feat: add changes from this pr: rust-lang/rust#128283

Author: ratankaliani

src/bootstrap/src/core/config/config.rs

@@ -514,6 +514,11 @@ impl TargetSelection {
     pub fn is_windows(&self) -> bool {
         self.contains("windows")
     }
+
+    /// Path to the file defining the custom target, if any.
+    pub fn filepath(&self) -> Option<&Path> {
+        self.file.as_ref().map(Path::new)
+    }
 }
 
 impl fmt::Display for TargetSelection {
@@ -2388,7 +2393,11 @@ impl Config {
             .get(&target)
             .and_then(|t| t.split_debuginfo)
             .or_else(|| {
-                if self.build == target { self.rust_split_debuginfo_for_build_triple } else { None }
+                if self.build == target {
+                    self.rust_split_debuginfo_for_build_triple
+                } else {
+                    None
+                }
             })
             .unwrap_or_else(|| SplitDebuginfo::default_for_platform(target))
     }

src/bootstrap/src/core/sanity.rs

@@ -262,7 +262,9 @@ than building it.
 
             if !has_target {
                 // This might also be a custom target, so check the target file that could have been specified by the user.
-                if let Some(custom_target_path) = env::var_os("RUST_TARGET_PATH") {
+                if target.filepath().is_some_and(|p| p.exists()) {
+                    has_target = true;
+                } else if let Some(custom_target_path) = env::var_os("RUST_TARGET_PATH") {
                     let mut target_filename = OsString::from(&target_str);
                     // Target filename ends with `.json`.
                     target_filename.push(".json");
@@ -277,8 +279,12 @@ than building it.
 
             if !has_target {
                 panic!(
-                    "No such target exists in the target list,
-                specify a correct location of the JSON specification file for custom targets!"
+                    "No such target exists in the target list,\n\
+                     make sure to correctly specify the location \
+                     of the JSON specification file \
+                     for custom targets!\n\
+                     Use BOOTSTRAP_SKIP_TARGET_SANITY=1 to \
+                     bypass this check."
                 );
             }
         }

tests/ui/check-cfg/well-known-values.stderr

@@ -230,7 +230,7 @@ warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
 LL |     target_vendor = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_vendor` are: `apple`, `espressif`, `fortanix`, `ibm`, `kmc`, `nintendo`, `nvidia`, `pc`, `risc0`, `sony`, `sun`, `unikraft`, `unknown`, `uwp`, `win7`, and `wrs`
+   = note: expected values for `target_vendor` are: `apple`, `espressif`, `fortanix`, `ibm`, `kmc`, `nintendo`, `nvidia`, `pc`, `succinct`, `sony`, `sun`, `unikraft`, `unknown`, `uwp`, `win7`, and `wrs`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`