handle file paths to libraries

Typically pkgconfig files specify cflags for linking with -L and -l,
however, pkgconfig files can also specify paths to library files. For
example, building Qt5 statically on macOS generates the following
pkgconfig file. Notice the absolute path to libqtpcre.a in Libs.private:

prefix=/Users/be/qt5-installed
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include

host_bins=${prefix}/bin
qt_config=debug_and_release release debug build_all c++11 c++14 c++17 c++1z concurrent dbus no-pkg-config reduce_exports release_tools static stl

Name: Qt5 Core
Description: Qt Core module
Version: 5.15.5
Libs: -L${libdir} -lQt5Core
Libs.private: -framework DiskArbitration -framework IOKit -lm -framework AppKit -framework Security -framework ApplicationServices -framework CoreServices -framework CoreFoundation -framework Foundation -lz /Users/be/sw/qt-everywhere-src-5.15.5/qtbase/lib/libqtpcre2.a
Cflags: -DQT_CORE_LIB -I${includedir}/QtCore -I${includedir}

Building Qt5 statically on macOS with vcpkg generates this pkgconfig
file which has a handful of file paths for libraries:

prefix=${pcfiledir}/../..

exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include/qt5

host_bins=${prefix}/tools/qt5/bin
qt_config=release c++11 c++14 c++17 c++1z concurrent dbus no-pkg-config reduce_exports static stl properties animation textcodec big_codecs codecs itemmodel proxymodel concatenatetablesproxymodel textdate datestring doubleconversion filesystemiterator filesystemwatcher gestures identityproxymodel library mimetype process statemachine regularexpression settings sharedmemory sortfilterproxymodel stringlistmodel systemsemaphore temporaryfile translation transposeproxymodel xmlstream xmlstreamreader xmlstreamwriter

Name: Qt5 Core
Description: Qt Core module
Version: 5.15.3

Libs: -L"${libdir}" -lQt5Core  -L"${prefix}/lib" -L"${prefix}/lib/manual-link" -framework DiskArbitration -framework IOKit -lm -framework AppKit -framework Security -framework ApplicationServices -framework CoreServices -framework CoreFoundation -framework Foundation ${prefix}/lib/libz.a -ldouble-conversion ${prefix}/lib/libicui18n.a ${prefix}/lib/libicutu.a ${prefix}/lib/libicuuc.a ${prefix}/lib/libicuio.a ${prefix}/lib/libicudata.a ${prefix}/lib/libpcre2-16.a -lzstd  ${prefix}/lib/libbz2.a ${prefix}/lib/libpng16.a ${prefix}/lib/libicui18n.a ${prefix}/lib/libicutu.a ${prefix}/lib/libicuuc.a ${prefix}/lib/libicuio.a ${prefix}/lib/libicudata.a ${prefix}/lib/libzstd.a
Cflags: -DQT_CORE_LIB -I"${includedir}/QtCore" -I"${includedir}"

Author: Be-ing

src/lib.rs

@@ -95,6 +95,7 @@ pub struct Config {
 pub struct Library {
     pub libs: Vec<String>,
     pub link_paths: Vec<PathBuf>,
+    pub link_files: Vec<PathBuf>,
     pub frameworks: Vec<String>,
     pub framework_paths: Vec<PathBuf>,
     pub include_paths: Vec<PathBuf>,
@@ -558,6 +559,7 @@ impl Library {
         Library {
             libs: Vec::new(),
             link_paths: Vec::new(),
+            link_files: Vec::new(),
             include_paths: Vec::new(),
             ld_args: Vec::new(),
             frameworks: Vec::new(),
@@ -568,9 +570,67 @@ impl Library {
         }
     }
 
+    /// Extract the &str to pass to cargo:rustc-link-lib from a filename (just the file name, not including directories)
+    /// using target-specific logic.
+    fn extract_lib_from_filename<'a>(target: &str, filename: &'a str) -> Option<&'a str> {
+        fn test_suffixes<'b>(filename: &'b str, suffixes: &[&str]) -> Option<&'b str> {
+            for suffix in suffixes {
+                if filename.ends_with(suffix) {
+                    return Some(&filename[..filename.len() - suffix.len()]);
+                }
+            }
+            None
+        }
+
+        let prefix = "lib";
+        if target.contains("msvc") {
+            // According to link.exe documentation:
+            // https://learn.microsoft.com/en-us/cpp/build/reference/link-input-files?view=msvc-170
+            //
+            //   LINK doesn't use file extensions to make assumptions about the contents of a file.
+            //   Instead, LINK examines each input file to determine what kind of file it is.
+            //
+            // However, rustc appends `.lib` to the string it receives from the -l command line argument,
+            // which it receives from Cargo via cargo:rustc-link-lib:
+            // https://github.com/rust-lang/rust/blob/657f246812ab2684e3c3954b1c77f98fd59e0b21/compiler/rustc_codegen_ssa/src/back/linker.rs#L828
+            // https://github.com/rust-lang/rust/blob/657f246812ab2684e3c3954b1c77f98fd59e0b21/compiler/rustc_codegen_ssa/src/back/linker.rs#L843
+            // So the only file extension that works for MSVC targets is `.lib`
+            return test_suffixes(filename, &[".lib"]);
+        } else if target.contains("windows") && target.contains("gnu") {
+            // GNU targets for Windows, including gnullvm, use `LinkerFlavor::Gcc` internally in rustc,
+            // which tells rustc to use the GNU linker. rustc does not prepend/append to the string it
+            // receives via the -l command line argument before passing it to the linker:
+            // https://github.com/rust-lang/rust/blob/657f246812ab2684e3c3954b1c77f98fd59e0b21/compiler/rustc_codegen_ssa/src/back/linker.rs#L446
+            // https://github.com/rust-lang/rust/blob/657f246812ab2684e3c3954b1c77f98fd59e0b21/compiler/rustc_codegen_ssa/src/back/linker.rs#L457
+            // GNU ld can work with more types of files than just the .lib files that MSVC's link.exe needs.
+            // GNU ld will prepend the `lib` prefix to the filename if necessary, so it is okay to remove
+            // the `lib` prefix from the filename. The `.a` suffix *requires* the `lib` prefix.
+            // https://sourceware.org/binutils/docs-2.39/ld.html#index-direct-linking-to-a-dll
+            if filename.starts_with(prefix) {
+                let filename = &filename[prefix.len()..];
+                return test_suffixes(filename, &[".dll.a", ".dll", ".lib", ".a"]);
+            } else {
+                return test_suffixes(filename, &[".dll.a", ".dll", ".lib"]);
+            }
+        } else if target.contains("apple") {
+            if filename.starts_with(prefix) {
+                let filename = &filename[prefix.len()..];
+                return test_suffixes(filename, &[".a", ".so", ".dylib"]);
+            }
+            return None;
+        } else {
+            if filename.starts_with(prefix) {
+                let filename = &filename[prefix.len()..];
+                return test_suffixes(filename, &[".a", ".so"]);
+            }
+            return None;
+        }
+    }
+
     fn parse_libs_cflags(&mut self, name: &str, output: &[u8], config: &Config) {
         let mut is_msvc = false;
-        if let Ok(target) = env::var("TARGET") {
+        let target = env::var("TARGET");
+        if let Ok(target) = &target {
             if target.contains("msvc") {
                 is_msvc = true;
             }
@@ -670,7 +730,36 @@ impl Library {
                         self.include_paths.push(PathBuf::from(inc));
                     }
                 }
-                _ => (),
+                _ => {
+                    let path = std::path::Path::new(part);
+                    if path.is_file() {
+                        // Cargo doesn't have a means to directly specify a file path to link,
+                        // so split up the path into the parent directory and library name.
+                        // TODO: pass file path directly when link-arg library type is stabilized
+                        // https://github.com/rust-lang/rust/issues/99427
+                        if let (Some(dir), Some(file_name), Ok(target)) =
+                            (path.parent(), path.file_name(), &target)
+                        {
+                            match Self::extract_lib_from_filename(
+                                target,
+                                &file_name.to_string_lossy(),
+                            ) {
+                                Some(lib_basename) => {
+                                    let link_search =
+                                        format!("rustc-link-search={}", dir.display());
+                                    config.print_metadata(&link_search);
+
+                                    let link_lib = format!("rustc-link-lib={}", lib_basename);
+                                    config.print_metadata(&link_lib);
+                                    self.link_files.push(PathBuf::from(path));
+                                }
+                                None => {
+                                    println!("cargo:warning=File path {} found in pkg-config file for {}, but could not extract library base name to pass to linker command line", path.display(), name);
+                                }
+                            }
+                        }
+                    }
+                }
             }
         }
 
@@ -776,60 +865,104 @@ fn split_flags(output: &[u8]) -> Vec<String> {
     words
 }
 
-#[test]
-#[cfg(target_os = "macos")]
-fn system_library_mac_test() {
-    use std::path::Path;
-
-    let system_roots = vec![PathBuf::from("/Library"), PathBuf::from("/System")];
-
-    assert!(!is_static_available(
-        "PluginManager",
-        &system_roots,
-        &[PathBuf::from("/Library/Frameworks")]
-    ));
-    assert!(!is_static_available(
-        "python2.7",
-        &system_roots,
-        &[PathBuf::from(
-            "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config"
-        )]
-    ));
-    assert!(!is_static_available(
-        "ffi_convenience",
-        &system_roots,
-        &[PathBuf::from(
-            "/Library/Ruby/Gems/2.0.0/gems/ffi-1.9.10/ext/ffi_c/libffi-x86_64/.libs"
-        )]
-    ));
-
-    // Homebrew is in /usr/local, and it's not a part of the OS
-    if Path::new("/usr/local/lib/libpng16.a").exists() {
-        assert!(is_static_available(
-            "png16",
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[cfg(target_os = "macos")]
+    fn system_library_mac_test() {
+        use std::path::Path;
+
+        let system_roots = vec![PathBuf::from("/Library"), PathBuf::from("/System")];
+
+        assert!(!is_static_available(
+            "PluginManager",
+            &system_roots,
+            &[PathBuf::from("/Library/Frameworks")]
+        ));
+        assert!(!is_static_available(
+            "python2.7",
+            &system_roots,
+            &[PathBuf::from(
+                "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config"
+            )]
+        ));
+        assert!(!is_static_available(
+            "ffi_convenience",
             &system_roots,
-            &[PathBuf::from("/usr/local/lib")]
+            &[PathBuf::from(
+                "/Library/Ruby/Gems/2.0.0/gems/ffi-1.9.10/ext/ffi_c/libffi-x86_64/.libs"
+            )]
         ));
 
-        let libpng = Config::new()
-            .range_version("1".."99")
-            .probe("libpng16")
-            .unwrap();
-        assert!(libpng.version.find('\n').is_none());
+        // Homebrew is in /usr/local, and it's not a part of the OS
+        if Path::new("/usr/local/lib/libpng16.a").exists() {
+            assert!(is_static_available(
+                "png16",
+                &system_roots,
+                &[PathBuf::from("/usr/local/lib")]
+            ));
+
+            let libpng = Config::new()
+                .range_version("1".."99")
+                .probe("libpng16")
+                .unwrap();
+            assert!(libpng.version.find('\n').is_none());
+        }
+    }
+
+    #[test]
+    #[cfg(target_os = "linux")]
+    fn system_library_linux_test() {
+        assert!(!is_static_available(
+            "util",
+            &[PathBuf::from("/usr")],
+            &[PathBuf::from("/usr/lib/x86_64-linux-gnu")]
+        ));
+        assert!(!is_static_available(
+            "dialog",
+            &[PathBuf::from("/usr")],
+            &[PathBuf::from("/usr/lib")]
+        ));
     }
-}
 
-#[test]
-#[cfg(target_os = "linux")]
-fn system_library_linux_test() {
-    assert!(!is_static_available(
-        "util",
-        &[PathBuf::from("/usr")],
-        &[PathBuf::from("/usr/lib/x86_64-linux-gnu")]
-    ));
-    assert!(!is_static_available(
-        "dialog",
-        &[PathBuf::from("/usr")],
-        &[PathBuf::from("/usr/lib")]
-    ));
+    #[track_caller]
+    fn test_library_filename(target: &str, filename: &str) {
+        assert_eq!(
+            Library::extract_lib_from_filename(target, filename),
+            Some("foo")
+        );
+    }
+
+    #[test]
+    fn link_filename_linux() {
+        let target = "x86_64-unknown-linux-gnu";
+        test_library_filename(target, "libfoo.a");
+        test_library_filename(target, "libfoo.so");
+    }
+
+    #[test]
+    fn link_filename_apple() {
+        let target = "x86_64-apple-darwin";
+        test_library_filename(target, "libfoo.a");
+        test_library_filename(target, "libfoo.so");
+        test_library_filename(target, "libfoo.dylib");
+    }
+
+    #[test]
+    fn link_filename_msvc() {
+        let target = "x86_64-pc-windows-msvc";
+        // static and dynamic libraries have the same .lib suffix
+        test_library_filename(target, "foo.lib");
+    }
+
+    #[test]
+    fn link_filename_mingw() {
+        let target = "x86_64-pc-windows-gnu";
+        test_library_filename(target, "foo.lib");
+        test_library_filename(target, "libfoo.a");
+        test_library_filename(target, "foo.dll");
+        test_library_filename(target, "foo.dll.a");
+    }
 }