Auto merge of #12607 - epage:target, r=weihanglo

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

### What does this PR try to resolve?

I was needing to do some more cross-compilation and forgot what the target triple was that I needed to run.  I realized i had to re-remember the command yet again.   Especially with #12585 in memory, I realized that `--target` isn't working like `--package` and other arguments that can report supported values.

In working on it, I realized I probably didn't want to report supported values yet out of concern for how big the list is (see also #12585), so I decided to just list the relevant commands for now.  We *might* be able to parse the rustup output to report those targets but I didn't receive a glowing endorsement from the rustup team about parsing the list (more of "yes, hyrums law and at least its interactive rather than CI").

Before:
```
error: a value is required for '--target <TRIPLE>' but none was supplied

For more information, try '--help'.
```

After:
```
error: "--target" takes a target architecture as an argument.

Run `rustup target list` to see possible targets.
```
(quotes were used because the other "list available" use them, we should probably work to be uniform in how we quote)

### How should we test and review this PR?

First PR adds a test showing the existing output and then through the rest you can see how the output changed

Author: bors

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)