Auto merge of #9848 - nipunn1313:desc, r=alexcrichton

Show desc of well known subcommands (fmt, clippy) in cargo --list

Fixes #8680

An approach to #8680 that shows these in `cargo --list` without showing them directly in the `cargo --help`.

```
➜  cargo git:(desc) target/debug/cargo --list | grep clippy
    clippy               Checks a package to catch common mistakes and improve your Rust code.
```

Here's what mine looks like visually now:
![image](https://user-images.githubusercontent.com/1300387/131178775-2255ef0d-1993-47dd-bc73-9015394b967c.png)

src/bin/cargo/cli.rs

@@ -3,12 +3,23 @@ use cargo::core::{features, CliUnstable};
 use cargo::{self, drop_print, drop_println, CliResult, Config};
 use clap::{AppSettings, Arg, ArgMatches};
 use itertools::Itertools;
+use std::collections::HashMap;
 
 use super::commands;
 use super::list_commands;
 use crate::command_prelude::*;
 use cargo::core::features::HIDDEN;
 
+lazy_static::lazy_static! {
+    // Maps from commonly known external commands (not builtin to cargo) to their
+    // description, for the help page. Reserved for external subcommands that are
+    // core within the rust ecosystem (esp ones that might become internal in the future).
+    static ref KNOWN_EXTERNAL_COMMAND_DESCRIPTIONS: HashMap<&'static str, &'static str> = vec![
+        ("clippy", "Checks a package to catch common mistakes and improve your Rust code."),
+        ("fmt", "Formats all bin and lib files of the current crate using rustfmt."),
+    ].into_iter().collect();
+}
+
 pub fn main(config: &mut Config) -> CliResult {
     // CAUTION: Be careful with using `config` until it is configured below.
     // In general, try to avoid loading config values unless necessary (like
@@ -100,14 +111,22 @@ Run with 'cargo -Z [FLAG] [SUBCOMMAND]'",
     if args.is_present("list") {
         drop_println!(config, "Installed Commands:");
         for (name, command) in list_commands(config) {
+            let known_external_desc = KNOWN_EXTERNAL_COMMAND_DESCRIPTIONS.get(name.as_str());
             match command {
                 CommandInfo::BuiltIn { about } => {
+                    assert!(
+                        known_external_desc.is_none(),
+                        "KNOWN_EXTERNAL_COMMANDS shouldn't contain builtin \"{}\"",
+                        name
+                    );
                     let summary = about.unwrap_or_default();
                     let summary = summary.lines().next().unwrap_or(&summary); // display only the first line
                     drop_println!(config, "    {:<20} {}", name, summary);
                 }
                 CommandInfo::External { path } => {
-                    if is_verbose {
+                    if let Some(desc) = known_external_desc {
+                        drop_println!(config, "    {:<20} {}", name, desc);
+                    } else if is_verbose {
                         drop_println!(config, "    {:<20} {}", name, path.display());
                     } else {
                         drop_println!(config, "    {}", name);

tests/testsuite/cargo_command.rs

@@ -101,6 +101,31 @@ fn list_command_looks_at_path() {
     );
 }
 
+#[cargo_test]
+fn list_command_handles_known_external_commands() {
+    let p = project()
+        .executable(Path::new("path-test").join("cargo-fmt"), "")
+        .build();
+
+    let fmt_desc = "    fmt                  Formats all bin and lib files of the current crate using rustfmt.";
+
+    // Without path - fmt isn't there
+    p.cargo("--list")
+        .env("PATH", "")
+        .with_stdout_does_not_contain(fmt_desc)
+        .run();
+
+    // With path - fmt is there with known description
+    let mut path = path();
+    path.push(p.root().join("path-test"));
+    let path = env::join_paths(path.iter()).unwrap();
+
+    p.cargo("--list")
+        .env("PATH", &path)
+        .with_stdout_contains(fmt_desc)
+        .run();
+}
+
 #[cargo_test]
 fn list_command_resolves_symlinks() {
     let proj = project()