Adjust target search algorithm for rustlib path

With this the concerns expressed in #83800 should be addressed.

compiler/rustc_target/src/lib.rs

@@ -15,6 +15,8 @@
 #![feature(associated_type_bounds)]
 #![feature(exhaustive_patterns)]
 
+use std::path::{Path, PathBuf};
+
 #[macro_use]
 extern crate rustc_macros;
 
@@ -29,3 +31,52 @@ pub mod spec;
 /// This is a hack to allow using the `HashStable_Generic` derive macro
 /// instead of implementing everything in `rustc_middle`.
 pub trait HashStableContext {}
+
+/// The name of rustc's own place to organize libraries.
+///
+/// Used to be `rustc`, now the default is `rustlib`.
+const RUST_LIB_DIR: &str = "rustlib";
+
+/// Returns a `rustlib` path for this particular target, relative to the provided sysroot.
+///
+/// For example: `target_sysroot_path("/usr", "x86_64-unknown-linux-gnu")` =>
+/// `"lib*/rustlib/x86_64-unknown-linux-gnu"`.
+pub fn target_rustlib_path(sysroot: &Path, target_triple: &str) -> PathBuf {
+    let libdir = find_libdir(sysroot);
+    std::array::IntoIter::new([
+        Path::new(libdir.as_ref()),
+        Path::new(RUST_LIB_DIR),
+        Path::new(target_triple),
+    ])
+    .collect::<PathBuf>()
+}
+
+/// The name of the directory rustc expects libraries to be located.
+fn find_libdir(sysroot: &Path) -> std::borrow::Cow<'static, str> {
+    // FIXME: This is a quick hack to make the rustc binary able to locate
+    // Rust libraries in Linux environments where libraries might be installed
+    // to lib64/lib32. This would be more foolproof by basing the sysroot off
+    // of the directory where `librustc_driver` is located, rather than
+    // where the rustc binary is.
+    // If --libdir is set during configuration to the value other than
+    // "lib" (i.e., non-default), this value is used (see issue #16552).
+
+    #[cfg(target_pointer_width = "64")]
+    const PRIMARY_LIB_DIR: &str = "lib64";
+
+    #[cfg(target_pointer_width = "32")]
+    const PRIMARY_LIB_DIR: &str = "lib32";
+
+    const SECONDARY_LIB_DIR: &str = "lib";
+
+    match option_env!("CFG_LIBDIR_RELATIVE") {
+        None | Some("lib") => {
+            if sysroot.join(PRIMARY_LIB_DIR).join(RUST_LIB_DIR).exists() {
+                PRIMARY_LIB_DIR.into()
+            } else {
+                SECONDARY_LIB_DIR.into()
+            }
+        }
+        Some(libdir) => libdir.into(),
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -1897,15 +1897,15 @@ impl Target {
         Ok(base)
     }
 
-    /// Search RUST_TARGET_PATH for a JSON file specifying the given target
-    /// triple. If none is found, look for a file called `target.json` inside
-    /// the sysroot under the target-triple's `rustlib` directory.
-    /// Note that it could also just be a bare filename already, so also
-    /// check for that. If one of the hardcoded targets we know about, just
-    /// return it directly.
+    /// Search for a JSON file specifying the given target triple.
     ///
-    /// The error string could come from any of the APIs called, including
-    /// filesystem access and JSON decoding.
+    /// If none is found in `$RUST_TARGET_PATH`, look for a file called `target.json` inside the
+    /// sysroot under the target-triple's `rustlib` directory.  Note that it could also just be a
+    /// bare filename already, so also check for that. If one of the hardcoded targets we know
+    /// about, just return it directly.
+    ///
+    /// The error string could come from any of the APIs called, including filesystem access and
+    /// JSON decoding.
     pub fn search(target_triple: &TargetTriple, sysroot: &PathBuf) -> Result<Target, String> {
         use rustc_serialize::json;
         use std::env;
@@ -1942,8 +1942,13 @@ impl Target {
 
                 // Additionally look in the sysroot under `lib/rustlib/<triple>/target.json`
                 // as a fallback.
-                let p =
-                    sysroot.join("lib").join("rustlib").join(&target_triple).join("target.json");
+                let rustlib_path = crate::target_rustlib_path(&sysroot, &target_triple);
+                let p = std::array::IntoIter::new([
+                    Path::new(sysroot),
+                    Path::new(&rustlib_path),
+                    Path::new("target.json"),
+                ])
+                .collect::<PathBuf>();
                 if p.is_file() {
                     return load_file(&p);
                 }