Fix rust-docs in CI.

Author: ehuss

ci/azure-install-rust.yml

@@ -5,7 +5,7 @@ steps:
       rustup component remove --toolchain=$TOOLCHAIN rust-docs || echo "already removed"
       rustup update --no-self-update $TOOLCHAIN
       if [[ "$TOOLCHAIN" == "nightly"* ]]; then
-        rustup component add --toolchain=$TOOLCHAIN rustc-dev llvm-tools-preview
+        rustup component add --toolchain=$TOOLCHAIN rustc-dev llvm-tools-preview rust-docs
       fi
       rustup default $TOOLCHAIN
     displayName: Install rust

tests/testsuite/rustdoc_extern_html.rs

@@ -78,10 +78,12 @@ fn std_docs() {
         // --extern-html-root-url is unstable
         return;
     }
-    if !cargo::util::is_ci() {
-        // For local developers, skip this test if docs aren't installed.
-        let docs = std::path::Path::new(&paths::sysroot()).join("share/doc/rust/html");
-        if !docs.exists() {
+    // For local developers, skip this test if docs aren't installed.
+    let docs = std::path::Path::new(&paths::sysroot()).join("share/doc/rust/html");
+    if !docs.exists() {
+        if cargo::util::is_ci() {
+            panic!("std docs are not installed, check that the rust-docs component is installed");
+        } else {
             eprintln!(
                 "documentation not found at {}, \
                 skipping test (run `rustdoc component add rust-docs` to install",