clippy-driver: if --sysroot is specified on the command line, use that

If the user explicitly sets sysroot on the command line, then use that
value.

Issue #3663

src/driver.rs

@@ -71,8 +71,17 @@ pub fn main() {
                 exit(0);
             }
 
-            let sys_root = option_env!("SYSROOT")
-                .map(String::from)
+            let mut orig_args: Vec<String> = env::args().collect();
+
+            // Get the sysroot, looking from most specific to this invocation to the least:
+            // - command line
+            // - runtime environment
+            //    - SYSROOT
+            //    - RUSTUP_HOME, MULTIRUST_HOME, RUSTUP_TOOLCHAIN, MULTIRUST_TOOLCHAIN
+            // - sysroot from rustc in the path
+            // - compile-time environment
+            let sys_root = arg_value(&orig_args, "--sysroot", |_| true)
+                .map(|s| s.to_string())
                 .or_else(|| std::env::var("SYSROOT").ok())
                 .or_else(|| {
                     let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
@@ -88,11 +97,11 @@ pub fn main() {
                         .and_then(|out| String::from_utf8(out.stdout).ok())
                         .map(|s| s.trim().to_owned())
                 })
+                .or_else(|| option_env!("SYSROOT").map(String::from))
                 .expect("need to specify SYSROOT env var during clippy compilation, or use rustup or multirust");
 
             // Setting RUSTC_WRAPPER causes Cargo to pass 'rustc' as the first argument.
             // We're invoking the compiler programmatically, so we ignore this/
-            let mut orig_args: Vec<String> = env::args().collect();
             if orig_args.len() <= 1 {
                 std::process::exit(1);
             }