List installed tools when no command is provided to uv tool run (#5553)

## Summary

Part of #4024 

## Test Plan

Test cases included.

crates/uv-cli/src/lib.rs

@@ -2264,8 +2264,10 @@ pub struct ToolRunArgs {
     ///
     /// If more complex version specification is desired or if the command is provided by a different
     /// package, use `--from`.
+    ///
+    /// If omitted, lists the available tools.
     #[command(subcommand)]
-    pub command: ExternalCommand,
+    pub command: Option<ExternalCommand>,
 
     /// Use the given package to provide the command.
     ///

crates/uv/src/commands/tool/run.rs

@@ -31,7 +31,7 @@ use crate::commands::reporters::PythonDownloadReporter;
 
 use crate::commands::project::resolve_names;
 use crate::commands::{
-    project, project::environment::CachedEnvironment, tool::common::matching_packages,
+    project, project::environment::CachedEnvironment, tool::common::matching_packages, tool_list,
 };
 use crate::commands::{ExitStatus, SharedState};
 use crate::printer::Printer;
@@ -56,7 +56,7 @@ impl Display for ToolRunCommand {
 
 /// Run a command.
 pub(crate) async fn run(
-    command: ExternalCommand,
+    command: Option<ExternalCommand>,
     from: Option<String>,
     with: &[RequirementsSource],
     python: Option<String>,
@@ -76,6 +76,11 @@ pub(crate) async fn run(
         warn_user_once!("`{invocation_source}` is experimental and may change without warning");
     }
 
+    // treat empty command as `uv tool list`
+    let Some(command) = command else {
+        return tool_list(false, PreviewMode::Enabled, cache, printer).await;
+    };
+
     let (target, args) = command.split();
     let Some(target) = target else {
         return Err(anyhow::anyhow!("No tool command provided"));

crates/uv/src/settings.rs

@@ -251,7 +251,7 @@ impl RunSettings {
 #[allow(clippy::struct_excessive_bools)]
 #[derive(Debug, Clone)]
 pub(crate) struct ToolRunSettings {
-    pub(crate) command: ExternalCommand,
+    pub(crate) command: Option<ExternalCommand>,
     pub(crate) from: Option<String>,
     pub(crate) with: Vec<String>,
     pub(crate) with_requirements: Vec<PathBuf>,

crates/uv/tests/tool_run.rs

@@ -766,3 +766,48 @@ fn tool_run_requirements_txt_arguments() {
      + werkzeug==3.0.1
     "###);
 }
+
+/// List installed tools when no command arg is given (e.g. `uv tool run`).
+#[test]
+fn tool_run_list_installed() {
+    let context = TestContext::new("3.12").with_filtered_exe_suffix();
+    let tool_dir = context.temp_dir.child("tools");
+    let bin_dir = context.temp_dir.child("bin");
+
+    // No tools installed.
+    uv_snapshot!(context.filters(), context.tool_run()
+        .env("UV_TOOL_DIR", tool_dir.as_os_str())
+        .env("XDG_BIN_HOME", bin_dir.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: `uv tool run` is experimental and may change without warning
+    No tools installed
+    "###);
+
+    // Install `black`.
+    context
+        .tool_install()
+        .arg("black==24.2.0")
+        .env("UV_TOOL_DIR", tool_dir.as_os_str())
+        .env("XDG_BIN_HOME", bin_dir.as_os_str())
+        .assert()
+        .success();
+
+    // List installed tools.
+    uv_snapshot!(context.filters(), context.tool_run()
+        .env("UV_TOOL_DIR", tool_dir.as_os_str())
+        .env("XDG_BIN_HOME", bin_dir.as_os_str()), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    black v24.2.0
+    - black
+    - blackd
+
+    ----- stderr -----
+    warning: `uv tool run` is experimental and may change without warning
+    "###);
+}