Add unstable --output-format option to "rustdoc"

We achieved this by:
   * Handle `--output-format` argument, accepting `html` or `json`
   * If `json` is passed, we append the following to
     `compile_opts.target_rustc_args`:
     	1. `-Zunstable-commands`
	2. `--output-format`

src/bin/cargo/commands/doc.rs

@@ -47,13 +47,15 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let ws = args.workspace(config)?;
     let mode = CompileMode::Doc {
         deps: !args.flag("no-deps"),
+        json: false,
     };
     let mut compile_opts =
         args.compile_options(config, mode, Some(&ws), ProfileChecking::Custom)?;
     compile_opts.rustdoc_document_private_items = args.flag("document-private-items");
 
     let doc_opts = DocOptions {
         open_result: args.flag("open"),
+        output_format: ops::OutputFormat::Html,
         compile_opts,
     };
     ops::doc(&ws, &doc_opts)?;

src/bin/cargo/commands/rustdoc.rs

@@ -1,4 +1,6 @@
-use cargo::ops::{self, DocOptions};
+use std::str::FromStr;
+
+use cargo::ops::{self, DocOptions, OutputFormat};
 
 use crate::command_prelude::*;
 
@@ -35,6 +37,10 @@ pub fn cli() -> Command {
         .arg_features()
         .arg_target_triple("Build for the target triple")
         .arg_target_dir()
+        .arg(
+            opt("output-format", "the output type to write (unstable)")
+                .value_parser(["html", "json"]),
+        )
         .arg_manifest_path()
         .arg_message_format()
         .arg_unit_graph()
@@ -45,20 +51,40 @@ pub fn cli() -> Command {
 
 pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let ws = args.workspace(config)?;
+    let output_format = if let Some(output_format) = args._value_of("output-format") {
+        OutputFormat::from_str(output_format).unwrap()
+    } else {
+        OutputFormat::Html
+    };
     let mut compile_opts = args.compile_options_for_single_package(
         config,
-        CompileMode::Doc { deps: false },
+        CompileMode::Doc {
+            deps: false,
+            json: matches!(output_format, OutputFormat::Json),
+        },
         Some(&ws),
         ProfileChecking::Custom,
     )?;
-    let target_args = values(args, "args");
+    let mut target_args = values(args, "args");
+
+    if matches!(output_format, OutputFormat::Json) {
+        config
+            .cli_unstable()
+            .fail_if_stable_opt("--output-format", 12103)?;
+        // Not sure about this. @charmitro
+        target_args.push("-Zunstable-options".to_string());
+        target_args.push("--output-format=json".to_string());
+    }
+
     compile_opts.target_rustdoc_args = if target_args.is_empty() {
         None
     } else {
         Some(target_args)
     };
+
     let doc_opts = DocOptions {
         open_result: args.flag("open"),
+        output_format: OutputFormat::Html,
         compile_opts,
     };
     ops::doc(&ws, &doc_opts)?;

src/cargo/core/compiler/build_config.rs

@@ -177,7 +177,8 @@ pub enum CompileMode {
     Bench,
     /// A target that will be documented with `rustdoc`.
     /// If `deps` is true, then it will also document all dependencies.
-    Doc { deps: bool },
+    /// if `json` is true, the documentation output is in json format.
+    Doc { deps: bool, json: bool },
     /// A target that will be tested with `rustdoc`.
     Doctest,
     /// An example or library that will be scraped for function calls by `rustdoc`.

src/cargo/core/compiler/context/compilation_files.rs

@@ -435,11 +435,16 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
         bcx: &BuildContext<'a, 'cfg>,
     ) -> CargoResult<Arc<Vec<OutputFile>>> {
         let ret = match unit.mode {
-            CompileMode::Doc { .. } => {
-                let path = self
-                    .out_dir(unit)
-                    .join(unit.target.crate_name())
-                    .join("index.html");
+            CompileMode::Doc { json, .. } => {
+                let path = if json {
+                    self.out_dir(unit)
+                        .join(format!("{}.json", unit.target.crate_name()))
+                } else {
+                    self.out_dir(unit)
+                        .join(unit.target.crate_name())
+                        .join("index.html")
+                };
+
                 vec![OutputFile {
                     path,
                     hardlink: None,

src/cargo/core/compiler/unit_dependencies.rs

@@ -632,7 +632,11 @@ fn compute_deps_doc(
         )?;
         ret.push(lib_unit_dep);
         if dep_lib.documented() {
-            if let CompileMode::Doc { deps: true } = unit.mode {
+            if let CompileMode::Doc {
+                deps: true,
+                json: true | false,
+            } = unit.mode
+            {
                 // Document this lib as well.
                 let doc_unit_dep = new_unit_dep(
                     state,

src/cargo/ops/cargo_compile/mod.rs

@@ -417,7 +417,12 @@ pub fn create_bcx<'a, 'cfg>(
 
     // TODO: In theory, Cargo should also dedupe the roots, but I'm uncertain
     // what heuristics to use in that case.
-    if build_config.mode == (CompileMode::Doc { deps: true }) {
+    if build_config.mode
+        == (CompileMode::Doc {
+            deps: true,
+            json: true || false,
+        })
+    {
         remove_duplicate_doc(build_config, &units, &mut unit_graph);
     }
 

src/cargo/ops/cargo_doc.rs

@@ -5,21 +5,50 @@ use crate::util::CargoResult;
 use std::path::Path;
 use std::path::PathBuf;
 use std::process::Command;
+use std::str::FromStr;
+
+/// Format of rustdoc [`--output-format`][1].
+///
+/// [1]: https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#-w--output-format-output-format
+#[derive(Debug, Default, Clone)]
+pub enum OutputFormat {
+    #[default]
+    Html,
+    Json,
+}
+
+impl FromStr for OutputFormat {
+    type Err = ();
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "json" => Ok(Self::Json),
+            "html" => Ok(Self::Html),
+            _ => Err(()),
+        }
+    }
+}
 
 /// Strongly typed options for the `cargo doc` command.
 #[derive(Debug)]
 pub struct DocOptions {
     /// Whether to attempt to open the browser after compiling the docs
     pub open_result: bool,
+    /// Same as `rustdoc --output-format`
+    pub output_format: OutputFormat,
     /// Options to pass through to the compiler
     pub compile_opts: ops::CompileOptions,
 }
 
 /// Main method for `cargo doc`.
 pub fn doc(ws: &Workspace<'_>, options: &DocOptions) -> CargoResult<()> {
+    if options.open_result && matches!(options.output_format, OutputFormat::Json) {
+        anyhow::bail!("`--open` is not allowed with `json` output format.");
+    }
+
     let compilation = ops::compile(ws, &options.compile_opts)?;
 
-    if options.open_result {
+    if options.open_result && matches!(options.output_format, OutputFormat::Html) {
         let name = &compilation
             .root_crate_names
             .get(0)

src/cargo/ops/mod.rs

@@ -5,7 +5,7 @@ pub use self::cargo_compile::{
     compile, compile_with_exec, compile_ws, create_bcx, print, resolve_all_features, CompileOptions,
 };
 pub use self::cargo_compile::{CompileFilter, FilterRule, LibRule, Packages};
-pub use self::cargo_doc::{doc, DocOptions};
+pub use self::cargo_doc::{doc, DocOptions, OutputFormat};
 pub use self::cargo_fetch::{fetch, FetchOptions};
 pub use self::cargo_generate_lockfile::generate_lockfile;
 pub use self::cargo_generate_lockfile::update_lockfile;

src/doc/man/cargo-doc.md

@@ -112,6 +112,7 @@ and supports common Unix glob patterns.
 {{#options}}
 {{> options-jobs }}
 {{> options-keep-going }}
+{{> options-output-format }}
 {{/options}}
 
 {{> section-environment }}

src/doc/man/cargo-rustdoc.md

@@ -100,6 +100,7 @@ if its name is the same as the lib target. Binaries are skipped if they have
 {{#options}}
 {{> options-jobs }}
 {{> options-keep-going }}
+{{> options-output-format }}
 {{/options}}
 
 {{> section-environment }}

src/doc/man/generated_txt/cargo-doc.txt

@@ -305,6 +305,9 @@ OPTIONS
            than aborting the build on the first one that fails to build.
            Unstable, requires -Zunstable-options.
 
+       --output-format
+           The output type to write. Unstable, requires -Zunstable-options.
+
 ENVIRONMENT
        See the reference
        <https://doc.rust-lang.org/cargo/reference/environment-variables.html>

src/doc/man/generated_txt/cargo-rustdoc.txt

@@ -321,6 +321,9 @@ OPTIONS
            than aborting the build on the first one that fails to build.
            Unstable, requires -Zunstable-options.
 
+       --output-format
+           The output type to write. Unstable, requires -Zunstable-options.
+
 ENVIRONMENT
        See the reference
        <https://doc.rust-lang.org/cargo/reference/environment-variables.html>

src/doc/man/includes/options-output-format.md

@@ -0,0 +1,4 @@
+{{#option "`--output-format`"}}
+The output type to write.
+Unstable, requires `-Zunstable-options`.
+{{/option}}

src/doc/src/commands/cargo-doc.md

@@ -368,6 +368,11 @@ the build on the first one that fails to build. Unstable, requires
 <code>-Zunstable-options</code>.</dd>
 
 
+<dt class="option-term" id="option-cargo-doc---output-format"><a class="option-anchor" href="#option-cargo-doc---output-format"></a><code>--output-format</code></dt>
+<dd class="option-desc">The output type to write.
+Unstable, requires <code>-Zunstable-options</code>.</dd>
+
+
 </dl>
 
 ## ENVIRONMENT

src/doc/src/commands/cargo-rustdoc.md

@@ -387,6 +387,11 @@ the build on the first one that fails to build. Unstable, requires
 <code>-Zunstable-options</code>.</dd>
 
 
+<dt class="option-term" id="option-cargo-rustdoc---output-format"><a class="option-anchor" href="#option-cargo-rustdoc---output-format"></a><code>--output-format</code></dt>
+<dd class="option-desc">The output type to write.
+Unstable, requires <code>-Zunstable-options</code>.</dd>
+
+
 </dl>
 
 ## ENVIRONMENT

src/etc/man/cargo-doc.1

@@ -371,6 +371,12 @@ Build as many crates in the dependency graph as possible, rather than aborting
 the build on the first one that fails to build. Unstable, requires
 \fB\-Zunstable\-options\fR\&.
 .RE
+.sp
+\fB\-\-output\-format\fR
+.RS 4
+The output type to write.
+Unstable, requires \fB\-Zunstable\-options\fR\&.
+.RE
 .SH "ENVIRONMENT"
 See \fIthe reference\fR <https://doc.rust\-lang.org/cargo/reference/environment\-variables.html> for
 details on environment variables that Cargo reads.

src/etc/man/cargo-rustdoc.1

@@ -390,6 +390,12 @@ Build as many crates in the dependency graph as possible, rather than aborting
 the build on the first one that fails to build. Unstable, requires
 \fB\-Zunstable\-options\fR\&.
 .RE
+.sp
+\fB\-\-output\-format\fR
+.RS 4
+The output type to write.
+Unstable, requires \fB\-Zunstable\-options\fR\&.
+.RE
 .SH "ENVIRONMENT"
 See \fIthe reference\fR <https://doc.rust\-lang.org/cargo/reference/environment\-variables.html> for
 details on environment variables that Cargo reads.

tests/testsuite/rustdoc.rs

@@ -20,6 +20,58 @@ fn rustdoc_simple() {
         .run();
 }
 
+#[cargo_test(nightly, reason = "--output-format is unstable")]
+fn rustdoc_simple_json() {
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("rustdoc -Z unstable-options --output-format json -v")
+        .masquerade_as_nightly_cargo(&["-Zunstable-options"])
+        .with_stderr(
+            "\
+[DOCUMENTING] foo v0.0.1 ([CWD])
+[RUNNING] `rustdoc [..]--crate-name foo src/lib.rs [..]\
+        -o [CWD]/target/doc \
+        [..] \
+        -L dependency=[CWD]/target/debug/deps [..]`
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}
+
+#[cargo_test(nightly, reason = "--output-format is unstable")]
+fn rustdoc_invalid_output_format() {
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("rustdoc -Z unstable-options --output-format pdf -v")
+        .masquerade_as_nightly_cargo(&["unstable-options"])
+        .with_status(1)
+        .with_stderr(
+            "\
+error: invalid value 'pdf' for '--output-format <output-format>'
+  [possible values: html, json]
+
+For more information, try '--help'.
+",
+        )
+        .run();
+}
+
+#[cargo_test(nightly, reason = "--output-format is unstable")]
+fn rustdoc_simple_json_fail() {
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("rustdoc -Z unstable-options --output-format json -v")
+        .with_status(101)
+        .with_stderr(
+            "\
+error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
+[..]
+",
+	)
+        .run();
+}
+
 #[cargo_test]
 fn rustdoc_args() {
     let p = project().file("src/lib.rs", "").build();