Use a DWARF version consistent with rustc

Rustc defaults to DWARF-2 on some targets, and DWARF-4 on others.
However using -g with the C compiler yields whatever default version the
C compiler prefers.

One side effect is that the DWARF debug info shipped in some libraries
with rustc itself (e.g. libcompiler_builtins and others) have recently
switched to DWARF-5 as a side effect of upgrading the clang version
used on rustc CI. (rust-lang/rust#98746)

Ideally, the preferred DWARF version would be given by the rust compiler
and/or cargo, but that's not the case at the moment, so the next best
thing is something that aligns with the current defaults, although
work in under way to add a rustc flag that would allow to pick the
preferred DWARF version (rust-lang/rust#98350)

src/lib.rs

@@ -212,13 +212,17 @@ enum ToolFamily {
 
 impl ToolFamily {
     /// What the flag to request debug info for this family of tools look like
-    fn add_debug_flags(&self, cmd: &mut Tool) {
+    fn add_debug_flags(&self, cmd: &mut Tool, dwarf_version: Option<u32>) {
         match *self {
             ToolFamily::Msvc { .. } => {
                 cmd.push_cc_arg("-Z7".into());
             }
             ToolFamily::Gnu | ToolFamily::Clang => {
-                cmd.push_cc_arg("-g".into());
+                cmd.push_cc_arg(
+                    dwarf_version
+                        .map_or_else(|| "-g".into(), |v| format!("-gdwarf-{}", v))
+                        .into(),
+                );
             }
         }
     }
@@ -1555,7 +1559,7 @@ impl Build {
                 cmd.args.push("-G".into());
             }
             let family = cmd.family;
-            family.add_debug_flags(cmd);
+            family.add_debug_flags(cmd, self.get_dwarf_version());
         }
 
         if self.get_force_frame_pointer() {
@@ -2771,6 +2775,25 @@ impl Build {
         })
     }
 
+    fn get_dwarf_version(&self) -> Option<u32> {
+        // Tentatively matches the DWARF version defaults as of rustc 1.62.
+        let target = self.get_target().ok()?;
+        if target.contains("android")
+            || target.contains("apple")
+            || target.contains("dragonfly")
+            || target.contains("freebsd")
+            || target.contains("netbsd")
+            || target.contains("openbsd")
+            || target.contains("windows-gnu")
+        {
+            Some(2)
+        } else if target.contains("linux") {
+            Some(4)
+        } else {
+            None
+        }
+    }
+
     fn get_force_frame_pointer(&self) -> bool {
         self.force_frame_pointer.unwrap_or_else(|| self.get_debug())
     }

tests/test.rs

@@ -20,7 +20,7 @@ fn gnu_smoke() {
     test.cmd(0)
         .must_have("-O2")
         .must_have("foo.c")
-        .must_not_have("-g")
+        .must_not_have("-gdwarf-4")
         .must_have("-c")
         .must_have("-ffunction-sections")
         .must_have("-fdata-sections");
@@ -52,19 +52,34 @@ fn gnu_opt_level_s() {
         .must_not_have("-Oz");
 }
 
+#[test]
+fn gnu_debug() {
+    let test = Test::gnu();
+    test.gcc().debug(true).file("foo.c").compile("foo");
+    test.cmd(0).must_have("-gdwarf-4");
+
+    let test = Test::gnu();
+    test.gcc()
+        .target("x86_64-apple-darwin")
+        .debug(true)
+        .file("foo.c")
+        .compile("foo");
+    test.cmd(0).must_have("-gdwarf-2");
+}
+
 #[test]
 fn gnu_debug_fp_auto() {
     let test = Test::gnu();
     test.gcc().debug(true).file("foo.c").compile("foo");
-    test.cmd(0).must_have("-g");
+    test.cmd(0).must_have("-gdwarf-4");
     test.cmd(0).must_have("-fno-omit-frame-pointer");
 }
 
 #[test]
 fn gnu_debug_fp() {
     let test = Test::gnu();
     test.gcc().debug(true).file("foo.c").compile("foo");
-    test.cmd(0).must_have("-g");
+    test.cmd(0).must_have("-gdwarf-4");
     test.cmd(0).must_have("-fno-omit-frame-pointer");
 }
 
@@ -78,7 +93,7 @@ fn gnu_debug_nofp() {
         .force_frame_pointer(false)
         .file("foo.c")
         .compile("foo");
-    test.cmd(0).must_have("-g");
+    test.cmd(0).must_have("-gdwarf-4");
     test.cmd(0).must_not_have("-fno-omit-frame-pointer");
 
     let test = Test::gnu();
@@ -87,7 +102,7 @@ fn gnu_debug_nofp() {
         .debug(true)
         .file("foo.c")
         .compile("foo");
-    test.cmd(0).must_have("-g");
+    test.cmd(0).must_have("-gdwarf-4");
     test.cmd(0).must_not_have("-fno-omit-frame-pointer");
 }