Add `--dry-run` support to `uv self update` (#9829)

See commentary at
https://github.com/astral-sh/uv/issues/9828#issuecomment-2537542100
regarding the limitations and future upstream changes needed.

```
❯ cargo build --features self-update
   Compiling uv v0.5.8 (/Users/zb/workspace/uv/crates/uv)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 7.28s
❯ cp ./target/debug/uv ~/.cargo/bin
❯ uv self update --dry-run
info: Checking for updates...
Nothing to do. You're on the latest version of uv (v0.5.8)
❯ uv self update --dry-run 0.5.7
info: Checking for updates...
Would update uv from v0.5.8 to v0.5.7
❯ vi ~/.config/uv/uv-receipt.json  # Edit the receipt to think its on an older version
❯ uv self update --dry-run
info: Checking for updates...
Would update uv from v0.5.8 to the latest version
```

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cli/src/lib.rs

@@ -589,6 +589,10 @@ pub struct SelfUpdateArgs {
     /// A token is not required but can be used to reduce the chance of encountering rate limits.
     #[arg(long, env = EnvVars::UV_GITHUB_TOKEN)]
     pub token: Option<String>,
+
+    /// Run without performing the update.
+    #[arg(long)]
+    pub dry_run: bool,
 }
 
 #[derive(Args)]

crates/uv/src/commands/self_update.rs

@@ -15,6 +15,7 @@ use crate::printer::Printer;
 pub(crate) async fn self_update(
     version: Option<String>,
     token: Option<String>,
+    dry_run: bool,
     printer: Printer,
 ) -> Result<ExitStatus> {
     let mut updater = AxoUpdater::new_for("uv");
@@ -87,7 +88,38 @@ pub(crate) async fn self_update(
         UpdateRequest::Latest
     };
 
-    updater.configure_version_specifier(update_request);
+    updater.configure_version_specifier(update_request.clone());
+
+    if dry_run {
+        // TODO(charlie): `updater.fetch_release` isn't public, so we can't say what the latest
+        // version is.
+        if updater.is_update_needed().await? {
+            let version = match update_request {
+                UpdateRequest::Latest | UpdateRequest::LatestMaybePrerelease => {
+                    "the latest version".to_string()
+                }
+                UpdateRequest::SpecificTag(version) | UpdateRequest::SpecificVersion(version) => {
+                    format!("v{version}")
+                }
+            };
+            writeln!(
+                printer.stderr(),
+                "Would update uv from {} to {}",
+                format!("v{}", env!("CARGO_PKG_VERSION")).bold().white(),
+                version.bold().white(),
+            )?;
+        } else {
+            writeln!(
+                printer.stderr(),
+                "{}",
+                format_args!(
+                    "You're on the latest version of uv ({})",
+                    format!("v{}", env!("CARGO_PKG_VERSION")).bold().white()
+                )
+            )?;
+        }
+        return Ok(ExitStatus::Success);
+    }
 
     // Run the updater. This involves a network request, since we need to determine the latest
     // available version of uv.

crates/uv/src/lib.rs

@@ -1018,8 +1018,9 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
                 SelfCommand::Update(SelfUpdateArgs {
                     target_version,
                     token,
+                    dry_run,
                 }),
-        }) => commands::self_update(target_version, token, printer).await,
+        }) => commands::self_update(target_version, token, dry_run, printer).await,
         Commands::Self_(SelfNamespace {
             command:
                 SelfCommand::Version {

docs/reference/cli.md

@@ -9371,6 +9371,8 @@ uv self update [OPTIONS] [TARGET_VERSION]
 
 <p>See <code>--project</code> to only change the project root directory.</p>
 
+</dd><dt id="uv-self-update--dry-run"><a href="#uv-self-update--dry-run"><code>--dry-run</code></a></dt><dd><p>Run without performing the update</p>
+
 </dd><dt id="uv-self-update--help"><a href="#uv-self-update--help"><code>--help</code></a>, <code>-h</code></dt><dd><p>Display the concise help for this command</p>
 
 </dd><dt id="uv-self-update--managed-python"><a href="#uv-self-update--managed-python"><code>--managed-python</code></a></dt><dd><p>Require use of uv-managed Python versions.</p>