build: Inherit flags from rustc

Where applicable, detect which RUSTFLAGS were set for rustc and convert
them into their corresponding cc flags in order to ensure consistent
codegen across Rust and non-Rust modules.

Author: mrkajetanp

src/lib.rs

@@ -312,6 +312,7 @@ pub struct Build {
     emit_rerun_if_env_changed: bool,
     cached_compiler_family: Arc<RwLock<HashMap<Box<Path>, ToolFamily>>>,
     shell_escaped_flags: Option<bool>,
+    inherit_rustflags: bool,
 }
 
 /// Represents the types of errors that may occur while using cc-rs.
@@ -437,6 +438,7 @@ impl Build {
             emit_rerun_if_env_changed: true,
             cached_compiler_family: Arc::default(),
             shell_escaped_flags: None,
+            inherit_rustflags: true,
         }
     }
 
@@ -1313,6 +1315,15 @@ impl Build {
         self
     }
 
+    /// Configure whether cc should automatically inherit compatible flags passed to rustc
+    /// from `CARGO_ENCODED_RUSTFLAGS`.
+    ///
+    /// This option defaults to `true`.
+    pub fn inherit_rustflags(&mut self, inherit_rustflags: bool) -> &mut Build {
+        self.inherit_rustflags = inherit_rustflags;
+        self
+    }
+
     #[doc(hidden)]
     pub fn __set_env<A, B>(&mut self, a: A, b: B) -> &mut Build
     where
@@ -1904,6 +1915,11 @@ impl Build {
             cmd.args.push((**flag).into());
         }
 
+        // Add cc flags inherited from matching rustc flags
+        if self.inherit_rustflags {
+            self.add_inherited_rustflags(&mut cmd)?;
+        }
+
         for flag in self.flags_supported.iter() {
             if self
                 .is_flag_supported_inner(flag, &cmd.path, &target)
@@ -2439,6 +2455,32 @@ impl Build {
         Ok(())
     }
 
+    fn add_inherited_rustflags(&self, cmd: &mut Tool) -> Result<(), Error> {
+        let env_os = match self.getenv("CARGO_ENCODED_RUSTFLAGS") {
+            Some(env) => env,
+            // No encoded RUSTFLAGS -> nothing to do
+            None => return Ok(()),
+        };
+        let mut cc_flags: Vec<OsString> = env_os
+            .to_string_lossy()
+            .split("\u{1f}")
+            // Strip prefixes from rustc flags
+            .flat_map(|flag| {
+                if flag == "-Z" || flag == "-C" {
+                    None
+                } else if flag.starts_with("-Z") || flag.starts_with("-C") {
+                    Some(&flag[2..])
+                } else {
+                    Some(flag)
+                }
+            })
+            .flat_map(|flag| rustc_to_cc_flag(flag, cmd.family))
+            .collect();
+
+        cmd.args.append(&mut cc_flags);
+        Ok(())
+    }
+
     fn has_flags(&self) -> bool {
         let flags_env_var_name = if self.cpp { "CXXFLAGS" } else { "CFLAGS" };
         let flags_env_var_value = self.getenv_with_target_prefixes(flags_env_var_name);
@@ -4221,6 +4263,161 @@ fn map_darwin_target_from_rust_to_compiler_architecture(target: &Target) -> &str
     }
 }
 
+// Rust and clang/cc don't agree on what equivalent flags should look like either.
+fn rustc_to_cc_flag(flag: &str, family: ToolFamily) -> Option<OsString> {
+    match family {
+        ToolFamily::Clang { .. } | ToolFamily::Gnu => match flag {
+            _ if flag.starts_with("branch-protection=") => {
+                Some(format!("-m{}", flag.replace(",", "+")).into())
+            }
+            _ if flag.starts_with("code-model=") => {
+                Some(flag.replace("code-model", "-mcmodel").into())
+            }
+            _ if flag.starts_with("no-vectorize-loops") => Some("-fno-vectorize".into()),
+            _ if flag.starts_with("no-vectorize-slp") => Some("-fno-slp-vectorize".into()),
+            _ if flag.starts_with("profile-generate") => Some(format!("-f{}", flag).into()),
+            _ if flag.starts_with("profile-use") => Some(format!("-f{}", flag).into()),
+            _ if flag.starts_with("control-flow-guard=") => {
+                let (_, rustc_val) = flag.split_once("=").unwrap();
+                let cc_val = match rustc_val {
+                    "y" | "yes" | "on" | "true" | "checks" => "cf",
+                    "nochecks" => "cf-nochecks",
+                    "n" | "no" | "off" | "false" => "none",
+                    _ => return None,
+                };
+                Some(format!("-mguard={cc_val}").into())
+            }
+            _ if flag.starts_with("embed-bitcode=") => {
+                let (_, rustc_val) = flag.split_once("=").unwrap();
+                let cc_val = match rustc_val {
+                    "y" | "yes" | "on" | "true" => "all",
+                    "n" | "no" | "off" | "false" => "off",
+                    _ => return None,
+                };
+                Some(format!("-fembed-bitcode={cc_val}").into())
+            }
+            _ if flag.starts_with("force-frame-pointers") => {
+                let force_frame_pointers = if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "y" | "yes" | "on" | "true" => true,
+                        "n" | "no" | "off" | "false" => false,
+                        _ => return None,
+                    }
+                } else {
+                    true
+                };
+                let cc_flag = if force_frame_pointers {
+                    "-fno-omit-frame-pointer"
+                } else {
+                    "-fomit-frame-pointer"
+                };
+                Some(cc_flag.into())
+            }
+            _ if flag.starts_with("link-dead-code") => {
+                if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "n" | "no" | "off" | "false" => Some("-dead_strip".into()),
+                        _ => None,
+                    }
+                } else {
+                    None
+                }
+            }
+            _ if flag.starts_with("lto") => {
+                let lto_mode = if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "y" | "yes" | "on" | "true" | "fat" => "full",
+                        "thin" => "thin",
+                        _ => return None,
+                    }
+                } else {
+                    "full"
+                };
+
+                Some(format!("-flto={}", lto_mode).into())
+            }
+            _ if flag.starts_with("no-redzone") => {
+                let no_redzone = if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "y" | "yes" | "on" | "true" => true,
+                        "n" | "no" | "off" | "false" => false,
+                        _ => return None,
+                    }
+                } else {
+                    true
+                };
+
+                let cc_flag = if no_redzone {
+                    "-mno-red-zone"
+                } else {
+                    "-mred-zone"
+                };
+                Some(cc_flag.into())
+            }
+            _ if flag.starts_with("relocation-model=") => {
+                let (_, rustc_val) = flag.split_once("=").unwrap();
+                let cc_flag = match rustc_val {
+                    "pic" => "-fPIC",
+                    "pie" => "-fPIE",
+                    "dynamic-no-pic" => "-mdynamic-no-pic",
+                    _ => return None,
+                };
+                Some(cc_flag.into())
+            }
+            _ if flag.starts_with("soft-float") => {
+                let soft_float = if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "y" | "yes" | "on" | "true" => true,
+                        "n" | "no" | "off" | "false" => false,
+                        _ => return None,
+                    }
+                } else {
+                    true
+                };
+
+                let cc_flag = if soft_float {
+                    "-msoft-float"
+                } else {
+                    "-mno-soft-float"
+                };
+                Some(cc_flag.into())
+            }
+            _ => None,
+        },
+        ToolFamily::Msvc { .. } => match flag {
+            _ if flag.starts_with("control-flow-guard=") => {
+                let (_, rustc_val) = flag.split_once("=").unwrap();
+                let cc_val = match rustc_val {
+                    "y" | "yes" | "on" | "true" | "checks" => "cf",
+                    "n" | "no" | "off" | "false" => "cf-",
+                    _ => return None,
+                };
+                Some(format!("/guard:{cc_val}").into())
+            }
+            _ if flag.starts_with("force-frame-pointers") => {
+                let force_frame_pointers = if flag.contains("=") {
+                    let (_, rustc_val) = flag.split_once("=").unwrap();
+                    match rustc_val {
+                        "y" | "yes" | "on" | "true" => true,
+                        "n" | "no" | "off" | "false" => false,
+                        _ => return None,
+                    }
+                } else {
+                    true
+                };
+                let cc_flag = if force_frame_pointers { "/Oy-" } else { "/Oy" };
+                Some(cc_flag.into())
+            }
+            _ => None,
+        },
+    }
+}
+
 #[derive(Clone, Copy, PartialEq)]
 enum AsmFileExt {
     /// `.asm` files. On MSVC targets, we assume these should be passed to MASM