fix panic when rustc tries to reduce intermediate filenames len with utf8

compiler/rustc_session/src/config.rs

@@ -1211,13 +1211,22 @@ fn maybe_strip_file_name(mut path: PathBuf) -> PathBuf {
     if path.file_name().map_or(0, |name| name.len()) > MAX_FILENAME_LENGTH {
         let filename = path.file_name().unwrap().to_string_lossy();
         let hash_len = 64 / 4; // Hash64 is 64 bits encoded in hex
-        let stripped_len = filename.len() - MAX_FILENAME_LENGTH + hash_len;
+        let hyphen_len = 1; // the '-' we insert between hash and suffix
+
+        // number of bytes of suffix we can keep so that "hash-<suffix>" fits
+        let allowed_suffix = MAX_FILENAME_LENGTH.saturating_sub(hash_len + hyphen_len);
+
+        // number of bytes to remove from the start
+        let stripped_bytes = filename.len().saturating_sub(allowed_suffix);
+
+        // ensure we don't cut in a middle of a char
+        let split_at = filename.ceil_char_boundary(stripped_bytes);
 
         let mut hasher = StableHasher::new();
-        filename[..stripped_len].hash(&mut hasher);
+        filename[..split_at].hash(&mut hasher);
         let hash = hasher.finish::<Hash64>();
 
-        path.set_file_name(format!("{:x}-{}", hash, &filename[stripped_len..]));
+        path.set_file_name(format!("{:x}-{}", hash, &filename[split_at..]));
     }
     path
 }

tests/run-make/lto-long-filenames/rmake.rs

@@ -9,8 +9,6 @@
 
 //@ ignore-cross-compile
 
-use std::fs;
-
 use run_make_support::{rfs, rustc};
 
 // This test make sure we don't get such following error:

tests/run-make/lto-long-filenames_cn/rmake.rs

@@ -0,0 +1,25 @@
+//@ ignore-cross-compile
+// gnu ld is confused with intermediate files having multibytes characters in their names:
+// = note: ld.exe: cannot find f0d5ff18d6510ebc-???_???_??????????_?_?????_?_???????.d50c2 \
+// 4c0c4ea93cc-cgu.0.rcgu.o: Invalid argument
+// as this is not something rustc can fix by itself,
+// we just skip the test on windows-gnu for now. Hence:
+//@ ignore-windows-gnu
+
+use run_make_support::{rfs, rustc};
+
+// This test make sure we don't crash when lto creates output files with long names.
+// cn characters can be multi-byte and thus trigger the long filename reduction code more easily.
+// we need to make sure that the code is properly generating names at char boundaries.
+// as reported in issue #147975
+fn main() {
+    let lto_flags = ["-Clto", "-Clto=yes", "-Clto=off", "-Clto=thin", "-Clto=fat"];
+    for prefix_len in 0..4 {
+        let prefix: String = std::iter::repeat("_").take(prefix_len).collect();
+        let main_file = format!("{}ⵅⴻⵎⵎⴻⵎ_ⴷⵉⵎⴰ_ⵖⴻⴼ_ⵢⵉⵙⴻⴽⴽⵉⵍⴻⵏ_ⵏ_ⵡⴰⵟⴰⵙ_ⵏ_ⵢⵉⴱⵢⵜⴻⵏ.rs", prefix);
+        rfs::write(&main_file, "fn main() {}\n");
+        for flag in lto_flags {
+            rustc().input(&main_file).arg(flag).run();
+        }
+    }
+}