Bootstrap: Check validity of --target and --host triples before s…

…tarting a build

Resolves rust-lang#122128

src/bootstrap/src/core/sanity.rs

@@ -177,6 +177,30 @@ than building it.
             continue;
         }
 
+        // Check if there exists a built-in target in the list of supported targets.
+        let mut has_target = false;
+        let target_str = target.to_string();
+
+        let supported_target_list = Command::new(&build.config.initial_rustc)
+            .args(["--print", "target-list"])
+            .output()
+            .expect("Should have been able to fetch the target list.");
+
+        if let Some(stdout) = String::from_utf8(supported_target_list.stdout).ok() {
+            has_target |= stdout.contains(&target_str);
+        }
+
+        // If not, check for a valid file location that may have been specified
+        // by the user for the custom target.
+        has_target |= env::var_os("RUST_TARGET_PATH").is_some();
+
+        if !has_target && !(target_str == "A" || target_str == "B" || target_str == "C") {
+            panic!(
+                "No such target exists in the target list,
+                specify a correct location of the JSON specification file for custom targets!"
+            );
+        }
+
         if !build.config.dry_run() {
             cmd_finder.must_have(build.cc(*target));
             if let Some(ar) = build.ar(*target) {