Allowing '--extern' usage in build scripts and in config files

src/cargo/core/compiler/custom_build.rs

@@ -67,6 +67,8 @@ pub struct BuildOutput {
     pub library_paths: Vec<PathBuf>,
     /// Names and link kinds of libraries, suitable for the `-l` flag.
     pub library_links: Vec<String>,
+    /// External direct library dependencies passed to rustc with the `--extern` flag.
+    pub library_externs: Vec<String>,
     /// Linker arguments suitable to be passed to `-C link-arg=<args>`
     pub linker_args: Vec<(LinkArgTarget, String)>,
     /// Various `--cfg` flags to pass to the compiler.
@@ -684,6 +686,7 @@ impl BuildOutput {
     ) -> CargoResult<BuildOutput> {
         let mut library_paths = Vec::new();
         let mut library_links = Vec::new();
+        let mut library_externs = Vec::new();
         let mut linker_args = Vec::new();
         let mut cfgs = Vec::new();
         let mut check_cfgs = Vec::new();
@@ -855,9 +858,10 @@ impl BuildOutput {
             // Keep in sync with TargetConfig::parse_links_overrides.
             match key {
                 "rustc-flags" => {
-                    let (paths, links) = BuildOutput::parse_rustc_flags(&value, &whence)?;
+                    let (paths, links, externs) = BuildOutput::parse_rustc_flags(&value, &whence)?;
                     library_links.extend(links.into_iter());
                     library_paths.extend(paths.into_iter());
+                    library_externs.extend(externs.into_iter());
                 }
                 "rustc-link-lib" => library_links.push(value.to_string()),
                 "rustc-link-search" => library_paths.push(PathBuf::from(value)),
@@ -1001,6 +1005,7 @@ impl BuildOutput {
         Ok(BuildOutput {
             library_paths,
             library_links,
+            library_externs,
             linker_args,
             cfgs,
             check_cfgs,
@@ -1018,19 +1023,22 @@ impl BuildOutput {
     pub fn parse_rustc_flags(
         value: &str,
         whence: &str,
-    ) -> CargoResult<(Vec<PathBuf>, Vec<String>)> {
+    ) -> CargoResult<(Vec<PathBuf>, Vec<String>, Vec<String>)> {
         let value = value.trim();
         let mut flags_iter = value
             .split(|c: char| c.is_whitespace())
             .filter(|w| w.chars().any(|c| !c.is_whitespace()));
-        let (mut library_paths, mut library_links) = (Vec::new(), Vec::new());
+        let (mut library_paths, mut library_links, mut externs) =
+            (Vec::new(), Vec::new(), Vec::new());
 
         while let Some(flag) = flags_iter.next() {
-            if flag.starts_with("-l") || flag.starts_with("-L") {
+            let is_extern = flag.starts_with("--extern");
+            if flag.starts_with("-l") || flag.starts_with("-L") || is_extern {
                 // Check if this flag has no space before the value as is
                 // common with tools like pkg-config
                 // e.g. -L/some/dir/local/lib or -licui18n
-                let (flag, mut value) = flag.split_at(2);
+                let split_location = if is_extern { 8 } else { 2 };
+                let (flag, mut value) = flag.split_at(split_location);
                 if value.is_empty() {
                     value = match flags_iter.next() {
                         Some(v) => v,
@@ -1045,19 +1053,20 @@ impl BuildOutput {
                 match flag {
                     "-l" => library_links.push(value.to_string()),
                     "-L" => library_paths.push(PathBuf::from(value)),
+                    "--extern" => externs.push(value.to_string()),
 
                     // This was already checked above
                     _ => unreachable!(),
                 };
             } else {
                 bail!(
-                    "Only `-l` and `-L` flags are allowed in {}: `{}`",
+                    "Only `-l`, `-L` and `--extern` flags are allowed in {}: `{}`",
                     whence,
                     value
                 )
             }
         }
-        Ok((library_paths, library_links))
+        Ok((library_paths, library_links, externs))
     }
 
     /// Parses [`cargo::rustc-env`] instruction.

src/cargo/core/compiler/mod.rs

@@ -462,7 +462,7 @@ fn rustc(
         Ok(())
     }));
 
-    // Add all relevant `-L` and `-l` flags from dependencies (now calculated and
+    // Add all relevant `-L`, `-l` and `--extern` flags from dependencies (now calculated and
     // present in `state`) to the command provided.
     fn add_native_deps(
         rustc: &mut ProcessBuilder,
@@ -484,6 +484,11 @@ fn rustc(
             }
 
             if key.0 == current_id {
+                for extern_ in output.library_externs.iter() {
+                    println!("Adding extern {}", extern_);
+                    rustc.arg("--extern").arg(extern_);
+                }
+
                 if pass_l_flag {
                     for name in output.library_links.iter() {
                         rustc.arg("-l").arg(name);

src/cargo/util/context/target.rs

@@ -158,9 +158,10 @@ fn parse_links_overrides(
                 "rustc-flags" => {
                     let flags = value.string(key)?;
                     let whence = format!("target config `{}.{}` (in {})", target_key, key, flags.1);
-                    let (paths, links) = BuildOutput::parse_rustc_flags(flags.0, &whence)?;
+                    let (paths, links, externs) = BuildOutput::parse_rustc_flags(flags.0, &whence)?;
                     output.library_paths.extend(paths);
                     output.library_links.extend(links);
+                    output.library_externs.extend(externs);
                 }
                 "rustc-link-lib" => {
                     let list = value.list(key)?;

tests/testsuite/bad_config.rs

@@ -2960,7 +2960,7 @@ fn bad_target_links_overrides() {
     p.cargo("check")
         .with_status(101)
         .with_stderr_data(str![[r"
-[ERROR] Only `-l` and `-L` flags are allowed in target config `target.[..].rustc-flags` (in [..]foo/.cargo/config.toml): `foo`
+[ERROR] Only `-l`, `-L` and `--extern` flags are allowed in target config `target.[..].rustc-flags` (in [..]foo/.cargo/config.toml): `foo`
 
 "]])
         .run();

tests/testsuite/build_script.rs

@@ -842,7 +842,7 @@ fn custom_build_script_wrong_rustc_flags() {
     p.cargo("build")
         .with_status(101)
         .with_stderr_contains(
-            "[ERROR] Only `-l` and `-L` flags are allowed in build script of `foo v0.5.0 ([CWD])`: \
+            "[ERROR] Only `-l`, `-L` and `--extern` flags are allowed in build script of `foo v0.5.0 ([CWD])`: \
              `-aaa -bbb`",
         )
         .run();