Deduplicate handling of environnment variables

Author: djc

src/config.rs

@@ -1,9 +1,9 @@
 use std::borrow::Cow;
 use std::fmt::{self, Debug, Display};
-use std::io;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::sync::Arc;
+use std::{env, io};
 
 use anyhow::{anyhow, bail, Context, Result};
 use serde::Deserialize;
@@ -288,26 +288,20 @@ impl<'a> Cfg<'a> {
         let default_host_triple =
             settings_file.with(|s| Ok(get_default_host_triple(s, process)))?;
         // Environment override
-        let env_override = process
-            .var("RUSTUP_TOOLCHAIN")
-            .ok()
-            .and_then(if_not_empty)
+        let env_override = non_empty_env_var("RUSTUP_TOOLCHAIN", process)?
             .map(ResolvableLocalToolchainName::try_from)
             .transpose()?
             .map(|t| t.resolve(&default_host_triple))
             .transpose()?;
 
-        let dist_root_server = match process.var("RUSTUP_DIST_SERVER") {
-            Ok(s) if !s.is_empty() => {
+        let dist_root_server = match non_empty_env_var("RUSTUP_DIST_SERVER", process)? {
+            Some(s) => {
                 trace!("`RUSTUP_DIST_SERVER` has been set to `{s}`");
                 s
             }
-            _ => {
+            None => {
                 // For backward compatibility
-                process
-                    .var("RUSTUP_DIST_ROOT")
-                    .ok()
-                    .and_then(if_not_empty)
+                non_empty_env_var("RUSTUP_DIST_ROOT", process)?
                     .inspect(|url| trace!("`RUSTUP_DIST_ROOT` has been set to `{url}`"))
                     .map_or(Cow::Borrowed(dist::DEFAULT_DIST_ROOT), Cow::Owned)
                     .as_ref()
@@ -1000,11 +994,12 @@ fn get_default_host_triple(s: &Settings, process: &Process) -> dist::TargetTripl
         .unwrap_or_else(|| dist::TargetTriple::from_host_or_build(process))
 }
 
-fn if_not_empty<S: PartialEq<str>>(s: S) -> Option<S> {
-    if s == *"" {
-        None
-    } else {
-        Some(s)
+fn non_empty_env_var(name: &str, process: &Process) -> anyhow::Result<Option<String>> {
+    match process.var(name) {
+        Ok(s) if !s.is_empty() => Ok(Some(s)),
+        Ok(_) => Ok(None),
+        Err(env::VarError::NotPresent) => Ok(None),
+        Err(err) => Err(err.into()),
     }
 }