fix(cli): Help users know possible --target values

src/bin/cargo/cli.rs

@@ -12,6 +12,7 @@ use std::fmt::Write;
 use super::commands;
 use super::list_commands;
 use crate::command_prelude::*;
+use crate::util::is_rustup;
 use cargo::core::features::HIDDEN;
 
 pub fn main(config: &mut LazyConfig) -> CliResult {
@@ -511,11 +512,7 @@ impl GlobalArgs {
 }
 
 pub fn cli() -> Command {
-    // ALLOWED: `RUSTUP_HOME` should only be read from process env, otherwise
-    // other tools may point to executables from incompatible distributions.
-    #[allow(clippy::disallowed_methods)]
-    let is_rustup = std::env::var_os("RUSTUP_HOME").is_some();
-    let usage = if is_rustup {
+    let usage = if is_rustup() {
         "cargo [+toolchain] [OPTIONS] [COMMAND]\n       cargo [+toolchain] [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]..."
     } else {
         "cargo [OPTIONS] [COMMAND]\n       cargo [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]..."

src/bin/cargo/commands/clean.rs

@@ -27,7 +27,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let opts = CleanOptions {
         config,
         spec: values(args, "package"),
-        targets: args.targets(),
+        targets: args.targets()?,
         requested_profile: args.get_profile_name(config, "dev", ProfileChecking::Custom)?,
         profile_specified: args.contains_id("profile") || args.flag("release"),
         doc: args.flag("doc"),

src/bin/cargo/commands/fetch.rs

@@ -17,7 +17,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
 
     let opts = FetchOptions {
         config,
-        targets: args.targets(),
+        targets: args.targets()?,
     };
     let _ = ops::fetch(&ws, &opts)?;
     Ok(())

src/bin/cargo/commands/package.rs

@@ -58,7 +58,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
             check_metadata: !args.flag("no-metadata"),
             allow_dirty: args.flag("allow-dirty"),
             to_package: specs,
-            targets: args.targets(),
+            targets: args.targets()?,
             jobs: args.jobs()?,
             keep_going: args.keep_going(),
             cli_features: args.cli_features()?,

src/bin/cargo/commands/publish.rs

@@ -50,7 +50,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
             verify: !args.flag("no-verify"),
             allow_dirty: args.flag("allow-dirty"),
             to_publish: args.packages_from_flags()?,
-            targets: args.targets(),
+            targets: args.targets()?,
             jobs: args.jobs()?,
             keep_going: args.keep_going(),
             dry_run: args.dry_run(),

src/bin/cargo/commands/tree.rs

@@ -136,7 +136,7 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
             .warn("the --all-targets flag has been changed to --target=all")?;
         vec!["all".to_string()]
     } else {
-        args._values_of("target")
+        args.targets()?
     };
     let target = tree::Target::from_cli(targets);
 

src/cargo/util/command_prelude.rs

@@ -4,6 +4,7 @@ use crate::core::{Edition, Workspace};
 use crate::ops::{CompileFilter, CompileOptions, NewOptions, Packages, VersionControl};
 use crate::util::important_paths::find_root_manifest_for_wd;
 use crate::util::interning::InternedString;
+use crate::util::is_rustup;
 use crate::util::restricted_names::is_glob_pattern;
 use crate::util::toml::{StringOrVec, TomlProfile};
 use crate::util::validate_package_name;
@@ -218,7 +219,10 @@ pub trait CommandExt: Sized {
     }
 
     fn arg_target_triple(self, target: &'static str) -> Self {
-        self._arg(multi_opt("target", "TRIPLE", target).help_heading(heading::COMPILATION_OPTIONS))
+        self._arg(
+            optional_multi_opt("target", "TRIPLE", target)
+                .help_heading(heading::COMPILATION_OPTIONS),
+        )
     }
 
     fn arg_target_dir(self) -> Self {
@@ -440,8 +444,20 @@ pub trait ArgMatchesExt {
         self.maybe_flag("keep-going")
     }
 
-    fn targets(&self) -> Vec<String> {
-        self._values_of("target")
+    fn targets(&self) -> CargoResult<Vec<String>> {
+        if self.is_present_with_zero_values("target") {
+            let cmd = if is_rustup() {
+                "rustup target list"
+            } else {
+                "rustc --print target-list"
+            };
+            bail!(
+                "\"--target\" takes a target architecture as an argument.
+
+Run `{cmd}` to see possible targets."
+            );
+        }
+        Ok(self._values_of("target"))
     }
 
     fn get_profile_name(
@@ -590,7 +606,7 @@ pub trait ArgMatchesExt {
             config,
             self.jobs()?,
             self.keep_going(),
-            &self.targets(),
+            &self.targets()?,
             mode,
         )?;
         build_config.message_format = message_format.unwrap_or(MessageFormat::Human);

src/cargo/util/mod.rs

@@ -66,6 +66,13 @@ pub mod toml_mut;
 mod vcs;
 mod workspace;
 
+pub fn is_rustup() -> bool {
+    // ALLOWED: `RUSTUP_HOME` should only be read from process env, otherwise
+    // other tools may point to executables from incompatible distributions.
+    #[allow(clippy::disallowed_methods)]
+    std::env::var_os("RUSTUP_HOME").is_some()
+}
+
 pub fn elapsed(duration: Duration) -> String {
     let secs = duration.as_secs();
 

tests/testsuite/cargo_bench/help/stdout.log

@@ -45,7 +45,7 @@ Feature Selection:
 Compilation Options:
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_build/help/stdout.log

@@ -42,7 +42,7 @@ Compilation Options:
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
       --keep-going              Do not abort the build as soon as there is an error
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --out-dir <PATH>          Copy final artifacts to this directory (unstable)
       --build-plan              Output the build plan in JSON (unstable)

tests/testsuite/cargo_check/help/stdout.log

@@ -42,7 +42,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Check artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Check artifacts with the specified profile
-      --target <TRIPLE>         Check for the target triple
+      --target [<TRIPLE>]       Check for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_clean/help/stdout.log

@@ -17,7 +17,7 @@ Package Selection:
 Compilation Options:
   -r, --release                 Whether or not to clean release artifacts
       --profile <PROFILE-NAME>  Clean artifacts of the specified profile
-      --target <TRIPLE>         Target triple to clean output for
+      --target [<TRIPLE>]       Target triple to clean output for
       --target-dir <DIRECTORY>  Directory for all generated artifacts
 
 Manifest Options:

tests/testsuite/cargo_doc/help/stdout.log

@@ -39,7 +39,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Build artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_fetch/help/stdout.log

@@ -11,7 +11,7 @@ Options:
   -h, --help                Print help
 
 Compilation Options:
-      --target <TRIPLE>  Fetch dependencies for the target triple
+      --target [<TRIPLE>]  Fetch dependencies for the target triple
 
 Manifest Options:
       --manifest-path <PATH>  Path to Cargo.toml

tests/testsuite/cargo_fix/help/stdout.log

@@ -47,7 +47,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Fix artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Fix for the target triple
+      --target [<TRIPLE>]       Fix for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json
 

tests/testsuite/cargo_install/help/stdout.log

@@ -44,7 +44,7 @@ Compilation Options:
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
       --keep-going              Do not abort the build as soon as there is an error
       --profile <PROFILE-NAME>  Install artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json
 

tests/testsuite/cargo_package/help/stdout.log

@@ -25,7 +25,7 @@ Feature Selection:
       --no-default-features  Do not activate the `default` feature
 
 Compilation Options:
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
       --keep-going              Do not abort the build as soon as there is an error

tests/testsuite/cargo_publish/help/stdout.log

@@ -27,7 +27,7 @@ Feature Selection:
 Compilation Options:
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
       --keep-going              Do not abort the build as soon as there is an error
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
 
 Manifest Options:

tests/testsuite/cargo_run/help/stdout.log

@@ -33,7 +33,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Build artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_rustc/help/stdout.log

@@ -44,7 +44,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Build artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Target triple which compiles will be for
+      --target [<TRIPLE>]       Target triple which compiles will be for
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_rustdoc/help/stdout.log

@@ -42,7 +42,7 @@ Compilation Options:
       --keep-going              Do not abort the build as soon as there is an error
   -r, --release                 Build artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_test/help/stdout.log

@@ -48,7 +48,7 @@ Compilation Options:
   -j, --jobs <N>                Number of parallel jobs, defaults to # of CPUs.
   -r, --release                 Build artifacts in release mode, with optimizations
       --profile <PROFILE-NAME>  Build artifacts with the specified profile
-      --target <TRIPLE>         Build for the target triple
+      --target [<TRIPLE>]       Build for the target triple
       --target-dir <DIRECTORY>  Directory for all generated artifacts
       --unit-graph              Output build graph in JSON (unstable)
       --timings[=<FMTS>]        Timing output formats (unstable) (comma separated): html, json

tests/testsuite/cargo_tree/help/stdout.log

@@ -33,8 +33,8 @@ Feature Selection:
       --no-default-features  Do not activate the `default` feature
 
 Compilation Options:
-      --target <TRIPLE>  Filter dependencies matching the given target-triple (default host
-                         platform). Pass `all` to include all targets.
+      --target [<TRIPLE>]  Filter dependencies matching the given target-triple (default host
+                           platform). Pass `all` to include all targets.
 
 Manifest Options:
       --manifest-path <PATH>  Path to Cargo.toml

tests/testsuite/list_availables.rs

@@ -8,6 +8,7 @@ const BIN: u8 = 1 << 1;
 const TEST: u8 = 1 << 2;
 const BENCH: u8 = 1 << 3;
 const PACKAGE: u8 = 1 << 4;
+const TARGET: u8 = 1 << 5;
 
 fn list_availables_test(command: &str, targets: u8) {
     let full_project = project()
@@ -154,6 +155,20 @@ No benches available.
 error: \"--test\" takes one argument.
 No tests available.
 
+",
+            )
+            .with_status(101)
+            .run();
+    }
+
+    if targets & TARGET != 0 {
+        empty_project
+            .cargo(&format!("{} --target", command))
+            .with_stderr(
+                "\
+error: \"--target\" takes a target architecture as an argument.
+
+Run `[..]` to see possible targets.
 ",
             )
             .with_status(101)
@@ -163,52 +178,52 @@ No tests available.
 
 #[cargo_test]
 fn build_list_availables() {
-    list_availables_test("build", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("build", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn check_list_availables() {
-    list_availables_test("check", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("check", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn doc_list_availables() {
-    list_availables_test("doc", BIN | PACKAGE);
+    list_availables_test("doc", BIN | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn fix_list_availables() {
-    list_availables_test("fix", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("fix", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn run_list_availables() {
-    list_availables_test("run", EXAMPLE | BIN | PACKAGE);
+    list_availables_test("run", EXAMPLE | BIN | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn test_list_availables() {
-    list_availables_test("test", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("test", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn bench_list_availables() {
-    list_availables_test("bench", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("bench", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn install_list_availables() {
-    list_availables_test("install", EXAMPLE | BIN);
+    list_availables_test("install", EXAMPLE | BIN | TARGET);
 }
 
 #[cargo_test]
 fn rustdoc_list_availables() {
-    list_availables_test("rustdoc", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("rustdoc", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn rustc_list_availables() {
-    list_availables_test("rustc", EXAMPLE | BIN | TEST | BENCH | PACKAGE);
+    list_availables_test("rustc", EXAMPLE | BIN | TEST | BENCH | PACKAGE | TARGET);
 }
 
 #[cargo_test]
@@ -218,12 +233,12 @@ fn pkgid_list_availables() {
 
 #[cargo_test]
 fn tree_list_availables() {
-    list_availables_test("tree", PACKAGE);
+    list_availables_test("tree", PACKAGE | TARGET);
 }
 
 #[cargo_test]
 fn clean_list_availables() {
-    list_availables_test("clean", PACKAGE);
+    list_availables_test("clean", PACKAGE | TARGET);
 }
 
 #[cargo_test]