Auto merge of #49956 - QuietMisdreavus:rustdoc-codegen, r=GuillaumeGomez

rustdoc: port the -C option from rustc

Blocked on https://github.com/rust-lang/rust/pull/49864. The included test won't work without those changes, so this PR includes those commits as well.

When documenting items that require certain target features, it helps to be able to force those target features into existence. Rather than include a flag just to parse those features, i instead decided to port the `-C` flag from rustc in its entirety. It takes the same parameters, because it runs through the same parsing function. This has the added benefit of being able to control the codegen of doctests as well.

One concern i have with the flag is that i set it to stable here. My rationale is that it is a direct port of functionality on rustc that is currently stable, used only in mechanisms that it is originally used for. If needed, i can set it back to be unstable.

src/librustdoc/lib.rs

@@ -61,7 +61,8 @@ use std::sync::mpsc::channel;
 use syntax::edition::Edition;
 use externalfiles::ExternalHtml;
 use rustc::session::search_paths::SearchPaths;
-use rustc::session::config::{ErrorOutputType, RustcOptGroup, nightly_options, Externs};
+use rustc::session::config::{ErrorOutputType, RustcOptGroup, Externs, CodegenOptions};
+use rustc::session::config::{nightly_options, build_codegen_options};
 use rustc_back::target::TargetTriple;
 
 #[macro_use]
@@ -155,6 +156,9 @@ pub fn opts() -> Vec<RustcOptGroup> {
         stable("plugin-path", |o| {
             o.optmulti("", "plugin-path", "directory to load plugins from", "DIR")
         }),
+        stable("C", |o| {
+            o.optmulti("C", "codegen", "pass a codegen option to rustc", "OPT[=VALUE]")
+        }),
         stable("passes", |o| {
             o.optmulti("", "passes",
                        "list of passes to also run, you might want \
@@ -441,14 +445,16 @@ pub fn main_args(args: &[String]) -> isize {
         }
     };
 
+    let cg = build_codegen_options(&matches, ErrorOutputType::default());
+
     match (should_test, markdown_input) {
         (true, true) => {
             return markdown::test(input, cfgs, libs, externs, test_args, maybe_sysroot,
-                                  display_warnings, linker, edition)
+                                  display_warnings, linker, edition, cg)
         }
         (true, false) => {
             return test::run(Path::new(input), cfgs, libs, externs, test_args, crate_name,
-                             maybe_sysroot, display_warnings, linker, edition)
+                             maybe_sysroot, display_warnings, linker, edition, cg)
         }
         (false, true) => return markdown::render(Path::new(input),
                                                  output.unwrap_or(PathBuf::from("doc")),
@@ -458,7 +464,7 @@ pub fn main_args(args: &[String]) -> isize {
     }
 
     let output_format = matches.opt_str("w");
-    let res = acquire_input(PathBuf::from(input), externs, edition, &matches, move |out| {
+    let res = acquire_input(PathBuf::from(input), externs, edition, cg, &matches, move |out| {
         let Output { krate, passes, renderinfo } = out;
         info!("going to format");
         match output_format.as_ref().map(|s| &**s) {
@@ -500,14 +506,15 @@ fn print_error<T>(error_message: T) where T: Display {
 fn acquire_input<R, F>(input: PathBuf,
                        externs: Externs,
                        edition: Edition,
+                       cg: CodegenOptions,
                        matches: &getopts::Matches,
                        f: F)
                        -> Result<R, String>
 where R: 'static + Send, F: 'static + Send + FnOnce(Output) -> R {
     match matches.opt_str("r").as_ref().map(|s| &**s) {
-        Some("rust") => Ok(rust_input(input, externs, edition, matches, f)),
+        Some("rust") => Ok(rust_input(input, externs, edition, cg, matches, f)),
         Some(s) => Err(format!("unknown input format: {}", s)),
-        None => Ok(rust_input(input, externs, edition, matches, f))
+        None => Ok(rust_input(input, externs, edition, cg, matches, f))
     }
 }
 
@@ -536,6 +543,7 @@ fn parse_externs(matches: &getopts::Matches) -> Result<Externs, String> {
 fn rust_input<R, F>(cratefile: PathBuf,
                     externs: Externs,
                     edition: Edition,
+                    cg: CodegenOptions,
                     matches: &getopts::Matches,
                     f: F) -> R
 where R: 'static + Send,
@@ -589,7 +597,7 @@ where R: 'static + Send,
         let (mut krate, renderinfo) =
             core::run_core(paths, cfgs, externs, Input::File(cratefile), triple, maybe_sysroot,
                            display_warnings, crate_name.clone(),
-                           force_unstable_if_unmarked, edition);
+                           force_unstable_if_unmarked, edition, cg);
 
         info!("finished with rustc");