THRIFT-5158 Update Rust generator and Rust lib,test,tutorial to only support 2018 edition
Client: rs
Patch: Allen George

This closes #2078
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index 247bcf1..cfa4041 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -248,7 +248,7 @@
       ruby-bundler
 
 # Rust dependencies
-RUN curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.35.0 -y
+RUN curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.40.0 -y
 ENV PATH /root/.cargo/bin:$PATH
 
 # Swift on Linux for cross tests