Simplify the reading of RUST_LIBC_UNSTABLE_GNU_*_BITS variables

Use (almost) the same code in build.rs and libc-test/build.rs.
Make the decision chain clearer.
Have a single default value that can be overriden in various ways,
which should make switching the default much easier.

(backport <#4652>)
(cherry picked from commit 4a4a57b)

Author: snogge

build.rs

@@ -112,28 +112,37 @@ fn main() {
         && target_arch != "riscv32"
         && target_arch != "x86_64"
     {
-        match env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS") {
-            Ok(val) if val == "64" => {
-                set_cfg("gnu_file_offset_bits64");
-                set_cfg("linux_time_bits64");
-                set_cfg("gnu_time_bits64");
-            }
-            Ok(val) if val != "32" => {
-                panic!("RUST_LIBC_UNSTABLE_GNU_TIME_BITS may only be set to '32' or '64'")
-            }
-            _ => {
-                match env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS") {
-                    Ok(val) if val == "64" => {
-                        set_cfg("gnu_file_offset_bits64");
-                    }
-                    Ok(val) if val != "32" => {
-                        panic!("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS may only be set to '32' or '64'")
-                    }
-                    _ => {}
-                }
-            }
+        let defaultbits = "32".to_string();
+        let (timebits, filebits) = match (
+            env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS"),
+            env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+        ) {
+            (Ok(_), Ok(_)) => panic!("Do not set both RUST_LIBC_UNSTABLE_GNU_TIME_BITS and RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            (Err(_), Err(_)) => (defaultbits.clone(), defaultbits.clone()),
+            (Ok(tb), Err(_)) if tb == "64" => (tb.clone(), tb.clone()),
+            (Ok(tb), Err(_)) if tb == "32" => (tb, defaultbits.clone()),
+            (Ok(_), Err(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS, must be 32 or 64"),
+            (Err(_), Ok(fb)) if fb == "32" || fb == "64" => (defaultbits.clone(), fb),
+            (Err(_), Ok(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32 or 64"),
+        };
+        let valid_bits = ["32", "64"];
+        assert!(
+            valid_bits.contains(&filebits.as_str()) && valid_bits.contains(&timebits.as_str()),
+            "Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS or RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32, 64 or unset"
+        );
+        assert!(
+            !(filebits == "32" && timebits == "64"),
+            "RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS must be 64 or unset if RUST_LIBC_UNSTABLE_GNU_TIME_BITS is 64"
+        );
+        if timebits == "64" {
+            set_cfg("linux_time_bits64");
+            set_cfg("gnu_time_bits64");
+        }
+        if filebits == "64" {
+            set_cfg("gnu_file_offset_bits64");
         }
     }
+
     // On CI: deny all warnings
     if libc_ci {
         set_cfg("libc_deny_warnings");

libc-test/build.rs

@@ -3698,29 +3698,36 @@ fn config_gnu_bits(target: &str, cfg: &mut ctest::TestGenerator) {
         && !target.contains("riscv32")
         && pointer_width == "32"
     {
-        match env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS") {
-            Ok(val) if val == "64" => {
-                cfg.define("_FILE_OFFSET_BITS", Some("64"));
-                cfg.define("_TIME_BITS", Some("64"));
-                cfg.cfg("gnu_file_offset_bits64", None);
-                cfg.cfg("linux_time_bits64", None);
-                cfg.cfg("gnu_time_bits64", None);
-            }
-            Ok(val) if val != "32" => {
-                panic!("RUST_LIBC_UNSTABLE_GNU_TIME_BITS may only be set to '32' or '64'")
-            }
-            _ => {
-                match env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS") {
-                    Ok(val) if val == "64" => {
-                        cfg.define("_FILE_OFFSET_BITS", Some("64"));
-                        cfg.cfg("gnu_file_offset_bits64", None);
-                    }
-                    Ok(val) if val != "32" => {
-                        panic!("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS may only be set to '32' or '64'")
-                    }
-                    _ => {}
-                }
-            }
+        let defaultbits = "32".to_string();
+        let (timebits, filebits) = match (
+            env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS"),
+            env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+        ) {
+            (Ok(_), Ok(_)) => panic!("Do not set both RUST_LIBC_UNSTABLE_GNU_TIME_BITS and RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            (Err(_), Err(_)) => (defaultbits.clone(), defaultbits.clone()),
+            (Ok(tb), Err(_)) if tb == "64" => (tb.clone(), tb.clone()),
+            (Ok(tb), Err(_)) if tb == "32" => (tb, defaultbits.clone()),
+            (Ok(_), Err(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS, must be 32 or 64"),
+            (Err(_), Ok(fb)) if fb == "32" || fb == "64" => (defaultbits.clone(), fb),
+            (Err(_), Ok(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32 or 64"),
+        };
+        let valid_bits = ["32", "64"];
+        assert!(
+            valid_bits.contains(&filebits.as_str()) && valid_bits.contains(&timebits.as_str()),
+            "Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS or RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32, 64 or unset"
+        );
+        assert!(
+            !(filebits == "32" && timebits == "64"),
+            "RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS must be 64 or unset if RUST_LIBC_UNSTABLE_GNU_TIME_BITS is 64"
+        );
+        if timebits == "64" {
+            cfg.define("_TIME_BITS", Some("64"));
+            cfg.cfg("linux_time_bits64", None);
+            cfg.cfg("gnu_time_bits64", None);
+        }
+        if filebits == "64" {
+            cfg.define("_FILE_OFFSET_BITS", Some("64"));
+            cfg.cfg("gnu_file_offset_bits64", None);
         }
     }
 }