Fix rustc version when clippy-driver is used

This is a combined cherry-pick of the following two commits:

- 18b8da9 ("Handle rustc version output correctly...")
- cdf12d2 ("Update `rustc_version_cmd`")

These two commits are squashed for the backport to slightly reduce the
amount of conflict resolution needed going forward (some small tweaks
were still needed).

This backports the following:

- <#3893>
- <#3903>

Commit 1 original message:

    Handle rustc version output correctly when `clippy-driver` used

Commit 2 original message:

    Update `rustc_version_cmd`

    Change `if let` to a `match` because it is about the same complexity but
    also works with our MSRV for 0.2. This should allow backporting [1]
    easier, as well as future backports that touch this code.

    Additionally, add some new documentation comments.

    [1]: #3893

Author: nathaniel-bennett

build.rs

@@ -1,5 +1,5 @@
 use std::env;
-use std::process::Command;
+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
@@ -194,39 +194,59 @@ 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"),
-            }
-        };
-    }
-
+/// Run `rustc --version` and capture the output, adjusting arguments as needed if `clippy-driver`
+/// is used instead.
+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 = match env::var_os("RUSTC_WRAPPER").as_ref() {
-        Some(wrapper) if !wrapper.is_empty() => {
+
+    let mut cmd = match rustc_wrapper {
+        Some(wrapper) => {
             let mut cmd = Command::new(wrapper);
             cmd.arg(rustc);
+            if is_clippy_driver {
+                cmd.arg("--rustc");
+            }
+
             cmd
         }
-        _ => Command::new(rustc),
+        None => Command::new(rustc),
     };
 
-    let output = cmd
-        .arg("--version")
-        .output()
-        .ok()
-        .expect("Failed to get rustc version");
+    cmd.arg("--version");
+
+    let output = cmd.output().ok().expect("Failed to get rustc version");
+
     if !output.status.success() {
         panic!(
             "failed to run rustc: {}",
             String::from_utf8_lossy(output.stderr.as_slice())
         );
     }
 
+    output
+}
+
+/// Return the minor version of `rustc`, as well as a bool indicating whether or not the version
+/// is a nightly.
+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") {