" = linux -a "$QUICHE" ]; then
   ln -vnf $(find target/release -name libcrypto.a -o -name libssl.a) deps/boringssl/src/lib/
 fi
 
+if [ "$TRAVIS_OS_NAME" = linux -a "$RUSTLS" ]; then
+  cd $HOME
+  git clone --depth=1 --recursive https://github.com/abetterinternet/crustls.git
+  curl https://sh.rustup.rs -sSf | sh -s -- -y
+  source $HOME/.cargo/env
+  cargo install cbindgen
+  cd $HOME/crustls
+  make
+  make DESTDIR=$HOME/crust install
+fi
+
 # Install common libraries.
 # The library build directories are set to be cached by .travis.yml. If you are
 # changing a build directory name below (eg a version change) then you must