Do not allow JSON targets to set is-builtin: true

compiler/rustc_target/src/spec/mod.rs

@@ -2010,6 +2010,10 @@ impl Target {
         key!(supported_sanitizers, SanitizerSet)?;
         key!(default_adjusted_cabi, Option<Abi>)?;
 
+        if base.is_builtin {
+            // This can cause unfortunate ICEs later down the line.
+            return Err(format!("may not set is_builtin for targets not built-in"));
+        }
         // Each field should have been read using `Json::remove_key` so any keys remaining are unused.
         let remaining_keys = obj.as_object().ok_or("Expected JSON object for target")?.keys();
         Ok((

src/test/run-make-fulldeps/rustdoc-target-spec-json-path/target.json

@@ -8,7 +8,6 @@
   "executables": true,
   "has-elf-tls": true,
   "has-rpath": true,
-  "is-builtin": true,
   "linker-is-gnu": true,
   "llvm-target": "x86_64-unknown-linux-gnu",
   "max-atomic-width": 64,

src/test/run-make-fulldeps/target-specs/Makefile

@@ -7,3 +7,5 @@ all:
 	RUST_TARGET_PATH=. $(RUSTC) foo.rs --target=my-awesome-platform --crate-type=lib --emit=asm
 	RUST_TARGET_PATH=. $(RUSTC) foo.rs --target=my-x86_64-unknown-linux-gnu-platform --crate-type=lib --emit=asm
 	$(RUSTC) -Z unstable-options --target=my-awesome-platform.json --print target-spec-json > $(TMPDIR)/test-platform.json && $(RUSTC) -Z unstable-options --target=$(TMPDIR)/test-platform.json --print target-spec-json | diff -q $(TMPDIR)/test-platform.json -
+	$(RUSTC) foo.rs --target=definitely-not-builtin-target 2>&1 | $(CGREP) 'may not set is_builtin'
+	$(RUSTC) foo.rs --target=mismatching-data-layout

src/test/run-make-fulldeps/target-specs/definitely-not-builtin-target.json

@@ -0,0 +1,7 @@
+{
+  "arch": "x86_64",
+  "is-builtin": true,
+  "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
+  "llvm-target": "x86_64-unknown-unknown-gnu",
+  "target-pointer-width": "64"
+}

src/test/run-make-fulldeps/target-specs/mismatching-data-layout.json

@@ -0,0 +1,6 @@
+{
+  "arch": "x86_64",
+  "data-layout": "e-m:e-i64:16:32:64",
+  "llvm-target": "x86_64-unknown-unknown-gnu",
+  "target-pointer-width": "64"
+}