Auto merge of #75549 - tmandry:rollup-sxjwa0w, r=tmandry

Rollup of 4 pull requests

Successful merges:

 - #75376 (Set CMAKE_SYSTEM_NAME when cross-compiling)
 - #75448 (merge `as_local_hir_id` with `local_def_id_to_hir_id`)
 - #75513 (Recover gracefully from `struct` parse errors)
 - #75545 (std/sys/unix/time: make it easier for LLVM to optimize `Instant` subtraction.)

Failed merges:

 - #75514 (Replaced `log` with `tracing`)

r? @ghost

Author: bors

library/std/src/sys/unix/time.rs

@@ -20,17 +20,29 @@ impl Timespec {
 
     fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
         if self >= other {
-            Ok(if self.t.tv_nsec >= other.t.tv_nsec {
-                Duration::new(
-                    (self.t.tv_sec - other.t.tv_sec) as u64,
-                    (self.t.tv_nsec - other.t.tv_nsec) as u32,
-                )
+            // NOTE(eddyb) two aspects of this `if`-`else` are required for LLVM
+            // to optimize it into a branchless form (see also #75545):
+            //
+            // 1. `self.t.tv_sec - other.t.tv_sec` shows up as a common expression
+            //    in both branches, i.e. the `else` must have its `- 1`
+            //    subtraction after the common one, not interleaved with it
+            //    (it used to be `self.t.tv_sec - 1 - other.t.tv_sec`)
+            //
+            // 2. the `Duration::new` call (or any other additional complexity)
+            //    is outside of the `if`-`else`, not duplicated in both branches
+            //
+            // Ideally this code could be rearranged such that it more
+            // directly expresses the lower-cost behavior we want from it.
+            let (secs, nsec) = if self.t.tv_nsec >= other.t.tv_nsec {
+                ((self.t.tv_sec - other.t.tv_sec) as u64, (self.t.tv_nsec - other.t.tv_nsec) as u32)
             } else {
-                Duration::new(
-                    (self.t.tv_sec - 1 - other.t.tv_sec) as u64,
+                (
+                    (self.t.tv_sec - other.t.tv_sec - 1) as u64,
                     self.t.tv_nsec as u32 + (NSEC_PER_SEC as u32) - other.t.tv_nsec as u32,
                 )
-            })
+            };
+
+            Ok(Duration::new(secs, nsec))
         } else {
             match other.sub_timespec(self) {
                 Ok(d) => Err(d),

src/bootstrap/native.rs

@@ -282,14 +282,6 @@ impl Step for Llvm {
                 "LLVM_CONFIG_PATH",
                 host_bin.join("llvm-config").with_extension(EXE_EXTENSION),
             );
-
-            if target.contains("netbsd") {
-                cfg.define("CMAKE_SYSTEM_NAME", "NetBSD");
-            } else if target.contains("freebsd") {
-                cfg.define("CMAKE_SYSTEM_NAME", "FreeBSD");
-            } else if target.contains("windows") {
-                cfg.define("CMAKE_SYSTEM_NAME", "Windows");
-            }
         }
 
         if let Some(ref suffix) = builder.config.llvm_version_suffix {
@@ -378,6 +370,22 @@ fn configure_cmake(
     }
     cfg.target(&target.triple).host(&builder.config.build.triple);
 
+    if target != builder.config.build {
+        if target.contains("netbsd") {
+            cfg.define("CMAKE_SYSTEM_NAME", "NetBSD");
+        } else if target.contains("freebsd") {
+            cfg.define("CMAKE_SYSTEM_NAME", "FreeBSD");
+        } else if target.contains("windows") {
+            cfg.define("CMAKE_SYSTEM_NAME", "Windows");
+        }
+        // When cross-compiling we should also set CMAKE_SYSTEM_VERSION, but in
+        // that case like CMake we cannot easily determine system version either.
+        //
+        // Since, the LLVM itself makes rather limited use of version checks in
+        // CMakeFiles (and then only in tests), and so far no issues have been
+        // reported, the system version is currently left unset.
+    }
+
     let sanitize_cc = |cc: &Path| {
         if target.contains("msvc") {
             OsString::from(cc.to_str().unwrap().replace("\\", "/"))