Add experimental feature to link against local copy of std from cargo doc --open

src/bin/cargo/commands/doc.rs

@@ -27,6 +27,12 @@ pub fn cli() -> App {
         .arg_features()
         .arg_target_triple("Build for the target triple")
         .arg_target_dir()
+        .arg(
+            Arg::with_name("use-local-sysroot-docs")
+                .long("use-local-sysroot-docs")
+                .help("Attempt to use local sysroot file:// links if available")
+                .hidden(true),
+        )
         .arg_manifest_path()
         .arg_message_format()
         .after_help(
@@ -48,19 +54,34 @@ the `cargo help pkgid` command.
 
 pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
     let ws = args.workspace(config)?;
+
     let mode = CompileMode::Doc {
         deps: !args.is_present("no-deps"),
     };
+
     let mut compile_opts = args.compile_options(config, mode, Some(&ws))?;
     compile_opts.local_rustdoc_args = if args.is_present("document-private-items") {
         Some(vec!["--document-private-items".to_string()])
     } else {
         None
     };
+
+    let use_local_sysroot_docs = args.is_present("use-local-sysroot-docs");
+    if use_local_sysroot_docs && !config.cli_unstable().unstable_options {
+        return Err(failure::format_err!(
+            "`cargo doc --use-local-sysroot-docs` is unstable, pass `-Z unstable-options` to enable it"
+        )
+        .into());
+    }
+
+    compile_opts.build_config.use_local_sysroot_docs = use_local_sysroot_docs;
+
     let doc_opts = DocOptions {
         open_result: args.is_present("open"),
         compile_opts,
     };
+
     ops::doc(&ws, &doc_opts)?;
+
     Ok(())
 }

src/bin/cargo/commands/rustdoc.rs

@@ -30,6 +30,12 @@ pub fn cli() -> App {
         .arg_features()
         .arg_target_triple("Build for the target triple")
         .arg_target_dir()
+        .arg(
+            Arg::with_name("use-local-sysroot-docs")
+                .long("use-local-sysroot-docs")
+                .help("Attempt to use local sysroot file:// links if available")
+                .hidden(true),
+        )
         .arg_manifest_path()
         .arg_message_format()
         .after_help(
@@ -51,21 +57,36 @@ the `cargo help pkgid` command.
 
 pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
     let ws = args.workspace(config)?;
+
     let mut compile_opts = args.compile_options_for_single_package(
         config,
         CompileMode::Doc { deps: false },
         Some(&ws),
     )?;
+
     let target_args = values(args, "args");
     compile_opts.target_rustdoc_args = if target_args.is_empty() {
         None
     } else {
         Some(target_args)
     };
+
+    let use_local_sysroot_docs = args.is_present("use-local-sysroot-docs");
+    if use_local_sysroot_docs && !config.cli_unstable().unstable_options {
+        return Err(failure::format_err!(
+            "`cargo doc --use-local-sysroot-docs` is unstable, pass `-Z unstable-options` to enable it"
+        )
+        .into());
+    }
+
+    compile_opts.build_config.use_local_sysroot_docs = use_local_sysroot_docs;
+
     let doc_opts = DocOptions {
         open_result: args.is_present("open"),
         compile_opts,
     };
+
     ops::doc(&ws, &doc_opts)?;
+
     Ok(())
 }

src/cargo/core/compiler/build_config.rs

@@ -29,6 +29,8 @@ pub struct BuildConfig {
     pub rustfix_diagnostic_server: RefCell<Option<RustfixDiagnosticServer>>,
     /// Whether or not Cargo should cache compiler output on disk.
     cache_messages: bool,
+    /// Whether rustdoc should attempt to get local std docs
+    pub use_local_sysroot_docs: bool,
 }
 
 impl BuildConfig {
@@ -101,6 +103,7 @@ impl BuildConfig {
             rustc_wrapper: None,
             rustfix_diagnostic_server: RefCell::new(None),
             cache_messages: config.cli_unstable().cache_messages,
+            use_local_sysroot_docs: false,
         })
     }
 

src/cargo/core/compiler/build_context/target_info.rs

@@ -15,6 +15,7 @@ pub struct TargetInfo {
     crate_types: RefCell<HashMap<String, Option<(String, String)>>>,
     cfg: Option<Vec<Cfg>>,
     pub sysroot_libdir: Option<PathBuf>,
+    pub sysroot_dir: Option<PathBuf>,
     pub rustflags: Vec<String>,
     pub rustdocflags: Vec<String>,
 }
@@ -113,6 +114,7 @@ impl TargetInfo {
         }
 
         let mut sysroot_libdir = None;
+        let mut sysroot_dir = None;
         if has_cfg_and_sysroot {
             let line = match lines.next() {
                 Some(line) => line,
@@ -122,6 +124,7 @@ impl TargetInfo {
                 ),
             };
             let mut rustlib = PathBuf::from(line);
+            sysroot_dir = Some(rustlib.clone());
             if kind == Kind::Host {
                 if cfg!(windows) {
                     rustlib.push("bin");
@@ -148,6 +151,7 @@ impl TargetInfo {
             crate_type_process: Some(crate_type_process),
             crate_types: RefCell::new(map),
             sysroot_libdir,
+            sysroot_dir,
             // recalculate `rustflags` from above now that we have `cfg`
             // information
             rustflags: env_args(

src/cargo/core/compiler/mod.rs

@@ -600,9 +600,35 @@ fn prepare_rustc<'a, 'cfg>(
     Ok(base)
 }
 
+fn add_sysroot_doc_args(bcx: &BuildContext<'_, '_>, cmd: &mut ProcessBuilder) {
+    let sysroot_dir = bcx.target_info.sysroot_dir.as_ref();
+
+    if sysroot_dir.is_none() {
+        return;
+    }
+
+    let sysroot_docdir = sysroot_dir.unwrap().join("share/doc/rust/html");
+
+    if !sysroot_docdir.is_dir() || !sysroot_docdir.join("index.html").is_file() {
+        return;
+    }
+
+    let sysroot_docdir = sysroot_docdir.to_string_lossy();
+    cmd.arg("-Zunstable-options");
+    for crate_name in ["std", "core", "proc_macro", "alloc", "test"].iter() {
+        cmd.arg("--extern-html-root-url")
+            .arg(format!("{}={}", crate_name, sysroot_docdir));
+    }
+}
+
 fn rustdoc<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoResult<Work> {
     let bcx = cx.bcx;
     let mut rustdoc = cx.compilation.rustdoc_process(unit.pkg, unit.target)?;
+
+    if bcx.build_config.use_local_sysroot_docs {
+        add_sysroot_doc_args(bcx, &mut rustdoc);
+    }
+
     rustdoc.inherit_jobserver(&cx.jobserver);
     rustdoc.arg("--crate-name").arg(&unit.target.crate_name());
     add_path_args(bcx, unit, &mut rustdoc);