Show appropriate activation command based on shell detection (#2221)

## Summary

Closes #2174.

## Test Plan

On Nushell:

```
(uv) ~/workspace/uv> cargo run venv                                                                                                                                                                                      
Using Python 3.12.0 interpreter at: /Users/crmarsh/workspace/uv/.venv/bin/python3
Creating virtualenv at: .venv
Activate with: overlay use .venv/bin/activate.nu
```

On Bash:

```
❯ cargo run venv "foo bar"
Using Python 3.12.0 interpreter at: /Users/crmarsh/.local/share/rtx/installs/python/3.12.0/bin/python3
Creating virtualenv at: foo bar
Activate with: source 'foo bar/bin/activate'
```

crates/uv/src/commands/venv.rs

@@ -25,6 +25,7 @@ use uv_traits::{BuildContext, ConfigSettings, InFlight, NoBuild, SetupPyStrategy
 
 use crate::commands::ExitStatus;
 use crate::printer::Printer;
+use crate::shell::Shell;
 
 /// Create a virtual environment.
 #[allow(clippy::unnecessary_wraps, clippy::too_many_arguments)]
@@ -210,29 +211,53 @@ async fn venv_impl(
         }
     }
 
-    if cfg!(windows) {
-        writeln!(
-            printer,
-            // This should work whether the user is on CMD or PowerShell:
-            "Activate with: {}",
-            path.join("Scripts")
-                .join("activate")
-                .simplified_display()
-                .green()
-        )
-        .into_diagnostic()?;
-    } else {
-        writeln!(
-            printer,
-            "Activate with: {}",
-            format!(
-                "source {}",
-                path.join("bin").join("activate").simplified_display()
+    // Determine the appropriate activation command.
+    let activation = match Shell::from_env() {
+        None => None,
+        Some(Shell::Bash | Shell::Zsh) => Some(format!(
+            "source {}",
+            shlex(path.join("bin").join("activate"))
+        )),
+        Some(Shell::Fish) => Some(format!(
+            "source {}",
+            shlex(path.join("bin").join("activate.fish"))
+        )),
+        Some(Shell::Nushell) => Some(format!(
+            "overlay use {}",
+            shlex(path.join("bin").join("activate.nu"))
+        )),
+        Some(Shell::Csh) => Some(format!(
+            "source {}",
+            shlex(path.join("bin").join("activate.csh"))
+        )),
+        Some(Shell::Powershell) => {
+            // No need to quote the path for PowerShell.
+            Some(
+                path.join("Scripts")
+                    .join("activate")
+                    .simplified_display()
+                    .to_string(),
             )
-            .green()
-        )
-        .into_diagnostic()?;
+        }
     };
+    if let Some(act) = activation {
+        writeln!(printer, "Activate with: {}", act.green()).into_diagnostic()?;
+    }
 
     Ok(ExitStatus::Success)
 }
+
+/// Quote a path, if necessary, for safe use in a shell command.
+fn shlex(executable: impl AsRef<Path>) -> String {
+    // Convert to a display path.
+    let executable = executable.as_ref().simplified_display().to_string();
+
+    // Like Python's `shlex.quote`:
+    // > Use single quotes, and put single quotes into double quotes
+    // > The string $'b is then quoted as '$'"'"'b'
+    if executable.contains(' ') {
+        format!("'{}'", executable.replace('\'', r#"'"'"'"#))
+    } else {
+        executable
+    }
+}

crates/uv/src/main.rs

@@ -50,6 +50,7 @@ mod confirm;
 mod logging;
 mod printer;
 mod requirements;
+mod shell;
 mod version;
 
 const DEFAULT_VENV_NAME: &str = ".venv";

crates/uv/src/shell.rs

@@ -0,0 +1,69 @@
+use std::path::Path;
+
+/// Shells for which virtualenv activation scripts are available.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub(crate) enum Shell {
+    /// Bourne Again SHell (bash)
+    Bash,
+    /// Friendly Interactive SHell (fish)
+    Fish,
+    /// PowerShell
+    Powershell,
+    /// Z SHell (zsh)
+    Zsh,
+    /// Nushell
+    Nushell,
+    /// C SHell (csh)
+    Csh,
+}
+
+impl Shell {
+    /// Determine the user's current shell from the environment.
+    ///
+    /// This will read the `SHELL` environment variable and try to determine which shell is in use
+    /// from that.
+    ///
+    /// If `SHELL` is not set, then on windows, it will default to powershell, and on
+    /// other `OSes` it will return `None`.
+    ///
+    /// If `SHELL` is set, but contains a value that doesn't correspond to one of the supported
+    /// shell types, then return `None`.
+    pub(crate) fn from_env() -> Option<Shell> {
+        if std::env::var_os("NU_VERSION").is_some() {
+            Some(Shell::Nushell)
+        } else if let Some(env_shell) = std::env::var_os("SHELL") {
+            Shell::from_shell_path(env_shell)
+        } else if cfg!(windows) {
+            Some(Shell::Powershell)
+        } else {
+            None
+        }
+    }
+
+    /// Parse a shell from a path to the executable for the shell.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crate::shells::Shell;
+    ///
+    /// assert_eq!(Shell::from_shell_path("/bin/bash"), Some(Shell::Bash));
+    /// assert_eq!(Shell::from_shell_path("/usr/bin/zsh"), Some(Shell::Zsh));
+    /// assert_eq!(Shell::from_shell_path("/opt/my_custom_shell"), None);
+    /// ```
+    pub(crate) fn from_shell_path<P: AsRef<Path>>(path: P) -> Option<Shell> {
+        parse_shell_from_path(path.as_ref())
+    }
+}
+
+fn parse_shell_from_path(path: &Path) -> Option<Shell> {
+    let name = path.file_stem()?.to_str()?;
+    match name {
+        "bash" => Some(Shell::Bash),
+        "zsh" => Some(Shell::Zsh),
+        "fish" => Some(Shell::Fish),
+        "csh" => Some(Shell::Csh),
+        "powershell" | "powershell_ise" => Some(Shell::Powershell),
+        _ => None,
+    }
+}