Move current process name detection into helper

We're going to reuse it to get a better error message.

Author: K900

src/bin/rustup-init.rs

@@ -13,8 +13,6 @@
 
 #![recursion_limit = "1024"]
 
-use std::path::PathBuf;
-
 use anyhow::{anyhow, Result};
 use cfg_if::cfg_if;
 use rs_tracing::*;
@@ -62,18 +60,7 @@ fn run_rustup_inner() -> Result<utils::ExitCode> {
     utils::current_dir()?;
     utils::current_exe()?;
 
-    // The name of arg0 determines how the program is going to behave
-    let arg0 = match process().var("RUSTUP_FORCE_ARG0") {
-        Ok(v) => Some(v),
-        Err(_) => process().args().next(),
-    }
-    .map(PathBuf::from);
-    let name = arg0
-        .as_ref()
-        .and_then(|a| a.file_stem())
-        .and_then(std::ffi::OsStr::to_str);
-
-    match name {
+    match process().name().as_deref() {
         Some("rustup") => rustup_mode::main(),
         Some(n) if n.starts_with("rustup-setup") || n.starts_with("rustup-init") => {
             // NB: The above check is only for the prefix of the file

src/currentprocess.rs

@@ -87,6 +87,8 @@ pub trait CurrentProcess:
     + Debug
 {
     fn clone_boxed(&self) -> Box<dyn CurrentProcess>;
+
+    fn name(&self) -> Option<String>;
 }
 
 // Machinery for Cloning boxes
@@ -107,6 +109,19 @@ where
     fn clone_boxed(&self) -> Box<dyn CurrentProcess + 'static> {
         Box::new(T::clone(self))
     }
+
+    fn name(&self) -> Option<String> {
+        let arg0 = match self.var("RUSTUP_FORCE_ARG0") {
+            Ok(v) => Some(v),
+            Err(_) => self.args().next(),
+        }
+        .map(PathBuf::from);
+
+        arg0.as_ref()
+            .and_then(|a| a.file_stem())
+            .and_then(std::ffi::OsStr::to_str)
+            .map(String::from)
+    }
 }
 
 impl Clone for Box<dyn CurrentProcess + 'static> {