Rollup merge of #63155 - mfkl:uwp-msvc, r=alexcrichton

Add UWP MSVC targets

Hi,

- The README URI change is the correct one for VS2019 community edition, which I suspect most people would use. Doesn't _need_ to be merged though.
- This 5e6619edd1 fixes the UWP build (msvc or not, doesn't matter). I suspect it broke with recent changes unnoticed because no CI.
- Store lib location is found through the VCToolsInstallDir env variable. The end of the path is currently for the VS2019 store lib locations only.
- I could not test the aarch64_uwp_windows_msvc target because the rust build script does not currently support arm64 msvc AFAIU.

src/librustc_codegen_ssa/back/link.rs

@@ -32,6 +32,7 @@ use std::path::{Path, PathBuf};
 use std::process::{Output, Stdio, ExitStatus};
 use std::str;
 use std::env;
+use std::ffi::OsString;
 
 pub use rustc_codegen_utils::link::*;
 
@@ -158,6 +159,36 @@ pub fn get_linker(sess: &Session, linker: &Path, flavor: LinkerFlavor) -> (PathB
         }
     };
 
+    // UWP apps have API restrictions enforced during Store submissions.
+    // To comply with the Windows App Certification Kit,
+    // MSVC needs to link with the Store versions of the runtime libraries (vcruntime, msvcrt, etc).
+    let t = &sess.target.target;
+    if flavor == LinkerFlavor::Msvc && t.target_vendor == "uwp" {
+        if let Some(ref tool) = msvc_tool {
+            let original_path = tool.path();
+            if let Some(ref root_lib_path) = original_path.ancestors().skip(4).next() {
+                let arch = match t.arch.as_str() {
+                    "x86_64" => Some("x64".to_string()),
+                    "x86" => Some("x86".to_string()),
+                    "aarch64" => Some("arm64".to_string()),
+                    _ => None,
+                };
+                if let Some(ref a) = arch {
+                    let mut arg = OsString::from("/LIBPATH:");
+                    arg.push(format!("{}\\lib\\{}\\store", root_lib_path.display(), a.to_string()));
+                    cmd.arg(&arg);
+                }
+                else {
+                    warn!("arch is not supported");
+                }
+            } else {
+                warn!("MSVC root path lib location not found");
+            }
+        } else {
+            warn!("link.exe not found");
+        }
+    }
+
     // The compiler's sysroot often has some bundled tools, so add it to the
     // PATH for the child.
     let mut new_path = sess.host_filesearch(PathKind::All)
@@ -1027,6 +1058,7 @@ fn link_args<'a, B: ArchiveBuilder<'a>>(cmd: &mut dyn Linker,
     let t = &sess.target.target;
 
     cmd.include_path(&fix_windows_verbatim_for_gcc(&lib_path));
+
     for obj in codegen_results.modules.iter().filter_map(|m| m.object.as_ref()) {
         cmd.add_object(obj);
     }