add and document a new is_system_llvm abstraction

Author: jyn514

src/bootstrap/src/core/build_steps/dist.rs

@@ -2042,23 +2042,24 @@ fn install_llvm_file(builder: &Builder<'_>, source: &Path, destination: &Path) {
 ///
 /// Returns whether the files were actually copied.
 fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir: &Path) -> bool {
-    if let Some(config) = builder.config.target_config.get(&target) {
-        if config.llvm_config.is_some() && !builder.config.llvm_from_ci {
-            // If the LLVM was externally provided, then we don't currently copy
-            // artifacts into the sysroot. This is not necessarily the right
-            // choice (in particular, it will require the LLVM dylib to be in
-            // the linker's load path at runtime), but the common use case for
-            // external LLVMs is distribution provided LLVMs, and in that case
-            // they're usually in the standard search path (e.g., /usr/lib) and
-            // copying them here is going to cause problems as we may end up
-            // with the wrong files and isn't what distributions want.
-            //
-            // This behavior may be revisited in the future though.
-            //
-            // If the LLVM is coming from ourselves (just from CI) though, we
-            // still want to install it, as it otherwise won't be available.
-            return false;
-        }
+    // If the LLVM was externally provided, then we don't currently copy
+    // artifacts into the sysroot. This is not necessarily the right
+    // choice (in particular, it will require the LLVM dylib to be in
+    // the linker's load path at runtime), but the common use case for
+    // external LLVMs is distribution provided LLVMs, and in that case
+    // they're usually in the standard search path (e.g., /usr/lib) and
+    // copying them here is going to cause problems as we may end up
+    // with the wrong files and isn't what distributions want.
+    //
+    // This behavior may be revisited in the future though.
+    //
+    // NOTE: this intentionally doesn't use `is_rust_llvm`; whether this is patched or not doesn't matter,
+    // we only care if the shared object itself is managed by bootstrap.
+    //
+    // If the LLVM is coming from ourselves (just from CI) though, we
+    // still want to install it, as it otherwise won't be available.
+    if builder.is_system_llvm(target) {
+        return false;
     }
 
     // On macOS, rustc (and LLVM tools) link to an unversioned libLLVM.dylib

src/bootstrap/src/core/build_steps/test.rs

@@ -1848,6 +1848,8 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 llvm_components_passed = true;
             }
             if !builder.is_rust_llvm(target) {
+                // FIXME: missing Rust patches is not the same as being system llvm; we should rename the flag at some point.
+                // Inspecting the tests with `// no-system-llvm` in src/test *looks* like this is doing the right thing, though.
                 cmd.arg("--system-llvm");
             }
 

src/bootstrap/src/core/config/config.rs

@@ -1810,7 +1810,14 @@ impl Config {
                     }
                     target.llvm_config = Some(config.src.join(s));
                 }
-                target.llvm_has_rust_patches = cfg.llvm_has_rust_patches;
+                if let Some(patches) = cfg.llvm_has_rust_patches {
+                    assert_eq!(
+                        config.submodules,
+                        Some(false),
+                        "cannot set `llvm-has-rust-patches` for a managed submodule (set `build.submodules = false` if you want to apply patches)"
+                    );
+                    target.llvm_has_rust_patches = Some(patches);
+                }
                 if let Some(ref s) = cfg.llvm_filecheck {
                     target.llvm_filecheck = Some(config.src.join(s));
                 }

src/bootstrap/src/lib.rs

@@ -823,18 +823,34 @@ impl Build {
         INTERNER.intern_path(self.out.join(&*target.triple).join("md-doc"))
     }
 
-    /// Returns `true` if no custom `llvm-config` is set for the specified target.
+    /// Returns `true` if this is an external version of LLVM not managed by bootstrap.
+    /// In particular, we expect llvm sources to be available when this is false.
     ///
-    /// If no custom `llvm-config` was specified then Rust's llvm will be used.
+    /// NOTE: this is not the same as `!is_rust_llvm` when `llvm_has_patches` is set.
+    fn is_system_llvm(&self, target: TargetSelection) -> bool {
+        match self.config.target_config.get(&target) {
+            Some(Target { llvm_config: Some(_), .. }) => {
+                let ci_llvm = self.config.llvm_from_ci && target == self.config.build;
+                !ci_llvm
+            }
+            // We're building from the in-tree src/llvm-project sources.
+            Some(Target { llvm_config: None, .. }) => false,
+            None => false,
+        }
+    }
+
+    /// Returns `true` if this is our custom, patched, version of LLVM.
+    ///
+    /// This does not necessarily imply that we're managing the `llvm-project` submodule.
     fn is_rust_llvm(&self, target: TargetSelection) -> bool {
         match self.config.target_config.get(&target) {
+            // We're using a user-controlled version of LLVM. The user has explicitly told us whether the version has our patches.
+            // (They might be wrong, but that's not a supported use-case.)
+            // In particular, this tries to support `submodules = false` and `patches = false`, for using a newer version of LLVM that's not through `rust-lang/llvm-project`.
             Some(Target { llvm_has_rust_patches: Some(patched), .. }) => *patched,
-            Some(Target { llvm_config, .. }) => {
-                // If the user set llvm-config we assume Rust is not patched,
-                // but first check to see if it was configured by llvm-from-ci.
-                (self.config.llvm_from_ci && target == self.config.build) || llvm_config.is_none()
-            }
-            None => true,
+            // The user hasn't promised the patches match.
+            // This only has our patches if it's downloaded from CI or built from source.
+            _ => !self.is_system_llvm(target),
         }
     }