Don't allow -Zunstable-options to take a value

Passing an explicit boolean value (`on`, `off` etc.) appears to work, but
actually puts the compiler into an unintended state where unstable _options_
are still forbidden, but unstable values of _some_ stable options are allowed.

compiler/rustc_session/src/options.rs

@@ -2171,8 +2171,14 @@ written to standard error output)"),
         "enable unsound and buggy MIR optimizations (default: no)"),
     /// This name is kind of confusing: Most unstable options enable something themselves, while
     /// this just allows "normal" options to be feature-gated.
+    ///
+    /// The main check for `-Zunstable-options` takes place separately from the
+    /// usual parsing of `-Z` options (see [`crate::config::nightly_options`]),
+    /// so this boolean value is mostly used for enabling unstable _values_ of
+    /// stable options. That separate check doesn't handle boolean values, so
+    /// to avoid an inconsistent state we also forbid them here.
     #[rustc_lint_opt_deny_field_access("use `Session::unstable_options` instead of this field")]
-    unstable_options: bool = (false, parse_bool, [UNTRACKED],
+    unstable_options: bool = (false, parse_no_value, [UNTRACKED],
         "adds unstable command line options to rustc interface (default: no)"),
     use_ctors_section: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "use legacy .ctors section for initializers rather than .init_array"),

tests/ui/codemap_tests/huge_multispan_highlight.rs

@@ -1,7 +1,7 @@
 //@ revisions: ascii unicode
 //@ compile-flags: --color=always
 //@[ascii] compile-flags: --error-format=human
-//@[unicode] compile-flags: -Zunstable-options=yes --error-format=human-unicode
+//@[unicode] compile-flags: -Zunstable-options --error-format=human-unicode
 //@ ignore-windows
 fn main() {
     let _ = match true {

tests/ui/diagnostic-width/E0271.rs

@@ -1,6 +1,6 @@
 //@ revisions: ascii unicode
 //@[ascii] compile-flags: --diagnostic-width=40
-//@[unicode] compile-flags: -Zunstable-options=yes --error-format=human-unicode --diagnostic-width=40
+//@[unicode] compile-flags: -Zunstable-options --error-format=human-unicode --diagnostic-width=40
 //@ normalize-stderr-test: "long-type-\d+" -> "long-type-hash"
 trait Future {
     type Error;

tests/ui/diagnostic-width/flag-human.rs

@@ -1,6 +1,6 @@
 //@ revisions: ascii unicode
 //@[ascii] compile-flags: --diagnostic-width=20
-//@[unicode] compile-flags: -Zunstable-options=yes --error-format=human-unicode --diagnostic-width=20
+//@[unicode] compile-flags: -Zunstable-options --error-format=human-unicode --diagnostic-width=20
 
 // This test checks that `-Z output-width` effects the human error output by restricting it to an
 // arbitrarily low value so that the effect is visible.

tests/ui/diagnostic-width/long-E0308.rs

@@ -1,6 +1,6 @@
 //@ revisions: ascii unicode
 //@[ascii] compile-flags: --diagnostic-width=60 -Zwrite-long-types-to-disk=yes
-//@[unicode] compile-flags: -Zunstable-options=yes --json=diagnostic-unicode --diagnostic-width=60 -Zwrite-long-types-to-disk=yes
+//@[unicode] compile-flags: -Zunstable-options --json=diagnostic-unicode --diagnostic-width=60 -Zwrite-long-types-to-disk=yes
 //@ normalize-stderr-test: "long-type-\d+" -> "long-type-hash"
 
 mod a {

tests/ui/diagnostic-width/non-1-width-unicode-multiline-label.rs

@@ -1,5 +1,5 @@
 //@ revisions: ascii unicode
-//@[unicode] compile-flags: -Zunstable-options=yes --error-format=human-unicode
+//@[unicode] compile-flags: -Zunstable-options --error-format=human-unicode
 // ignore-tidy-linelength
 
 fn main() {

tests/ui/diagnostic-width/non-whitespace-trimming-2.rs

@@ -1,5 +1,5 @@
 //@ revisions: ascii unicode
-//@[unicode] compile-flags: -Zunstable-options=yes --error-format=human-unicode
+//@[unicode] compile-flags: -Zunstable-options --error-format=human-unicode
 // ignore-tidy-linelength
 
 fn main() {

tests/ui/error-emitter/unicode-output.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -Zunstable-options=yes --error-format=human-unicode --color=always
+//@ compile-flags: -Zunstable-options --error-format=human-unicode --color=always
 //@ edition:2018
 //@ only-linux