bootstrap: Default to a sensible llvm-suffix.

I used version-channel-sha, hopefully that should work.

I checked that bootstrap builds, but I cannot check anything else since the llvm
build process is started from cargo, and thus calls clang, and thus I hit the
same bug I hope to fix with this change.

Hopefully fixes #59034.

src/bootstrap/lib.rs

@@ -241,6 +241,8 @@ pub struct Build {
     clippy_info: channel::GitInfo,
     miri_info: channel::GitInfo,
     rustfmt_info: channel::GitInfo,
+    in_tree_llvm_info: channel::GitInfo,
+    emscripten_llvm_info: channel::GitInfo,
     local_rebuild: bool,
     fail_fast: bool,
     doc_tests: DocTests,
@@ -363,6 +365,8 @@ impl Build {
         let clippy_info = channel::GitInfo::new(&config, &src.join("src/tools/clippy"));
         let miri_info = channel::GitInfo::new(&config, &src.join("src/tools/miri"));
         let rustfmt_info = channel::GitInfo::new(&config, &src.join("src/tools/rustfmt"));
+        let in_tree_llvm_info = channel::GitInfo::new(&config, &src.join("src/llvm-project"));
+        let emscripten_llvm_info = channel::GitInfo::new(&config, &src.join("src/llvm-emscripten"));
 
         let mut build = Build {
             initial_rustc: config.initial_rustc.clone(),
@@ -386,6 +390,8 @@ impl Build {
             clippy_info,
             miri_info,
             rustfmt_info,
+            in_tree_llvm_info,
+            emscripten_llvm_info,
             cc: HashMap::new(),
             cxx: HashMap::new(),
             ar: HashMap::new(),

src/bootstrap/native.rs

@@ -18,6 +18,7 @@ use build_helper::output;
 use cmake;
 use cc;
 
+use crate::channel;
 use crate::util::{self, exe};
 use build_helper::up_to_date;
 use crate::builder::{Builder, RunConfig, ShouldRun, Step};
@@ -231,7 +232,26 @@ impl Step for Llvm {
         }
 
         if let Some(ref suffix) = builder.config.llvm_version_suffix {
-            cfg.define("LLVM_VERSION_SUFFIX", suffix);
+            // Allow version-suffix="" to not define a version suffix at all.
+            if !suffix.is_empty() {
+                cfg.define("LLVM_VERSION_SUFFIX", suffix);
+            }
+        } else {
+            let mut default_suffix = format!(
+                "-rust-{}-{}",
+                channel::CFG_RELEASE_NUM,
+                builder.config.channel,
+            );
+            let llvm_info = if self.emscripten {
+                &builder.emscripten_llvm_info
+            } else {
+                &builder.in_tree_llvm_info
+            };
+            if let Some(sha) = llvm_info.sha_short() {
+                default_suffix.push_str("-");
+                default_suffix.push_str(sha);
+            }
+            cfg.define("LLVM_VERSION_SUFFIX", default_suffix);
         }
 
         if let Some(ref linker) = builder.config.llvm_use_linker {