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 invokes rustc with the --print=split-debuginfo to query
supported values and ignores the Cargo.toml entry if not supported.

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

@@ -55,8 +55,8 @@ 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,
+    /// Supported values for `-Csplit-debuginfo=` flag, queried from rustc
+    pub support_split_debuginfo: Vec<String>,
 }
 
 /// Kind of each file generated by a Unit, part of `FileType`.
@@ -167,6 +167,18 @@ impl TargetInfo {
         loop {
             let extra_fingerprint = kind.fingerprint_hash();
 
+            // Query rustc for supported -Csplit-debuginfo values
+            let support_split_debuginfo = rustc
+                .cached_output(
+                    rustc.workspace_process().arg("--print=split-debuginfo"),
+                    extra_fingerprint,
+                )
+                .unwrap_or_default()
+                .0
+                .lines()
+                .map(String::from)
+                .collect();
+
             // Query rustc for several kinds of info from each line of output:
             // 0) file-names (to determine output file prefix/suffix for given crate type)
             // 1) sysroot
@@ -199,14 +211,6 @@ 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
-                .cached_output(
-                    process.clone().arg("-Csplit-debuginfo=packed"),
-                    extra_fingerprint,
-                )
-                .is_ok();
-
             process.arg("--print=sysroot");
             process.arg("--print=cfg");
 
@@ -303,7 +307,7 @@ impl TargetInfo {
                     Flags::Rustdoc,
                 )?,
                 cfg,
-                supports_split_debuginfo,
+                support_split_debuginfo,
             });
         }
     }

src/cargo/core/compiler/mod.rs

@@ -894,10 +894,20 @@ 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 {
+        if cx
+            .bcx
+            .target_data
+            .info(unit.kind)
+            .support_split_debuginfo
+            .iter()
+            .find(|sup| sup.as_str() == split.as_str())
+            .is_some()
+        {
             cmd.arg("-C").arg(format!("split-debuginfo={}", split));
         }
     }