Handle rustc version output correctly when clippy-driver used

Author: nathaniel-bennett

build.rs

@@ -1,5 +1,6 @@
 use std::env;
-use std::process::Command;
+use std::ffi::{OsStr, OsString};
+use std::process::{Command, Output};
 use std::str;
 
 // List of cfgs this build script is allowed to set. The list is needed to support check-cfg, as we
@@ -111,37 +112,56 @@ fn main() {
     }
 }
 
-fn rustc_minor_nightly() -> (u32, bool) {
-    macro_rules! otry {
-        ($e:expr) => {
-            match $e {
-                Some(e) => e,
-                None => panic!("Failed to get rustc version"),
-            }
-        };
-    }
-
+fn rustc_version_cmd(is_clippy_driver: bool) -> Output {
+    let rustc_wrapper = env::var_os("RUSTC_WRAPPER").filter(|w| !w.is_empty());
     let rustc = env::var_os("RUSTC").expect("Failed to get rustc version: missing RUSTC env");
-    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER").filter(|w| !w.is_empty()) {
+
+    let mut cmd = if let Some(wrapper) = rustc_wrapper {
         let mut cmd = Command::new(wrapper);
         cmd.arg(rustc);
+        if is_clippy_driver {
+            cmd.arg("--rustc");
+        }
+
         cmd
     } else {
         Command::new(rustc)
     };
 
+    cmd.arg("--version");
+
     let output = cmd
-        .arg("--version")
         .output()
         .expect("Failed to get rustc version");
+
     if !output.status.success() {
         panic!(
             "failed to run rustc: {}",
             String::from_utf8_lossy(output.stderr.as_slice())
         );
     }
 
+    output
+}
+
+fn rustc_minor_nightly() -> (u32, bool) {
+    macro_rules! otry {
+        ($e:expr) => {
+            match $e {
+                Some(e) => e,
+                None => panic!("Failed to get rustc version"),
+            }
+        };
+    }
+
+    let mut output = rustc_version_cmd(false);
+
+    if otry!(str::from_utf8(&output.stdout).ok()).starts_with("clippy") {
+        output = rustc_version_cmd(true);
+    }
+
     let version = otry!(str::from_utf8(&output.stdout).ok());
+
     let mut pieces = version.split('.');
 
     if pieces.next() != Some("rustc 1") {