remove new allocations from imported_source_files

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -41,7 +41,7 @@ use std::io;
 use std::iter::TrustedLen;
 use std::mem;
 use std::num::NonZeroUsize;
-use std::path::PathBuf;
+use std::path::Path;
 use tracing::debug;
 
 pub(super) use cstore_impl::provide;
@@ -1474,30 +1474,30 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     /// Proc macro crates don't currently export spans, so this function does not have
     /// to work for them.
     fn imported_source_files(self, sess: &Session) -> &'a [ImportedSourceFile] {
+        fn filter<'a>(sess: &Session, path: Option<&'a Path>) -> Option<&'a Path> {
+            path.filter(|_| {
+                // Only spend time on further checks if we have what to translate *to*.
+                sess.opts.real_rust_source_base_dir.is_some()
+                // Some tests need the translation to be always skipped.
+                && sess.opts.unstable_opts.translate_remapped_path_to_local_path
+            })
+            .filter(|virtual_dir| {
+                // Don't translate away `/rustc/$hash` if we're still remapping to it,
+                // since that means we're still building `std`/`rustc` that need it,
+                // and we don't want the real path to leak into codegen/debuginfo.
+                !sess.opts.remap_path_prefix.iter().any(|(_from, to)| to == virtual_dir)
+            })
+        }
+
         // Translate the virtual `/rustc/$hash` prefix back to a real directory
         // that should hold actual sources, where possible.
         //
         // NOTE: if you update this, you might need to also update bootstrap's code for generating
         // the `rust-src` component in `Src::run` in `src/bootstrap/dist.rs`.
         let virtual_rust_source_base_dir = [
-            option_env!("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR").map(PathBuf::from),
-            sess.opts.unstable_opts.simulate_remapped_rust_src_base.clone(),
-        ]
-        .into_iter()
-        .filter(|_| {
-            // Only spend time on further checks if we have what to translate *to*.
-            sess.opts.real_rust_source_base_dir.is_some()
-                // Some tests need the translation to be always skipped.
-                && sess.opts.unstable_opts.translate_remapped_path_to_local_path
-        })
-        .flatten()
-        .filter(|virtual_dir| {
-            // Don't translate away `/rustc/$hash` if we're still remapping to it,
-            // since that means we're still building `std`/`rustc` that need it,
-            // and we don't want the real path to leak into codegen/debuginfo.
-            !sess.opts.remap_path_prefix.iter().any(|(_from, to)| to == virtual_dir)
-        })
-        .collect::<Vec<_>>();
+            filter(sess, option_env!("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR").map(Path::new)),
+            filter(sess, sess.opts.unstable_opts.simulate_remapped_rust_src_base.as_deref()),
+        ];
 
         let try_to_translate_virtual_to_real = |name: &mut rustc_span::FileName| {
             debug!(
@@ -1506,7 +1506,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 name, virtual_rust_source_base_dir, sess.opts.real_rust_source_base_dir,
             );
 
-            for virtual_dir in &virtual_rust_source_base_dir {
+            for virtual_dir in virtual_rust_source_base_dir.iter().flatten() {
                 if let Some(real_dir) = &sess.opts.real_rust_source_base_dir {
                     if let rustc_span::FileName::Real(old_name) = name {
                         if let rustc_span::RealFileName::Remapped { local_path: _, virtual_name } =