Support multiple --target flags on the CLI

This commit refactors the internals of Cargo to no longer have a
singular `--target` flag (and singular `requested_target` kind throught)
but to instead have a list. The semantics of multiple `--target` flags
is to build the selected targets for each of the input `--target` flag
inputs.

For now this is gated behind `-Zmultitarget` as an unstable features,
since I'm not entirely sure this is the interface we want. In general
it'd be great if we had a way to simply specify `Unit` structures of
what to build on the CLI, but we're in general very far away from that,
so I figured that this is probably sufficient at least for testing for
now.

cc rust-lang#8156

src/bin/cargo/commands/clean.rs

@@ -28,7 +28,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
     let opts = CleanOptions {
         config,
         spec: values(args, "package"),
-        target: args.target(),
+        targets: args.targets(),
         requested_profile: args.get_profile_name(config, "dev", ProfileChecking::Checked)?,
         profile_specified: args.is_present("profile") || args.is_present("release"),
         doc: args.is_present("doc"),

src/bin/cargo/commands/fetch.rs

@@ -28,7 +28,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
 
     let opts = FetchOptions {
         config,
-        target: args.target(),
+        targets: args.targets(),
     };
     let _ = ops::fetch(&ws, &opts)?;
     Ok(())

src/bin/cargo/commands/metadata.rs

@@ -12,13 +12,11 @@ pub fn cli() -> App {
         )
         .arg(opt("quiet", "No output printed to stdout").short("q"))
         .arg_features()
-        .arg(
-            opt(
-                "filter-platform",
-                "Only include resolve dependencies matching the given target-triple",
-            )
-            .value_name("TRIPLE"),
-        )
+        .arg(multi_opt(
+            "filter-platform",
+            "TRIPLE",
+            "Only include resolve dependencies matching the given target-triple",
+        ))
         .arg(opt(
             "no-deps",
             "Output information only about the workspace members \
@@ -51,7 +49,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
         all_features: args.is_present("all-features"),
         no_default_features: args.is_present("no-default-features"),
         no_deps: args.is_present("no-deps"),
-        filter_platform: args.value_of("filter-platform").map(|s| s.to_string()),
+        filter_platforms: args._values_of("filter-platform"),
         version,
     };
 

src/bin/cargo/commands/package.rs

@@ -42,7 +42,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
             list: args.is_present("list"),
             check_metadata: !args.is_present("no-metadata"),
             allow_dirty: args.is_present("allow-dirty"),
-            target: args.target(),
+            targets: args.targets(),
             jobs: args.jobs()?,
             features: args._values_of("features"),
             all_features: args.is_present("all-features"),

src/bin/cargo/commands/publish.rs

@@ -40,7 +40,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
             index,
             verify: !args.is_present("no-verify"),
             allow_dirty: args.is_present("allow-dirty"),
-            target: args.target(),
+            targets: args.targets(),
             jobs: args.jobs()?,
             dry_run: args.is_present("dry-run"),
             registry,

src/bin/cargo/commands/tree.rs

@@ -129,15 +129,15 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
         )?;
     }
 
-    let target = if args.is_present("all-targets") {
+    let targets = if args.is_present("all-targets") {
         config
             .shell()
             .warn("the --all-targets flag has been changed to --target=all")?;
-        Some("all")
+        vec!["all".to_string()]
     } else {
-        args.value_of("target")
+        args._values_of("target")
     };
-    let target = tree::Target::from_cli(target);
+    let target = tree::Target::from_cli(targets);
 
     let edge_kinds = parse_edge_kinds(config, args)?;
     let graph_features = edge_kinds.contains(&EdgeKind::Feature);

src/cargo/core/compiler/build_config.rs

@@ -2,6 +2,7 @@ use crate::core::compiler::CompileKind;
 use crate::core::interning::InternedString;
 use crate::util::ProcessBuilder;
 use crate::util::{CargoResult, Config, RustfixDiagnosticServer};
+use anyhow::bail;
 use serde::ser;
 use std::cell::RefCell;
 use std::path::PathBuf;
@@ -10,7 +11,7 @@ use std::path::PathBuf;
 #[derive(Debug)]
 pub struct BuildConfig {
     /// The requested kind of compilation for this session
-    pub requested_kind: CompileKind,
+    pub requested_kinds: Vec<CompileKind>,
     /// Number of rustc jobs to run in parallel.
     pub jobs: u32,
     /// Build profile
@@ -50,12 +51,11 @@ impl BuildConfig {
     pub fn new(
         config: &Config,
         jobs: Option<u32>,
-        requested_target: &Option<String>,
+        requested_targets: &[String],
         mode: CompileMode,
     ) -> CargoResult<BuildConfig> {
         let cfg = config.build_config()?;
-        let requested_kind =
-            CompileKind::from_requested_target(config, requested_target.as_deref())?;
+        let requested_kinds = CompileKind::from_requested_targets(config, requested_targets)?;
         if jobs == Some(0) {
             anyhow::bail!("jobs must be at least 1")
         }
@@ -69,7 +69,7 @@ impl BuildConfig {
         let jobs = jobs.or(cfg.jobs).unwrap_or(::num_cpus::get() as u32);
 
         Ok(BuildConfig {
-            requested_kind,
+            requested_kinds,
             jobs,
             requested_profile: InternedString::new("dev"),
             mode,
@@ -95,6 +95,13 @@ impl BuildConfig {
     pub fn test(&self) -> bool {
         self.mode == CompileMode::Test || self.mode == CompileMode::Bench
     }
+
+    pub fn single_requested_kind(&self) -> CargoResult<CompileKind> {
+        match self.requested_kinds.len() {
+            1 => Ok(self.requested_kinds[0]),
+            _ => bail!("only one `--target` argument is supported"),
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]

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

@@ -90,11 +90,18 @@ impl FileType {
 impl TargetInfo {
     pub fn new(
         config: &Config,
-        requested_kind: CompileKind,
+        requested_kinds: &[CompileKind],
         rustc: &Rustc,
         kind: CompileKind,
     ) -> CargoResult<TargetInfo> {
-        let rustflags = env_args(config, requested_kind, &rustc.host, None, kind, "RUSTFLAGS")?;
+        let rustflags = env_args(
+            config,
+            requested_kinds,
+            &rustc.host,
+            None,
+            kind,
+            "RUSTFLAGS",
+        )?;
         let mut process = rustc.process();
         process
             .arg("-")
@@ -180,15 +187,15 @@ impl TargetInfo {
             // information
             rustflags: env_args(
                 config,
-                requested_kind,
+                requested_kinds,
                 &rustc.host,
                 Some(&cfg),
                 kind,
                 "RUSTFLAGS",
             )?,
             rustdocflags: env_args(
                 config,
-                requested_kind,
+                requested_kinds,
                 &rustc.host,
                 Some(&cfg),
                 kind,
@@ -416,7 +423,7 @@ fn output_err_info(cmd: &ProcessBuilder, stdout: &str, stderr: &str) -> String {
 /// scripts, ...), even if it is the same as the target.
 fn env_args(
     config: &Config,
-    requested_kind: CompileKind,
+    requested_kinds: &[CompileKind],
     host_triple: &str,
     target_cfg: Option<&[Cfg]>,
     kind: CompileKind,
@@ -441,7 +448,7 @@ fn env_args(
     // This means that, e.g., even if the specified --target is the
     // same as the host, build scripts in plugins won't get
     // RUSTFLAGS.
-    if !requested_kind.is_host() && kind.is_host() {
+    if requested_kinds != &[CompileKind::Host] && kind.is_host() {
         // This is probably a build script or plugin and we're
         // compiling with --target. In this scenario there are
         // no rustflags we can apply.
@@ -526,20 +533,25 @@ pub struct RustcTargetData {
 }
 
 impl RustcTargetData {
-    pub fn new(ws: &Workspace<'_>, requested_kind: CompileKind) -> CargoResult<RustcTargetData> {
+    pub fn new(
+        ws: &Workspace<'_>,
+        requested_kinds: &[CompileKind],
+    ) -> CargoResult<RustcTargetData> {
         let config = ws.config();
         let rustc = config.load_global_rustc(Some(ws))?;
         let host_config = config.target_cfg_triple(&rustc.host)?;
-        let host_info = TargetInfo::new(config, requested_kind, &rustc, CompileKind::Host)?;
+        let host_info = TargetInfo::new(config, requested_kinds, &rustc, CompileKind::Host)?;
         let mut target_config = HashMap::new();
         let mut target_info = HashMap::new();
-        if let CompileKind::Target(target) = requested_kind {
-            let tcfg = config.target_cfg_triple(target.short_name())?;
-            target_config.insert(target, tcfg);
-            target_info.insert(
-                target,
-                TargetInfo::new(config, requested_kind, &rustc, CompileKind::Target(target))?,
-            );
+        for kind in requested_kinds {
+            if let CompileKind::Target(target) = *kind {
+                let tcfg = config.target_cfg_triple(target.short_name())?;
+                target_config.insert(target, tcfg);
+                target_info.insert(
+                    target,
+                    TargetInfo::new(config, requested_kinds, &rustc, *kind)?,
+                );
+            }
         }
 
         Ok(RustcTargetData {