Add support for setting -gdwarf-{version} based on RUSTFLAGS

Detect if `-Zdwarf-version` (which will probably be stabilized soon
as `-Cdwarf-version`) was passed in RUSTFLAGS and set the corresponding
Clang/GCC option to the same value.

Author: wesleywiser

src/flags.rs

@@ -18,6 +18,7 @@ pub(crate) struct RustcCodegenFlags<'a> {
     force_frame_pointers: Option<bool>,
     no_redzone: Option<bool>,
     soft_float: Option<bool>,
+    dwarf_version: Option<&'a str>,
 }
 
 impl<'this> RustcCodegenFlags<'this> {
@@ -148,6 +149,11 @@ impl<'this> RustcCodegenFlags<'this> {
                 self.branch_protection =
                     Some(flag_ok_or(value, "-Zbranch-protection must have a value")?);
             }
+            // https://doc.rust-lang.org/beta/unstable-book/compiler-flags/dwarf-version.html
+            // FIXME: Drop the -Z variant and update the doc link once the option is stablized
+            "-Zdwarf-version" | "-Cdwarf-version" => {
+                self.dwarf_version = Some(flag_ok_or(value, "-Zdwarf-version must have a value")?);
+            }
             _ => {}
         }
         Ok(())
@@ -250,6 +256,11 @@ impl<'this> RustcCodegenFlags<'this> {
                 };
                 push_if_supported(cc_flag.into());
             }
+            // https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-gdwarf-2
+            // https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#index-gdwarf
+            if let Some(value) = self.dwarf_version {
+                push_if_supported(format!("-gdwarf-{value}").into());
+            }
         }
 
         // Compiler-exclusive flags
@@ -390,6 +401,7 @@ mod tests {
             "-Crelocation-model=pic",
             "-Csoft-float=yes",
             "-Zbranch-protection=bti,pac-ret,leaf",
+            "-Zdwarf-version=5",
             // Set flags we don't recognise but rustc supports next
             // rustc flags
             "--cfg",
@@ -496,6 +508,7 @@ mod tests {
                 relocation_model: Some("pic"),
                 soft_float: Some(true),
                 branch_protection: Some("bti,pac-ret,leaf"),
+                dwarf_version: Some("5"),
             },
         );
     }

tests/rustflags.rs

@@ -19,12 +19,13 @@ fn inherits_rustflags() {
     // Correctly inherits flags from rustc
     std::env::set_var(
         "CARGO_ENCODED_RUSTFLAGS",
-        "-Cforce-frame-pointers=true\u{1f}-Ccode-model=small\u{1f}-Csoft-float",
+        "-Cforce-frame-pointers=true\u{1f}-Ccode-model=small\u{1f}-Csoft-float\u{1f}-Cdwarf-version=5",
     );
     let test = Test::gnu();
     test.gcc().file("foo.c").compile("foo");
     test.cmd(0)
         .must_have("-fno-omit-frame-pointer")
         .must_have("-mcmodel=small")
-        .must_have("-msoft-float");
+        .must_have("-msoft-float")
+        .must_have("-gdwarf-5");
 }