From 30fe923ca2cea2937210c7a8c7966444f271d2e2 Mon Sep 17 00:00:00 2001
From: Weihang Lo <me@weihanglo.tw>
Date: Thu, 26 Jan 2023 23:58:54 +0000
Subject: [PATCH] perf: reduce target info rustc query call to one

This is kinda a hack since we abuse `--print=crate-name`
where the crate name is `___` and used as a delimiter.
---
 .../compiler/build_context/target_info.rs     | 51 ++++++++++++++-----
 1 file changed, 38 insertions(+), 13 deletions(-)

diff --git a/src/cargo/core/compiler/build_context/target_info.rs b/src/cargo/core/compiler/build_context/target_info.rs
index 61d1f3316..7ac48f735 100644
--- a/src/cargo/core/compiler/build_context/target_info.rs
+++ b/src/cargo/core/compiler/build_context/target_info.rs
@@ -168,22 +168,11 @@ impl TargetInfo {
         loop {
             let extra_fingerprint = kind.fingerprint_hash();
 
-            // Query rustc for supported -Csplit-debuginfo values
-            let support_split_debuginfo = rustc
-                .cached_output(
-                    rustc.workspace_process().arg("--print=split-debuginfo"),
-                    extra_fingerprint,
-                )
-                .unwrap_or_default()
-                .0
-                .lines()
-                .map(String::from)
-                .collect();
-
             // Query rustc for several kinds of info from each line of output:
             // 0) file-names (to determine output file prefix/suffix for given crate type)
             // 1) sysroot
-            // 2) cfg
+            // 2) split-debuginfo
+            // 3) cfg
             //
             // Search `--print` to see what we query so far.
             let mut process = rustc.workspace_process();
@@ -213,6 +202,8 @@ impl TargetInfo {
             }
 
             process.arg("--print=sysroot");
+            process.arg("--print=split-debuginfo");
+            process.arg("--print=crate-name"); // `___` as a delimiter.
             process.arg("--print=cfg");
 
             let (output, error) = rustc
@@ -251,6 +242,26 @@ impl TargetInfo {
             });
             sysroot_target_libdir.push("lib");
 
+            let support_split_debuginfo = {
+                // HACK: abuse `--print=crate-name` to use `___` as a delimiter.
+                let mut res = Vec::new();
+                loop {
+                    match lines.next() {
+                        Some(line) if line == "___" => break,
+                        Some(line) => res.push(line.into()),
+                        None => {
+                            return error_missing_print_output(
+                                "split-debuginfo",
+                                &process,
+                                &output,
+                                &error,
+                            )
+                        }
+                    }
+                }
+                res
+            };
+
             let cfg = lines
                 .map(|line| Ok(Cfg::from_str(line)?))
                 .filter(TargetInfo::not_user_specific_cfg)
@@ -601,6 +612,20 @@ fn parse_crate_type(
     Ok(Some((prefix.to_string(), suffix.to_string())))
 }
 
+/// Helper for creating an error message for missing output from a certain `--print` request.
+fn error_missing_print_output<T>(
+    request: &str,
+    cmd: &ProcessBuilder,
+    stdout: &str,
+    stderr: &str,
+) -> CargoResult<T> {
+    let err_info = output_err_info(cmd, stdout, stderr);
+    anyhow::bail!(
+        "output of --print={request} missing when learning about \
+     target-specific information from rustc\n{err_info}",
+    )
+}
+
 /// Helper for creating an error message when parsing rustc output fails.
 fn output_err_info(cmd: &ProcessBuilder, stdout: &str, stderr: &str) -> String {
     let mut result = format!("command was: {}\n", cmd);