rustdoc: display doc(cfg(false)) properly

before we had an extra 'on' that was
ungramatical.

fixes https://github.com/rust-lang/rust/issues/138112

src/librustdoc/clean/cfg.rs

@@ -171,10 +171,15 @@ impl Cfg {
 
     /// Renders the configuration for long display, as a long HTML description.
     pub(crate) fn render_long_html(&self) -> String {
-        let on = if self.should_use_with_in_description() { "with" } else { "on" };
+        let on = if self.omit_preposition() {
+            ""
+        } else if self.should_use_with_in_description() {
+            "with "
+        } else {
+            "on "
+        };
 
-        let mut msg =
+        let mut msg = format!("Available {on}<strong>{}</strong>", Display(self, Format::LongHtml));
-            format!("Available {on} <strong>{}</strong>", Display(self, Format::LongHtml));
         if self.should_append_only_to_description() {
             msg.push_str(" only");
         }
@@ -244,6 +249,10 @@ impl Cfg {
             Some(self.clone())
         }
     }
+
+    fn omit_preposition(&self) -> bool {
+        matches!(self, Cfg::True | Cfg::False)
+    }
 }
 
 impl ops::Not for Cfg {

tests/rustdoc/cfg-bool.rs

@@ -0,0 +1,13 @@
+#![feature(doc_cfg)]
+#![crate_name = "foo"]
+
+// regression test for https://github.com/rust-lang/rust/issues/138112
+
+//@ has 'foo/fn.foo.html' '//div[@class="stab portability"]' 'Available nowhere'
+#[doc(cfg(false))]
+pub fn foo() {}
+
+// a cfg(true) will simply be ommited, as it is the same as no cfg.
+//@ !has 'foo/fn.bar.html' '//div[@class="stab portability"]' ''
+#[doc(cfg(true))]
+pub fn bar() {}