Fix split-debuginfo support detection

cargo assumed that if -Csplit-debuginfo=packed worked, all values would
be correct. This however is not the case -- as of Rust 1.65.0, rustc
supports packed, but not unpacked or off on Windows. Because of this,
having split-debuginfo="unpacked" on Windows has caused builds to fail,
as cargo assumed that the option is fine (split-debuginfo=packed
worked), but rustc then failed when being passed
-Csplit-debuginfo=unpacked.

This patch splits split-debuginfo support detection for respective
options (off, packed, unpacked).

src/cargo/core/compiler/build_context/target_info.rs

@@ -55,8 +55,12 @@ pub struct TargetInfo {
     pub rustflags: Vec<String>,
     /// Extra flags to pass to `rustdoc`, see [`extra_args`].
     pub rustdocflags: Vec<String>,
-    /// Whether or not rustc supports the `-Csplit-debuginfo` flag.
-    pub supports_split_debuginfo: bool,
+    /// Whether or not rustc supports the `-Csplit-debuginfo=off` flag.
+    pub supports_split_debuginfo_off: bool,
+    /// Whether or not rustc supports the `-Csplit-debuginfo=packed` flag.
+    pub supports_split_debuginfo_packed: bool,
+    /// Whether or not rustc supports the `-Csplit-debuginfo=unpacked` flag.
+    pub supports_split_debuginfo_unpacked: bool,
 }
 
 /// Kind of each file generated by a Unit, part of `FileType`.
@@ -199,14 +203,28 @@ impl TargetInfo {
                 process.arg("--crate-type").arg(crate_type.as_str());
             }
 
-            // An extra `rustc` call to determine `-Csplit-debuginfo=packed` support.
-            let supports_split_debuginfo = rustc
+            // 3 extra `rustc` calls to determine `-Csplit-debuginfo={off,packed,unpacked}` support.
+            let supports_split_debuginfo_off = rustc
+                .cached_output(
+                    process.clone().arg("-Csplit-debuginfo=off"),
+                    extra_fingerprint,
+                )
+                .is_ok();
+
+            let supports_split_debuginfo_packed = rustc
                 .cached_output(
                     process.clone().arg("-Csplit-debuginfo=packed"),
                     extra_fingerprint,
                 )
                 .is_ok();
 
+            let supports_split_debuginfo_unpacked = rustc
+                .cached_output(
+                    process.clone().arg("-Csplit-debuginfo=unpacked"),
+                    extra_fingerprint,
+                )
+                .is_ok();
+
             process.arg("--print=sysroot");
             process.arg("--print=cfg");
 
@@ -303,7 +321,9 @@ impl TargetInfo {
                     Flags::Rustdoc,
                 )?,
                 cfg,
-                supports_split_debuginfo,
+                supports_split_debuginfo_off,
+                supports_split_debuginfo_packed,
+                supports_split_debuginfo_unpacked,
             });
         }
     }

src/cargo/core/compiler/mod.rs

@@ -894,10 +894,31 @@ fn build_base_args(
     cmd.args(&lto_args(cx, unit));
 
     // This is generally just an optimization on build time so if we don't pass
-    // it then it's ok. As of the time of this writing it's a very new flag, so
-    // we need to dynamically check if it's available.
-    if cx.bcx.target_data.info(unit.kind).supports_split_debuginfo {
-        if let Some(split) = split_debuginfo {
+    // it then it's ok. The values for the flag (off, packed, unpacked) may be supported
+    // or not depending on the platform, so availability is checked per-value.
+    // For example, at the time of writing this code, on Windows the only stable valid
+    // value for split-debuginfo is "packed", while on Linux "unpacked" is also stable.
+    if let Some(split) = split_debuginfo {
+        let is_supported_off = cx
+            .bcx
+            .target_data
+            .info(unit.kind)
+            .supports_split_debuginfo_off
+            && split == "off";
+        let is_supported_packed = cx
+            .bcx
+            .target_data
+            .info(unit.kind)
+            .supports_split_debuginfo_packed
+            && split == "packed";
+        let is_supported_unpacked = cx
+            .bcx
+            .target_data
+            .info(unit.kind)
+            .supports_split_debuginfo_unpacked
+            && split == "unpacked";
+
+        if is_supported_off || is_supported_packed || is_supported_unpacked {
             cmd.arg("-C").arg(format!("split-debuginfo={}", split));
         }
     }