Compile Servo with the latest version of rust stable (#30831)

This completes the transition to compiling Servo with rust stable. Some
nightly-only features are still used when compiling the `script` and
`crown` crates, as well as for some style unit tests. These will likely
break with newer compiler versions, but `crown` can be disabled for them
conditionally. This is just the first step.

This has some caveats:

1. We need to disable setting up the special linker on Linux. The -Z
   option isn't supported with stable rust so using this is out --
   meanwhile we can't be sure that lld is installed on most systems.
2. `cargo fmt` still uses some unstable options, so we need to rely on
   the unstable toolchain just for running `fmt`. The idea is to fix this
   gradually.

support/crown/tests/compile-fail/trace_in_no_trace_composable_works.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 // compile-flags: --error-format=human
+//@rustc-env:RUSTC_BOOTSTRAP=1
+
 /// Mock `JSTraceable`
 pub trait JSTraceable {}
 

support/crown/tests/compile-fail/trace_in_no_trace_works.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 /// Mock `JSTraceable`
 pub trait JSTraceable {}

support/crown/tests/compile-fail/unrooted_must_root_parameter.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 #[crown::unrooted_must_root_lint::must_root]
 struct Foo(i32);

support/crown/tests/compile-fail/unrooted_must_root_return_type.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 #[crown::unrooted_must_root_lint::must_root]
 struct Foo(i32);

support/crown/tests/compile-fail/unrooted_must_root_struct_field.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 #[crown::unrooted_must_root_lint::must_root]
 struct Foo(i32);

support/crown/tests/run-pass/trace_in_no_trace_composable.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 /// Mock `JSTraceable`
 pub trait JSTraceable {}

support/crown/tests/run-pass/trace_in_no_trace_ok.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 /// Mock `JSTraceable`
 pub trait JSTraceable {}

support/crown/tests/run-pass/unrooted_must_root_ok.rs

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 // compile-flags: --error-format=human
+//@rustc-env:RUSTC_BOOTSTRAP=1
+
 #[crown::unrooted_must_root_lint::must_root]
 struct Foo(i32);
 #[crown::unrooted_must_root_lint::must_root]

support/crown/tests/ui/trace_in_no_trace_primitive.rs

@@ -1,6 +1,7 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+//@rustc-env:RUSTC_BOOTSTRAP=1
 
 /// Mock `JSTraceable`
 pub trait JSTraceable {}

support/crown/tests/ui/trace_in_no_trace_primitive.stderr

@@ -1,7 +1,7 @@
 warning: must_not_have_traceable marked wrapper is not needed for types that implements empty Traceable (like primitive types). Consider removing the wrapper.
-  --> $DIR/trace_in_no_trace_primitive.rs:15:12
+  --> $DIR/trace_in_no_trace_primitive.rs:16:12
    |
-15 | struct Foo(NoTrace<i32>);
+16 | struct Foo(NoTrace<i32>);
    |            ^^^^^^^^^^^^
    |
    = note: `#[warn(crown::empty_trace_in_no_trace)]` on by default