diff --git a/src/bootstrap/src/core/build_steps/check.rs b/src/bootstrap/src/core/build_steps/check.rs
index ed5b9edc86d..3e0bb2884cd 100644
--- a/src/bootstrap/src/core/build_steps/check.rs
+++ b/src/bootstrap/src/core/build_steps/check.rs
@@ -9,7 +9,7 @@
 };
 use crate::core::config::TargetSelection;
 use crate::{Compiler, Mode, Subcommand};
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 pub fn cargo_subcommand(kind: Kind) -> &'static str {
     match kind {
@@ -52,7 +52,7 @@ fn make_run(run: RunConfig<'_>) {
     }
 
     fn run(self, builder: &Builder<'_>) {
-        builder.update_submodule(&Path::new("library").join("stdarch"));
+        builder.require_submodule("library/stdarch", None);
 
         let target = self.target;
         let compiler = builder.compiler(builder.top_stage, builder.config.build);
diff --git a/src/bootstrap/src/core/build_steps/clippy.rs b/src/bootstrap/src/core/build_steps/clippy.rs
index ee7fb368a8c..68abf1e464a 100644
--- a/src/bootstrap/src/core/build_steps/clippy.rs
+++ b/src/bootstrap/src/core/build_steps/clippy.rs
@@ -1,7 +1,5 @@
 //! Implementation of running clippy on the compiler, standard library and various tools.
 
-use std::path::Path;
-
 use crate::builder::Builder;
 use crate::builder::ShouldRun;
 use crate::core::builder;
@@ -127,7 +125,7 @@ fn make_run(run: RunConfig<'_>) {
     }
 
     fn run(self, builder: &Builder<'_>) {
-        builder.update_submodule(&Path::new("library").join("stdarch"));
+        builder.require_submodule("library/stdarch", None);
 
         let target = self.target;
         let compiler = builder.compiler(builder.top_stage, builder.config.build);
diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 9bbebf9b870..59649058046 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -182,11 +182,16 @@ fn run(self, builder: &Builder<'_>) {
             return;
         }
 
-        builder.update_submodule(&Path::new("library").join("stdarch"));
+        builder.require_submodule("library/stdarch", None);
 
         // Profiler information requires LLVM's compiler-rt
         if builder.config.profiler {
-            builder.update_submodule(Path::new("src/llvm-project"));
+            builder.require_submodule(
+                "src/llvm-project",
+                Some(
+                    "The `build.profiler` config option requires `compiler-rt` sources from LLVM.",
+                ),
+            );
         }
 
         let mut target_deps = builder.ensure(StartupObjects { compiler, target });
@@ -456,13 +461,15 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
         // That's probably ok? At least, the difference wasn't enforced before. There's a comment in
         // the compiler_builtins build script that makes me nervous, though:
         // https://github.com/rust-lang/compiler-builtins/blob/31ee4544dbe47903ce771270d6e3bea8654e9e50/build.rs#L575-L579
-        builder.update_submodule(&Path::new("src").join("llvm-project"));
+        builder.require_submodule(
+            "src/llvm-project",
+            Some(
+                "The `build.optimized-compiler-builtins` config option \
+                 requires `compiler-rt` sources from LLVM.",
+            ),
+        );
         let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
-        if !compiler_builtins_root.exists() {
-            panic!(
-                "need LLVM sources available to build `compiler-rt`, but they weren't present; consider enabling `build.submodules = true` or disabling `optimized-compiler-builtins`"
-            );
-        }
+        assert!(compiler_builtins_root.exists());
         // Note that `libprofiler_builtins/build.rs` also computes this so if
         // you're changing something here please also change that.
         cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);
diff --git a/src/bootstrap/src/core/build_steps/dist.rs b/src/bootstrap/src/core/build_steps/dist.rs
index 649bcdd8733..4f4ef29c1d7 100644
--- a/src/bootstrap/src/core/build_steps/dist.rs
+++ b/src/bootstrap/src/core/build_steps/dist.rs
@@ -907,7 +907,7 @@ fn make_run(run: RunConfig<'_>) {
     /// Creates the `rust-src` installer component
     fn run(self, builder: &Builder<'_>) -> GeneratedTarball {
         if !builder.config.dry_run() {
-            builder.update_submodule(Path::new("src/llvm-project"));
+            builder.require_submodule("src/llvm-project", None);
         }
 
         let tarball = Tarball::new_targetless(builder, "rust-src");
@@ -1022,10 +1022,7 @@ fn run(self, builder: &Builder<'_>) -> GeneratedTarball {
             // FIXME: This code looks _very_ similar to what we have in `src/core/build_steps/vendor.rs`
             // perhaps it should be removed in favor of making `dist` perform the `vendor` step?
 
-            // Ensure we have all submodules from src and other directories checked out.
-            for submodule in build_helper::util::parse_gitmodules(&builder.src) {
-                builder.update_submodule(Path::new(submodule));
-            }
+            builder.require_and_update_all_submodules();
 
             // Vendor all Cargo dependencies
             let mut cmd = command(&builder.initial_cargo);
diff --git a/src/bootstrap/src/core/build_steps/doc.rs b/src/bootstrap/src/core/build_steps/doc.rs
index d8204ea00f7..bdd48fa644a 100644
--- a/src/bootstrap/src/core/build_steps/doc.rs
+++ b/src/bootstrap/src/core/build_steps/doc.rs
@@ -9,14 +9,14 @@
 
 use std::io::{self, Write};
 use std::path::{Path, PathBuf};
-use std::{fs, mem};
+use std::{env, fs, mem};
 
 use crate::core::build_steps::compile;
 use crate::core::build_steps::tool::{self, prepare_tool_cargo, SourceType, Tool};
 use crate::core::builder::{self, crate_description};
 use crate::core::builder::{Alias, Builder, Compiler, Kind, RunConfig, ShouldRun, Step};
 use crate::core::config::{Config, TargetSelection};
-use crate::utils::helpers::{dir_is_empty, symlink_dir, t, up_to_date};
+use crate::utils::helpers::{symlink_dir, t, up_to_date};
 use crate::Mode;
 
 macro_rules! submodule_helper {
@@ -53,8 +53,8 @@ fn make_run(run: RunConfig<'_>) {
 
             fn run(self, builder: &Builder<'_>) {
                 $(
-                    let path = Path::new(submodule_helper!( $path, submodule $( = $submodule )? ));
-                    builder.update_submodule(&path);
+                    let path = submodule_helper!( $path, submodule $( = $submodule )? );
+                    builder.require_submodule(path, None);
                 )?
                 builder.ensure(RustbookSrc {
                     target: self.target,
@@ -62,6 +62,7 @@ fn run(self, builder: &Builder<'_>) {
                     src: builder.src.join($path),
                     parent: Some(self),
                     languages: $lang.into(),
+                    rustdoc: None,
                 })
             }
         }
@@ -80,7 +81,6 @@ fn run(self, builder: &Builder<'_>) {
     EditionGuide, "src/doc/edition-guide", "edition-guide", &[], submodule;
     EmbeddedBook, "src/doc/embedded-book", "embedded-book", &[], submodule;
     Nomicon, "src/doc/nomicon", "nomicon", &[], submodule;
-    Reference, "src/doc/reference", "reference", &[], submodule;
     RustByExample, "src/doc/rust-by-example", "rust-by-example", &["ja"], submodule;
     RustdocBook, "src/doc/rustdoc", "rustdoc", &[];
     StyleGuide, "src/doc/style-guide", "style-guide", &[];
@@ -112,6 +112,7 @@ fn run(self, builder: &Builder<'_>) {
             src: builder.md_doc_out(self.target).join("unstable-book"),
             parent: Some(self),
             languages: vec![],
+            rustdoc: None,
         })
     }
 }
@@ -123,6 +124,7 @@ struct RustbookSrc<P: Step> {
     src: PathBuf,
     parent: Option<P>,
     languages: Vec<&'static str>,
+    rustdoc: Option<PathBuf>,
 }
 
 impl<P: Step> Step for RustbookSrc<P> {
@@ -153,13 +155,18 @@ fn run(self, builder: &Builder<'_>) {
             builder.info(&format!("Rustbook ({target}) - {name}"));
             let _ = fs::remove_dir_all(&out);
 
-            builder
-                .tool_cmd(Tool::Rustbook)
-                .arg("build")
-                .arg(&src)
-                .arg("-d")
-                .arg(&out)
-                .run(builder);
+            let mut rustbook_cmd = builder.tool_cmd(Tool::Rustbook);
+            if let Some(mut rustdoc) = self.rustdoc {
+                rustdoc.pop();
+                let old_path = env::var_os("PATH").unwrap_or_default();
+                let new_path =
+                    env::join_paths(std::iter::once(rustdoc).chain(env::split_paths(&old_path)))
+                        .expect("could not add rustdoc to PATH");
+
+                rustbook_cmd.env("PATH", new_path);
+            }
+
+            rustbook_cmd.arg("build").arg(&src).arg("-d").arg(&out).run(builder);
 
             for lang in &self.languages {
                 let out = out.join(lang);
@@ -217,22 +224,14 @@ fn make_run(run: RunConfig<'_>) {
     /// * Index page
     /// * Redirect pages
     fn run(self, builder: &Builder<'_>) {
-        let relative_path = Path::new("src").join("doc").join("book");
-        builder.update_submodule(&relative_path);
+        builder.require_submodule("src/doc/book", None);
 
         let compiler = self.compiler;
         let target = self.target;
 
-        let absolute_path = builder.src.join(&relative_path);
+        let absolute_path = builder.src.join("src/doc/book");
         let redirect_path = absolute_path.join("redirects");
-        if !absolute_path.exists()
-            || !redirect_path.exists()
-            || dir_is_empty(&absolute_path)
-            || dir_is_empty(&redirect_path)
-        {
-            eprintln!("Please checkout submodule: {}", relative_path.display());
-            crate::exit!(1);
-        }
+
         // build book
         builder.ensure(RustbookSrc {
             target,
@@ -240,6 +239,7 @@ fn run(self, builder: &Builder<'_>) {
             src: absolute_path.clone(),
             parent: Some(self),
             languages: vec![],
+            rustdoc: None,
         });
 
         // building older edition redirects
@@ -252,6 +252,7 @@ fn run(self, builder: &Builder<'_>) {
                 // treat the other editions as not having a parent.
                 parent: Option::<Self>::None,
                 languages: vec![],
+                rustdoc: None,
             });
         }
 
@@ -932,8 +933,8 @@ fn run(self, builder: &Builder<'_>) {
                     let _ = source_type; // silence the "unused variable" warning
                     let source_type = SourceType::Submodule;
 
-                    let path = Path::new(submodule_helper!( $path, submodule $( = $submodule )? ));
-                    builder.update_submodule(&path);
+                    let path = submodule_helper!( $path, submodule $( = $submodule )? );
+                    builder.require_submodule(path, None);
                 )?
 
                 let stage = builder.top_stage;
@@ -1172,12 +1173,6 @@ fn make_run(run: RunConfig<'_>) {
     /// in the "md-doc" directory in the build output directory. Then
     /// "rustbook" is used to convert it to HTML.
     fn run(self, builder: &Builder<'_>) {
-        // These submodules are required to be checked out to build rustbook
-        // because they have Cargo dependencies that are needed.
-        #[allow(clippy::single_element_loop)] // This will change soon.
-        for path in ["src/doc/book"] {
-            builder.update_submodule(Path::new(path));
-        }
         let out_base = builder.md_doc_out(self.target).join("rustc");
         t!(fs::create_dir_all(&out_base));
         let out_listing = out_base.join("src/lints");
@@ -1228,6 +1223,50 @@ fn run(self, builder: &Builder<'_>) {
             src: out_base,
             parent: Some(self),
             languages: vec![],
+            rustdoc: None,
+        });
+    }
+}
+
+#[derive(Ord, PartialOrd, Debug, Clone, Hash, PartialEq, Eq)]
+pub struct Reference {
+    pub compiler: Compiler,
+    pub target: TargetSelection,
+}
+
+impl Step for Reference {
+    type Output = ();
+    const DEFAULT: bool = true;
+
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        let builder = run.builder;
+        run.path("src/doc/reference").default_condition(builder.config.docs)
+    }
+
+    fn make_run(run: RunConfig<'_>) {
+        run.builder.ensure(Reference {
+            compiler: run.builder.compiler(run.builder.top_stage, run.builder.config.build),
+            target: run.target,
+        });
+    }
+
+    /// Builds the reference book.
+    fn run(self, builder: &Builder<'_>) {
+        builder.require_submodule("src/doc/reference", None);
+
+        // This is needed for generating links to the standard library using
+        // the mdbook-spec plugin.
+        builder.ensure(compile::Std::new(self.compiler, builder.config.build));
+        let rustdoc = builder.rustdoc(self.compiler);
+
+        // Run rustbook/mdbook to generate the HTML pages.
+        builder.ensure(RustbookSrc {
+            target: self.target,
+            name: "reference".to_owned(),
+            src: builder.src.join("src/doc/reference"),
+            parent: Some(self),
+            languages: vec![],
+            rustdoc: Some(rustdoc),
         });
     }
 }
diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index f234b08f5e2..369e14df14b 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -89,7 +89,7 @@ fn push_all(&mut self, s: impl AsRef<OsStr>) {
 /// if not).
 pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> LlvmBuildStatus {
     // If we have llvm submodule initialized already, sync it.
-    builder.update_existing_submodule(&Path::new("src").join("llvm-project"));
+    builder.update_existing_submodule("src/llvm-project");
 
     builder.config.maybe_download_ci_llvm();
 
@@ -110,7 +110,8 @@ pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> L
     }
 
     // Initialize the llvm submodule if not initialized already.
-    builder.update_submodule(&Path::new("src").join("llvm-project"));
+    // If submodules are disabled, this does nothing.
+    builder.update_submodule("src/llvm-project");
 
     let root = "src/llvm-project/llvm";
     let out_dir = builder.llvm_out(target);
@@ -1197,7 +1198,10 @@ fn make_run(run: RunConfig<'_>) {
 
     /// Build crtbegin.o/crtend.o for musl target.
     fn run(self, builder: &Builder<'_>) -> Self::Output {
-        builder.update_submodule(Path::new("src/llvm-project"));
+        builder.require_submodule(
+            "src/llvm-project",
+            Some("The LLVM sources are required for the CRT from `compiler-rt`."),
+        );
 
         let out_dir = builder.native_dir(self.target).join("crt");
 
@@ -1270,7 +1274,10 @@ fn make_run(run: RunConfig<'_>) {
 
     /// Build libunwind.a
     fn run(self, builder: &Builder<'_>) -> Self::Output {
-        builder.update_submodule(Path::new("src/llvm-project"));
+        builder.require_submodule(
+            "src/llvm-project",
+            Some("The LLVM sources are required for libunwind."),
+        );
 
         if builder.config.dry_run() {
             return PathBuf::new();
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index cc5931c68db..daafb41e9d8 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -2257,7 +2257,12 @@ fn run_local_doc(self, builder: &Builder<'_>) {
 }
 
 macro_rules! test_book {
-    ($($name:ident, $path:expr, $book_name:expr, default=$default:expr;)+) => {
+    ($(
+        $name:ident, $path:expr, $book_name:expr,
+        default=$default:expr
+        $(,submodules = $submodules:expr)?
+        ;
+    )+) => {
         $(
             #[derive(Debug, Clone, PartialEq, Eq, Hash)]
             pub struct $name {
@@ -2280,6 +2285,11 @@ fn make_run(run: RunConfig<'_>) {
                 }
 
                 fn run(self, builder: &Builder<'_>) {
+                    $(
+                        for submodule in $submodules {
+                            builder.require_submodule(submodule, None);
+                        }
+                    )*
                     builder.ensure(BookTest {
                         compiler: self.compiler,
                         path: PathBuf::from($path),
@@ -2293,15 +2303,15 @@ fn run(self, builder: &Builder<'_>) {
 }
 
 test_book!(
-    Nomicon, "src/doc/nomicon", "nomicon", default=false;
-    Reference, "src/doc/reference", "reference", default=false;
+    Nomicon, "src/doc/nomicon", "nomicon", default=false, submodules=["src/doc/nomicon"];
+    Reference, "src/doc/reference", "reference", default=false, submodules=["src/doc/reference"];
     RustdocBook, "src/doc/rustdoc", "rustdoc", default=true;
     RustcBook, "src/doc/rustc", "rustc", default=true;
-    RustByExample, "src/doc/rust-by-example", "rust-by-example", default=false;
-    EmbeddedBook, "src/doc/embedded-book", "embedded-book", default=false;
-    TheBook, "src/doc/book", "book", default=false;
+    RustByExample, "src/doc/rust-by-example", "rust-by-example", default=false, submodules=["src/doc/rust-by-example"];
+    EmbeddedBook, "src/doc/embedded-book", "embedded-book", default=false, submodules=["src/doc/embedded-book"];
+    TheBook, "src/doc/book", "book", default=false, submodules=["src/doc/book"];
     UnstableBook, "src/doc/unstable-book", "unstable-book", default=true;
-    EditionGuide, "src/doc/edition-guide", "edition-guide", default=false;
+    EditionGuide, "src/doc/edition-guide", "edition-guide", default=false, submodules=["src/doc/edition-guide"];
 );
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
@@ -2398,8 +2408,8 @@ fn make_run(run: RunConfig<'_>) {
     }
 
     fn run(self, builder: &Builder<'_>) {
-        let relative_path = Path::new("src").join("doc").join("rustc-dev-guide");
-        builder.update_submodule(&relative_path);
+        let relative_path = "src/doc/rustc-dev-guide";
+        builder.require_submodule(relative_path, None);
 
         let src = builder.src.join(relative_path);
         let mut rustbook_cmd = builder.tool_cmd(Tool::Rustbook).delay_failure();
@@ -3009,7 +3019,7 @@ fn run(self, builder: &Builder<'_>) {
         let _guard = builder.msg(Kind::Test, 0, "bootstrap", host, host);
 
         // Some tests require cargo submodule to be present.
-        builder.build.update_submodule(Path::new("src/tools/cargo"));
+        builder.build.require_submodule("src/tools/cargo", None);
 
         let mut check_bootstrap = command(builder.python());
         check_bootstrap
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index 087df2f8a88..a0cb0b0dbb9 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -1,6 +1,6 @@
 use std::env;
 use std::fs;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 use crate::core::build_steps::compile;
 use crate::core::build_steps::toolstate::ToolState;
@@ -241,6 +241,7 @@ macro_rules! bootstrap_tool {
         $(,is_external_tool = $external:expr)*
         $(,is_unstable_tool = $unstable:expr)*
         $(,allow_features = $allow_features:expr)?
+        $(,submodules = $submodules:expr)?
         ;
     )+) => {
         #[derive(PartialEq, Eq, Clone)]
@@ -287,6 +288,11 @@ fn make_run(run: RunConfig<'_>) {
             }
 
             fn run(self, builder: &Builder<'_>) -> PathBuf {
+                $(
+                    for submodule in $submodules {
+                        builder.require_submodule(submodule, None);
+                    }
+                )*
                 builder.ensure(ToolBuild {
                     compiler: self.compiler,
                     target: self.target,
@@ -314,7 +320,7 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
 }
 
 bootstrap_tool!(
-    Rustbook, "src/tools/rustbook", "rustbook";
+    Rustbook, "src/tools/rustbook", "rustbook", submodules = SUBMODULES_FOR_RUSTBOOK;
     UnstableBookGen, "src/tools/unstable-book-gen", "unstable-book-gen";
     Tidy, "src/tools/tidy", "tidy";
     Linkchecker, "src/tools/linkchecker", "linkchecker";
@@ -340,6 +346,10 @@ fn run(self, builder: &Builder<'_>) -> PathBuf {
     WasmComponentLd, "src/tools/wasm-component-ld", "wasm-component-ld", is_unstable_tool = true, allow_features = "min_specialization";
 );
 
+/// These are the submodules that are required for rustbook to work due to
+/// depending on mdbook plugins.
+pub static SUBMODULES_FOR_RUSTBOOK: &[&str] = &["src/doc/book", "src/doc/reference"];
+
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub struct OptimizedDist {
     pub compiler: Compiler,
@@ -363,7 +373,7 @@ fn make_run(run: RunConfig<'_>) {
     fn run(self, builder: &Builder<'_>) -> PathBuf {
         // We need to ensure the rustc-perf submodule is initialized when building opt-dist since
         // the tool requires it to be in place to run.
-        builder.update_submodule(Path::new("src/tools/rustc-perf"));
+        builder.require_submodule("src/tools/rustc-perf", None);
 
         builder.ensure(ToolBuild {
             compiler: self.compiler,
@@ -404,7 +414,7 @@ fn make_run(run: RunConfig<'_>) {
 
     fn run(self, builder: &Builder<'_>) -> PathBuf {
         // We need to ensure the rustc-perf submodule is initialized.
-        builder.update_submodule(Path::new("src/tools/rustc-perf"));
+        builder.require_submodule("src/tools/rustc-perf", None);
 
         let tool = ToolBuild {
             compiler: self.compiler,
@@ -705,7 +715,7 @@ fn make_run(run: RunConfig<'_>) {
     }
 
     fn run(self, builder: &Builder<'_>) -> PathBuf {
-        builder.build.update_submodule(Path::new("src/tools/cargo"));
+        builder.build.require_submodule("src/tools/cargo", None);
 
         builder.ensure(ToolBuild {
             compiler: self.compiler,
@@ -1087,8 +1097,6 @@ fn run(mut $sel, $builder: &Builder<'_>) -> PathBuf {
 
 // NOTE: tools need to be also added to `Builder::get_step_descriptions` in `builder.rs`
 // to make `./x.py build <tool>` work.
-// NOTE: Most submodule updates for tools are handled by bootstrap.py, since they're needed just to
-// invoke Cargo to build bootstrap. See the comment there for more details.
 tool_extended!((self, builder),
     Cargofmt, "src/tools/rustfmt", "cargo-fmt", stable=true;
     CargoClippy, "src/tools/clippy", "cargo-clippy", stable=true;
diff --git a/src/bootstrap/src/core/build_steps/vendor.rs b/src/bootstrap/src/core/build_steps/vendor.rs
index e6b3cb320cf..8732b30e940 100644
--- a/src/bootstrap/src/core/build_steps/vendor.rs
+++ b/src/bootstrap/src/core/build_steps/vendor.rs
@@ -1,6 +1,7 @@
+use crate::core::build_steps::tool::SUBMODULES_FOR_RUSTBOOK;
 use crate::core::builder::{Builder, RunConfig, ShouldRun, Step};
 use crate::utils::exec::command;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub(crate) struct Vendor {
@@ -35,8 +36,8 @@ fn run(self, builder: &Builder<'_>) -> Self::Output {
         }
 
         // These submodules must be present for `x vendor` to work.
-        for path in ["src/tools/cargo", "src/doc/book"] {
-            builder.build.update_submodule(Path::new(path));
+        for submodule in SUBMODULES_FOR_RUSTBOOK.iter().chain(["src/tools/cargo"].iter()) {
+            builder.build.require_submodule(submodule, None);
         }
 
         // Sync these paths by default.
diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index 97c9ece0036..ccabcad243f 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -13,15 +13,18 @@ fn configure_with_args(cmd: &[String], host: &[&str], target: &[&str]) -> Config
     config.save_toolstates = None;
     config.dry_run = DryRun::SelfCheck;
 
-    // Ignore most submodules, since we don't need them for a dry run.
-    // But make sure to check out the `doc` and `rust-analyzer` submodules, since some steps need them
-    // just to know which commands to run.
+    // Ignore most submodules, since we don't need them for a dry run, and the
+    // tests run much faster without them.
+    //
+    // The src/doc/book submodule is needed because TheBook step tries to
+    // access files even during a dry-run (may want to consider just skipping
+    // that in a dry run).
     let submodule_build = Build::new(Config {
         // don't include LLVM, so CI doesn't require ninja/cmake to be installed
         rust_codegen_backends: vec![],
         ..Config::parse(&["check".to_owned()])
     });
-    submodule_build.update_submodule(Path::new("src/doc/book"));
+    submodule_build.require_submodule("src/doc/book", None);
     config.submodules = Some(false);
 
     config.ninja_in_file = false;
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index e32288e2caf..3435225af7b 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -2404,8 +2404,11 @@ pub fn split_debuginfo(&self, target: TargetSelection) -> SplitDebuginfo {
             .unwrap_or_else(|| SplitDebuginfo::default_for_platform(target))
     }
 
-    pub fn submodules(&self, rust_info: &GitInfo) -> bool {
-        self.submodules.unwrap_or(rust_info.is_managed_git_subrepository())
+    /// Returns whether or not submodules should be managed by bootstrap.
+    pub fn submodules(&self) -> bool {
+        // If not specified in config, the default is to only manage
+        // submodules if we're currently inside a git repository.
+        self.submodules.unwrap_or(self.rust_info.is_managed_git_subrepository())
     }
 
     pub fn codegen_backends(&self, target: TargetSelection) -> &[String] {
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index 79bea50c626..d4303aab6d6 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -441,7 +441,13 @@ pub fn new(mut config: Config) -> Build {
             // Cargo.toml files.
             let rust_submodules = ["library/backtrace", "library/stdarch"];
             for s in rust_submodules {
-                build.update_submodule(Path::new(s));
+                build.require_submodule(
+                    s,
+                    Some(
+                        "The submodule is required for the standard library \
+                         and the main Cargo workspace.",
+                    ),
+                );
             }
             // Now, update all existing submodules.
             build.update_existing_submodules();
@@ -470,12 +476,17 @@ pub fn new(mut config: Config) -> Build {
         build
     }
 
-    // modified from `check_submodule` and `update_submodule` in bootstrap.py
     /// Given a path to the directory of a submodule, update it.
     ///
     /// `relative_path` should be relative to the root of the git repository, not an absolute path.
-    pub(crate) fn update_submodule(&self, relative_path: &Path) {
-        if !self.config.submodules(self.rust_info()) {
+    ///
+    /// This *does not* update the submodule if `config.toml` explicitly says
+    /// not to, or if we're not in a git repository (like a plain source
+    /// tarball). Typically [`Build::require_submodule`] should be
+    /// used instead to provide a nice error to the user if the submodule is
+    /// missing.
+    fn update_submodule(&self, relative_path: &str) {
+        if !self.config.submodules() {
             return;
         }
 
@@ -522,7 +533,7 @@ pub(crate) fn update_submodule(&self, relative_path: &Path) {
             return;
         }
 
-        println!("Updating submodule {}", relative_path.display());
+        println!("Updating submodule {relative_path}");
         helpers::git(Some(&self.src))
             .run_always()
             .args(["submodule", "-q", "sync"])
@@ -580,11 +591,53 @@ pub(crate) fn update_submodule(&self, relative_path: &Path) {
         }
     }
 
+    /// Updates a submodule, and exits with a failure if submodule management
+    /// is disabled and the submodule does not exist.
+    ///
+    /// The given submodule name should be its path relative to the root of
+    /// the main repository.
+    ///
+    /// The given `err_hint` will be shown to the user if the submodule is not
+    /// checked out and submodule management is disabled.
+    pub fn require_submodule(&self, submodule: &str, err_hint: Option<&str>) {
+        // When testing bootstrap itself, it is much faster to ignore
+        // submodules. Almost all Steps work fine without their submodules.
+        if cfg!(test) && !self.config.submodules() {
+            return;
+        }
+        self.update_submodule(submodule);
+        let absolute_path = self.config.src.join(submodule);
+        if dir_is_empty(&absolute_path) {
+            let maybe_enable = if !self.config.submodules()
+                && self.config.rust_info.is_managed_git_subrepository()
+            {
+                "\nConsider setting `build.submodules = true` or manually initializing the submodules."
+            } else {
+                ""
+            };
+            let err_hint = err_hint.map_or_else(String::new, |e| format!("\n{e}"));
+            eprintln!(
+                "submodule {submodule} does not appear to be checked out, \
+                 but it is required for this step{maybe_enable}{err_hint}"
+            );
+            exit!(1);
+        }
+    }
+
+    /// Updates all submodules, and exits with an error if submodule
+    /// management is disabled and the submodule does not exist.
+    pub fn require_and_update_all_submodules(&self) {
+        for submodule in build_helper::util::parse_gitmodules(&self.src) {
+            self.require_submodule(submodule, None);
+        }
+    }
+
     /// If any submodule has been initialized already, sync it unconditionally.
     /// This avoids contributors checking in a submodule change by accident.
-    pub fn update_existing_submodules(&self) {
-        // Avoid running git when there isn't a git checkout.
-        if !self.config.submodules(self.rust_info()) {
+    fn update_existing_submodules(&self) {
+        // Avoid running git when there isn't a git checkout, or the user has
+        // explicitly disabled submodules in `config.toml`.
+        if !self.config.submodules() {
             return;
         }
         let output = helpers::git(Some(&self.src))
@@ -597,22 +650,23 @@ pub fn update_existing_submodules(&self) {
         for line in output.lines() {
             // Look for `submodule.$name.path = $path`
             // Sample output: `submodule.src/rust-installer.path src/tools/rust-installer`
-            let submodule = Path::new(line.split_once(' ').unwrap().1);
+            let submodule = line.split_once(' ').unwrap().1;
+            let path = Path::new(submodule);
             // Don't update the submodule unless it's already been cloned.
-            if GitInfo::new(false, submodule).is_managed_git_subrepository() {
+            if GitInfo::new(false, path).is_managed_git_subrepository() {
                 self.update_submodule(submodule);
             }
         }
     }
 
     /// Updates the given submodule only if it's initialized already; nothing happens otherwise.
-    pub fn update_existing_submodule(&self, submodule: &Path) {
+    pub fn update_existing_submodule(&self, submodule: &str) {
         // Avoid running git when there isn't a git checkout.
-        if !self.config.submodules(self.rust_info()) {
+        if !self.config.submodules() {
             return;
         }
 
-        if GitInfo::new(false, submodule).is_managed_git_subrepository() {
+        if GitInfo::new(false, Path::new(submodule)).is_managed_git_subrepository() {
             self.update_submodule(submodule);
         }
     }
diff --git a/src/doc/reference b/src/doc/reference
index e2f0bdc4031..2e191814f16 160000
--- a/src/doc/reference
+++ b/src/doc/reference
@@ -1 +1 @@
-Subproject commit e2f0bdc4031866734661dcdb548184bde1450baf
+Subproject commit 2e191814f163ee1e77e2d6094eee4dd78a289c5b
diff --git a/src/tools/rustbook/Cargo.lock b/src/tools/rustbook/Cargo.lock
index 75b89a162e9..df051ed447e 100644
--- a/src/tools/rustbook/Cargo.lock
+++ b/src/tools/rustbook/Cargo.lock
@@ -661,6 +661,20 @@ dependencies = [
  "textwrap",
 ]
 
+[[package]]
+name = "mdbook-spec"
+version = "0.1.2"
+dependencies = [
+ "anyhow",
+ "mdbook",
+ "once_cell",
+ "pathdiff",
+ "regex",
+ "semver",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "mdbook-trpl-listing"
 version = "0.1.0"
@@ -794,6 +808,12 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "pathdiff"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8835116a5c179084a830efb3adc117ab007512b535bc1a21c991d3b32a6b44dd"
+
 [[package]]
 name = "percent-encoding"
 version = "2.3.1"
@@ -1079,6 +1099,7 @@ dependencies = [
  "env_logger",
  "mdbook",
  "mdbook-i18n-helpers",
+ "mdbook-spec",
  "mdbook-trpl-listing",
  "mdbook-trpl-note",
 ]
diff --git a/src/tools/rustbook/Cargo.toml b/src/tools/rustbook/Cargo.toml
index 51ba58483c5..2c29a2848b7 100644
--- a/src/tools/rustbook/Cargo.toml
+++ b/src/tools/rustbook/Cargo.toml
@@ -12,6 +12,7 @@ env_logger = "0.11"
 mdbook-trpl-listing = { path = "../../doc/book/packages/mdbook-trpl-listing" }
 mdbook-trpl-note = { path = "../../doc/book/packages/mdbook-trpl-note" }
 mdbook-i18n-helpers = "0.3.3"
+mdbook-spec = { path = "../../doc/reference/mdbook-spec"}
 
 [dependencies.mdbook]
 version = "0.4.37"
diff --git a/src/tools/rustbook/src/main.rs b/src/tools/rustbook/src/main.rs
index 31bba56adde..e94c2f5958e 100644
--- a/src/tools/rustbook/src/main.rs
+++ b/src/tools/rustbook/src/main.rs
@@ -9,6 +9,7 @@
 use mdbook::MDBook;
 use mdbook_i18n_helpers::preprocessors::Gettext;
 
+use mdbook_spec::Spec;
 use mdbook_trpl_listing::TrplListing;
 use mdbook_trpl_note::TrplNote;
 
@@ -83,6 +84,13 @@ pub fn build(args: &ArgMatches) -> Result3<()> {
         book.config.build.build_dir = dest_dir.into();
     }
 
+    // NOTE: Replacing preprocessors using this technique causes error
+    // messages to be displayed when the original preprocessor doesn't work
+    // (but it otherwise succeeds).
+    //
+    // This should probably be fixed in mdbook to remove the existing
+    // preprocessor, or this should modify the config and use
+    // MDBook::load_with_config.
     if book.config.get_preprocessor("trpl-note").is_some() {
         book.with_preprocessor(TrplNote);
     }
@@ -91,6 +99,10 @@ pub fn build(args: &ArgMatches) -> Result3<()> {
         book.with_preprocessor(TrplListing);
     }
 
+    if book.config.get_preprocessor("spec").is_some() {
+        book.with_preprocessor(Spec::new());
+    }
+
     book.build()?;
 
     Ok(())
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 499e735b1df..6898c4755d8 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -72,7 +72,7 @@
     //("src/tools/miri/test-cargo-miri", &[], None), // FIXME uncomment once all deps are vendored
     //("src/tools/miri/test_dependencies", &[], None), // FIXME uncomment once all deps are vendored
     ("src/tools/rust-analyzer", EXCEPTIONS_RUST_ANALYZER, None, &[]),
-    ("src/tools/rustbook", EXCEPTIONS_RUSTBOOK, None, &["src/doc/book"]),
+    ("src/tools/rustbook", EXCEPTIONS_RUSTBOOK, None, &["src/doc/book", "src/doc/reference"]),
     ("src/tools/rustc-perf", EXCEPTIONS_RUSTC_PERF, None, &["src/tools/rustc-perf"]),
     ("src/tools/x", &[], None, &[]),
     // tidy-alphabetical-end