Panic if rustdoc is built more than once

This prevents silent regressions where rustdoc is accidentally compiled
in more than one stage. We should always be building it just once
(likely in the final stage).

Author: Mark-Simulacrum

src/bootstrap/tool.rs

@@ -3,6 +3,7 @@ use std::env;
 use std::path::PathBuf;
 use std::process::{Command, exit};
 use std::collections::HashSet;
+use std::sync::Mutex;
 
 use crate::Mode;
 use crate::Compiler;
@@ -436,6 +437,7 @@ pub struct Rustdoc {
     pub compiler: Compiler,
 }
 
+
 impl Step for Rustdoc {
     type Output = PathBuf;
     const DEFAULT: bool = true;
@@ -452,6 +454,21 @@ impl Step for Rustdoc {
     }
 
     fn run(self, builder: &Builder<'_>) -> PathBuf {
+        lazy_static! {
+            static ref BUILT_RUSTDOC_IN: Mutex<Vec<u32>> = Mutex::new(Vec::new());
+        }
+        {
+            let mut built = BUILT_RUSTDOC_IN.lock().unwrap();
+            if !built.contains(&self.compiler.stage) {
+                built.push(self.compiler.stage);
+                assert_eq!(
+                    built.len(),
+                    1,
+                    "Only ever build one copy of rustdoc per run, currently built in stages {:?}",
+                    built,
+                );
+            }
+        }
         let target_compiler = self.compiler;
         if target_compiler.stage == 0 {
             if !target_compiler.is_snapshot(builder) {