rustbuild: Don't enable debuginfo in rustc

In rust-lang#37280 we enabled line number debugging information in release artifacts,
primarily to close out rust-lang#36452 where debugging information was critical for MSVC
builds of Rust to be useful in production. This commit, however, apparently had
some unfortunate side effects.

Namely it was noticed in rust-lang#37477 that if `RUST_BACKTRACE=1` was set then any
compiler error would take a very long time for the compiler to exit. The cause
of the problem here was somewhat deep:

* For all compiler errors, the compiler will `panic!` with a known value. This
  tears down the main compiler thread and allows cleaning up all the various
  resources. By default, however, this panic output is suppressed for "normal"
  compiler errors.
* When `RUST_BACKTRACE=1` was set this caused every compiler error to generate a
  backtrace.
* The libbacktrace library hits a pathological case where it spends a very long
  time in its custom allocation function, `backtrace_alloc`, because the
  compiler has so much debugging information. More information about this can be
  found in rust-lang#29293 with a summary at the end of rust-lang#37477.

To solve this problem this commit simply removes debuginfo from the compiler but
not from the standard library. This should allow us to keep rust-lang#36452 closed while
also closing rust-lang#37477. I've measured the difference to be orders of magnitude
faster than it was before, so we should see a much quicker time-to-exit after a
compile error when `RUST_BACKTRACE=1` is set.

Closes rust-lang#37477
Closes rust-lang#37571

configure

@@ -647,6 +647,7 @@ opt_nosave debug-assertions 0 "build with debugging assertions"
 opt_nosave llvm-release-debuginfo 0 "build LLVM with debugger metadata"
 opt_nosave debuginfo 0 "build with debugger metadata"
 opt_nosave debuginfo-lines 0 "build with line number debugger metadata"
+opt_nosave debuginfo-only-std 0 "build only libstd with debugging information"
 opt_nosave debug-jemalloc 0 "build jemalloc with --enable-debug --enable-fill"
 
 valopt localstatedir "/var/lib" "local state directory"
@@ -733,23 +734,26 @@ case "$CFG_RELEASE_CHANNEL" in
     nightly )
 	msg "overriding settings for $CFG_RELEASE_CHANNEL"
 	CFG_ENABLE_LLVM_ASSERTIONS=1
-
-        # FIXME(#37364) shouldn't have to disable this on windows-gnu
+        # FIXME(stage0) re-enable this on the next stage0 now that #35566 is
+        # fixed
         case "$CFG_BUILD" in
           *-pc-windows-gnu)
             ;;
           *)
-	    CFG_ENABLE_DEBUGINFO_LINES=1
+            CFG_ENABLE_DEBUGINFO_LINES=1
+            CFG_ENABLE_DEBUGINFO_ONLY_STD=1
             ;;
         esac
+
 	;;
     beta | stable)
 	msg "overriding settings for $CFG_RELEASE_CHANNEL"
         case "$CFG_BUILD" in
           *-pc-windows-gnu)
             ;;
           *)
-	    CFG_ENABLE_DEBUGINFO_LINES=1
+            CFG_ENABLE_DEBUGINFO_LINES=1
+            CFG_ENABLE_DEBUGINFO_ONLY_STD=1
             ;;
         esac
 	;;
@@ -785,6 +789,7 @@ if [ -n "$CFG_ENABLE_DEBUG_ASSERTIONS" ]; then putvar CFG_ENABLE_DEBUG_ASSERTION
 if [ -n "$CFG_ENABLE_LLVM_RELEASE_DEBUGINFO" ]; then putvar CFG_ENABLE_LLVM_RELEASE_DEBUGINFO; fi
 if [ -n "$CFG_ENABLE_DEBUGINFO" ]; then putvar CFG_ENABLE_DEBUGINFO; fi
 if [ -n "$CFG_ENABLE_DEBUGINFO_LINES" ]; then putvar CFG_ENABLE_DEBUGINFO_LINES; fi
+if [ -n "$CFG_ENABLE_DEBUGINFO_ONLY_STD" ]; then putvar CFG_ENABLE_DEBUGINFO_ONLY_STD; fi
 if [ -n "$CFG_ENABLE_DEBUG_JEMALLOC" ]; then putvar CFG_ENABLE_DEBUG_JEMALLOC; fi
 
 step_msg "looking for build programs"

src/bootstrap/compile.rs

@@ -189,6 +189,13 @@ pub fn rustc(build: &Build, target: &str, compiler: &Compiler) {
          .env("CFG_PREFIX", build.config.prefix.clone().unwrap_or(String::new()))
          .env("CFG_LIBDIR_RELATIVE", "lib");
 
+    // If we're not building a compiler with debugging information then remove
+    // these two env vars which would be set otherwise.
+    if build.config.rust_debuginfo_only_std {
+        cargo.env_remove("RUSTC_DEBUGINFO");
+        cargo.env_remove("RUSTC_DEBUGINFO_LINES");
+    }
+
     if let Some(ref ver_date) = build.ver_date {
         cargo.env("CFG_VER_DATE", ver_date);
     }

src/bootstrap/config.rs

@@ -63,6 +63,7 @@ pub struct Config {
     pub rust_debug_assertions: bool,
     pub rust_debuginfo: bool,
     pub rust_debuginfo_lines: bool,
+    pub rust_debuginfo_only_std: bool,
     pub rust_rpath: bool,
     pub rustc_default_linker: Option<String>,
     pub rustc_default_ar: Option<String>,
@@ -179,6 +180,7 @@ struct Rust {
     debug_assertions: Option<bool>,
     debuginfo: Option<bool>,
     debuginfo_lines: Option<bool>,
+    debuginfo_only_std: Option<bool>,
     debug_jemalloc: Option<bool>,
     use_jemalloc: Option<bool>,
     backtrace: Option<bool>,
@@ -298,6 +300,7 @@ impl Config {
             set(&mut config.rust_debug_assertions, rust.debug_assertions);
             set(&mut config.rust_debuginfo, rust.debuginfo);
             set(&mut config.rust_debuginfo_lines, rust.debuginfo_lines);
+            set(&mut config.rust_debuginfo_only_std, rust.debuginfo_only_std);
             set(&mut config.rust_optimize, rust.optimize);
             set(&mut config.rust_optimize_tests, rust.optimize_tests);
             set(&mut config.rust_debuginfo_tests, rust.debuginfo_tests);
@@ -390,6 +393,7 @@ impl Config {
                 ("DEBUG_ASSERTIONS", self.rust_debug_assertions),
                 ("DEBUGINFO", self.rust_debuginfo),
                 ("DEBUGINFO_LINES", self.rust_debuginfo_lines),
+                ("DEBUGINFO_ONLY_STD", self.rust_debuginfo_only_std),
                 ("JEMALLOC", self.use_jemalloc),
                 ("DEBUG_JEMALLOC", self.debug_jemalloc),
                 ("RPATH", self.rust_rpath),

src/bootstrap/config.toml.example

@@ -149,6 +149,11 @@
 # Whether or not line number debug information is emitted
 #debuginfo-lines = false
 
+# Whether or not to only build debuginfo for the standard library if enabled.
+# If enabled, this will not compile the compiler with debuginfo, just the
+# standard library.
+#debuginfo-only-std = false
+
 # Whether or not jemalloc is built and enabled
 #use-jemalloc = true