Auto merge of #85775 - adamrk:warn-unused-target-fields, r=nagisa

Emit warnings for unused fields in custom targets.

Add a warning which lists any fields in a custom target `json` file that aren't used. Currently unrecognized fields are ignored so, for example, a typo in the `json` will silently produce a target which isn't the one intended.

compiler/rustc_serialize/src/json.rs

@@ -1114,6 +1114,15 @@ impl Json {
         }
     }
 
+    /// If the Json value is an Object, deletes the value associated with the
+    /// provided key from the Object and returns it. Otherwise, returns None.
+    pub fn remove_key(&mut self, key: &str) -> Option<Json> {
+        match *self {
+            Json::Object(ref mut map) => map.remove(key),
+            _ => None,
+        }
+    }
+
     /// Attempts to get a nested Json Object for each key in `keys`.
     /// If any key is found not to exist, `find_path` will return `None`.
     /// Otherwise, it will return the Json value associated with the final key.

compiler/rustc_session/src/config.rs

@@ -12,7 +12,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::impl_stable_hash_via_hash;
 
 use rustc_target::abi::{Align, TargetDataLayout};
-use rustc_target::spec::{SplitDebuginfo, Target, TargetTriple};
+use rustc_target::spec::{SplitDebuginfo, Target, TargetTriple, TargetWarnings};
 
 use rustc_serialize::json;
 
@@ -899,9 +899,11 @@ pub(super) fn build_target_config(
     target_override: Option<Target>,
     sysroot: &PathBuf,
 ) -> Target {
-    let target_result =
-        target_override.map_or_else(|| Target::search(&opts.target_triple, sysroot), Ok);
-    let target = target_result.unwrap_or_else(|e| {
+    let target_result = target_override.map_or_else(
+        || Target::search(&opts.target_triple, sysroot),
+        |t| Ok((t, TargetWarnings::empty())),
+    );
+    let (target, target_warnings) = target_result.unwrap_or_else(|e| {
         early_error(
             opts.error_format,
             &format!(
@@ -911,6 +913,9 @@ pub(super) fn build_target_config(
             ),
         )
     });
+    for warning in target_warnings.warning_messages() {
+        early_warn(opts.error_format, &warning)
+    }
 
     if !matches!(target.pointer_width, 16 | 32 | 64) {
         early_error(

compiler/rustc_session/src/session.rs

@@ -1284,9 +1284,12 @@ pub fn build_session(
 
     let target_cfg = config::build_target_config(&sopts, target_override, &sysroot);
     let host_triple = TargetTriple::from_triple(config::host_triple());
-    let host = Target::search(&host_triple, &sysroot).unwrap_or_else(|e| {
+    let (host, target_warnings) = Target::search(&host_triple, &sysroot).unwrap_or_else(|e| {
         early_error(sopts.error_format, &format!("Error loading host specification: {}", e))
     });
+    for warning in target_warnings.warning_messages() {
+        early_warn(sopts.error_format, &warning)
+    }
 
     let loader = file_loader.unwrap_or_else(|| Box::new(RealFileLoader));
     let hash_kind = sopts.debugging_opts.src_hash_algorithm.unwrap_or_else(|| {

compiler/rustc_target/src/lib.rs

@@ -27,6 +27,9 @@ pub mod abi;
 pub mod asm;
 pub mod spec;
 
+#[cfg(test)]
+mod tests;
+
 /// Requirements for a `StableHashingContext` to be used in this crate.
 /// This is a hack to allow using the `HashStable_Generic` derive macro
 /// instead of implementing everything in `rustc_middle`.