Rollup merge of #79970 - bjorn3:no_unnecessary_llvm_checkout, r=Mark-Simulacrum

Misc rustbuild improvements when the LLVM backend isn't used

* Don't checkout llvm-project
* Don't require cmake and ninja

Fixes #78564

src/bootstrap/bootstrap.py

@@ -897,13 +897,17 @@ class RustBuild(object):
         filtered_submodules = []
         submodules_names = []
         llvm_checked_out = os.path.exists(os.path.join(self.rust_root, "src/llvm-project/.git"))
+        external_llvm_provided = self.get_toml('llvm-config') or self.downloading_llvm()
+        llvm_needed = not self.get_toml('codegen-backends', 'rust') \
+            or "llvm" in self.get_toml('codegen-backends', 'rust')
         for module in submodules:
             if module.endswith("llvm-project"):
-                # Don't sync the llvm-project submodule either if an external LLVM
+                # Don't sync the llvm-project submodule if an external LLVM was
-                # was provided, or if we are downloading LLVM. Also, if the
+                # provided, if we are downloading LLVM or if the LLVM backend is
-                # submodule has been initialized already, sync it anyways so that
+                # not being built. Also, if the submodule has been initialized
-                # it doesn't mess up contributor pull requests.
+                # already, sync it anyways so that it doesn't mess up contributor
-                if self.get_toml('llvm-config') or self.downloading_llvm():
+                # pull requests.
+                if external_llvm_provided or not llvm_needed:
                     if self.get_toml('lld') != 'true' and not llvm_checked_out:
                         continue
             check = self.check_submodule(module, slow_submodules)

src/bootstrap/sanity.rs

@@ -17,6 +17,7 @@ use std::process::Command;
 
 use build_helper::{output, t};
 
+use crate::cache::INTERNER;
 use crate::config::Target;
 use crate::Build;
 
@@ -79,7 +80,8 @@ pub fn check(build: &mut Build) {
     }
 
     // We need cmake, but only if we're actually building LLVM or sanitizers.
-    let building_llvm = build
+    let building_llvm = build.config.rust_codegen_backends.contains(&INTERNER.intern_str("llvm"))
+        && build
             .hosts
             .iter()
             .map(|host| {
@@ -147,11 +149,13 @@ pub fn check(build: &mut Build) {
         }
     }
 
+    if build.config.rust_codegen_backends.contains(&INTERNER.intern_str("llvm")) {
         // Externally configured LLVM requires FileCheck to exist
         let filecheck = build.llvm_filecheck(build.build);
         if !filecheck.starts_with(&build.out) && !filecheck.exists() && build.config.codegen_tests {
             panic!("FileCheck executable {:?} does not exist", filecheck);
         }
+    }
 
     for target in &build.targets {
         // Can't compile for iOS unless we're on macOS