Point standard library links to stable

Author: dtolnay

Cargo.toml

@@ -14,8 +14,15 @@ rust-version = "1.56"
 
 [package.metadata.docs.rs]
 rustc-args = ["--cfg=procmacro2_semver_exempt"]
-rustdoc-args = ["--cfg=procmacro2_semver_exempt", "--generate-link-to-definition"]
 targets = ["x86_64-unknown-linux-gnu"]
+rustdoc-args = [
+    "--cfg=procmacro2_semver_exempt",
+    "--generate-link-to-definition",
+    "--extern-html-root-url=core=https://doc.rust-lang.org",
+    "--extern-html-root-url=alloc=https://doc.rust-lang.org",
+    "--extern-html-root-url=std=https://doc.rust-lang.org",
+    "--extern-html-root-url=proc_macro=https://doc.rust-lang.org",
+]
 
 [package.metadata.playground]
 features = ["span-locations"]