Don't encode file information for span with a dummy location

Fixes #83112

The location information for a dummy span isn't real, so don't encode
it. This brings the incr comp cache code into line with the Span
`StableHash` impl, which doesn't hash the location information for dummy
spans.

Previously, we would attempt to load the 'original' file from a dummy
span - if the file id changed (e.g. due to being moved on disk), we would get an
ICE, since the Span was still valid due to its hash being unchanged.

Author: Aaron1011

compiler/rustc_middle/src/ty/query/on_disk_cache.rs

@@ -1045,12 +1045,12 @@ where
     E: 'a + OpaqueEncoder,
 {
     fn encode(&self, s: &mut CacheEncoder<'a, 'tcx, E>) -> Result<(), E::Error> {
-        if *self == DUMMY_SP {
+        let span_data = self.data();
+        if self.is_dummy() {
             TAG_PARTIAL_SPAN.encode(s)?;
-            return SyntaxContext::root().encode(s);
+            return span_data.ctxt.encode(s);
         }
 
-        let span_data = self.data();
         let pos = s.source_map.byte_pos_to_line_and_col(span_data.lo);
         let partial_span = match &pos {
             Some((file_lo, _, _)) => !file_lo.contains(span_data.hi),

src/test/run-make/issue-83112-incr-test-moved-file/Makefile

@@ -0,0 +1,25 @@
+include ../../run-make-fulldeps/tools.mk
+
+# FIXME https://github.com/rust-lang/rust/issues/78911
+# ignore-32bit wrong/no cross compiler and sometimes we pass wrong gcc args (-m64)
+
+# Regression test for issue #83112
+# The generated test harness code contains spans with a dummy location,
+# but a non-dummy SyntaxContext. Previously, the incremental cache was encoding
+# these spans as a full span (with a source file index), instead of skipping
+# the encoding of the location information. If the file gest moved, the hash
+# of the span will be unchanged (since it has a dummy location), so the incr
+# cache would end up try to load a non-existent file using the previously
+# enccoded source file id.
+
+SRC=$(TMPDIR)/src
+INCR=$(TMPDIR)/incr
+
+all:
+	mkdir $(SRC)
+	mkdir $(SRC)/mydir
+	mkdir $(INCR)
+	cp main.rs $(SRC)/main.rs
+	$(RUSTC) --test -C incremental=$(INCR) $(SRC)/main.rs
+	mv $(SRC)/main.rs $(SRC)/mydir/main.rs
+	$(RUSTC) --test -C incremental=$(INCR) $(SRC)/mydir/main.rs

src/test/run-make/issue-83112-incr-test-moved-file/main.rs

@@ -0,0 +1 @@
+fn main() {}