Remap paths from other crates

Previously, `remap-path-prefix` only applied to files loaded *in the current session*. File names
loaded in a previous invocation were used unchanged. This is unfortunate for UI testing when some of
the crates involved are in the standard library (we don't want to recompile the standard library for
arbitrary tests).

Change it to apply to existing files as well. I believe this will eventually allow removing the
special-casing for `-Z simulate-remapped-rust-src-base`.

Author: jyn514

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1726,6 +1726,12 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 // on `try_to_translate_virtual_to_real`).
                 try_to_translate_virtual_to_real(&mut name);
 
+                // We may have had different remap-path-prefix flags at the time this file was loaded.
+                // Apply the remapping for the current session.
+                // NOTE: this does not "undo and redo" the mapping - any existing remapping from the old
+                // crate is retained unmodified. Only files which were never remapped are considered.
+                name = sess.source_map().path_mapping().map_filename_prefix(&name).0;
+
                 let local_version = sess.source_map().new_imported_source_file(
                     name,
                     src_hash,

compiler/rustc_span/src/source_map.rs

@@ -1147,9 +1147,12 @@ impl FilePathMapping {
         }
     }
 
-    fn map_filename_prefix(&self, file: &FileName) -> (FileName, bool) {
+    /// Given a `file`, map it using the `remap-path-prefix` options for the current [`Session`].
+    ///
+    /// Public for use in rustc_metadata::decoder
+    pub fn map_filename_prefix(&self, file: &FileName) -> (FileName, bool) {
         match file {
-            FileName::Real(realfile) if let RealFileName::LocalPath(local_path) = realfile => {
+            FileName::Real(realfile @ RealFileName::LocalPath(local_path)) => {
                 let (mapped_path, mapped) = self.map_prefix(local_path);
                 let realfile = if mapped {
                     RealFileName::Remapped {
@@ -1161,7 +1164,13 @@ impl FilePathMapping {
                 };
                 (FileName::Real(realfile), mapped)
             }
-            FileName::Real(_) => unreachable!("attempted to remap an already remapped filename"),
+            existing @ FileName::Real(RealFileName::Remapped { .. }) => {
+                // This can happen if we are remapping the name of file that was loaded in a
+                // different Session, and inconsistent `remap-path-prefix` flags were passed between
+                // sessions. It's unclear what to do here. For now, respect the original flag, not
+                // the flag from the current session.
+                (existing.clone(), false)
+            }
             other => (other.clone(), false),
         }
     }

tests/run-make/remap-path-prefix-inconsistent/Makefile

@@ -0,0 +1,16 @@
+
+include ../tools.mk
+
+all: inconsistent-mapping last-crate-only
+
+last-crate-only:
+	$(RUSTC) is_true.rs --crate-type lib
+	$(RUSTC) main.rs --remap-path-prefix=$(CURDIR)=/TEST_DIR 2>$(TMPDIR)/last-crate.txt --extern is_true=$(TMPDIR)/libis_true.rlib || true
+	$(RUSTC_TEST_OP) $(TMPDIR)/last-crate.txt expected-last-crate.txt
+	$(CGREP) "/TEST_DIR" < $(TMPDIR)/last-crate.txt
+
+inconsistent-mapping:
+	$(RUSTC) is_true.rs --remap-path-prefix=$(CURDIR)=/TEST_DIR1 --crate-type lib
+	$(RUSTC) main.rs --remap-path-prefix=$(CURDIR)=/TEST_DIR2 2>$(TMPDIR)/inconsistent.txt --extern is_true=$(TMPDIR)/libis_true.rlib || true
+	$(RUSTC_TEST_OP) $(TMPDIR)/inconsistent.txt expected-inconsistent.txt
+	$(CGREP) "/TEST_DIR1" < $(TMPDIR)/inconsistent.txt

tests/run-make/remap-path-prefix-inconsistent/expected-inconsistent.txt

@@ -0,0 +1,16 @@
+error[E0061]: this function takes 1 argument but 0 arguments were supplied
+ --> main.rs:2:5
+  |
+2 |     is_true::query();
+  |     ^^^^^^^^^^^^^^-- an argument of type `bool` is missing
+  |
+note: function defined here
+ --> /TEST_DIR1/is_true.rs:1:8
+help: provide the argument
+  |
+2 |     is_true::query(/* bool */);
+  |                   ~~~~~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0061`.

tests/run-make/remap-path-prefix-inconsistent/expected-last-crate.txt

@@ -0,0 +1,19 @@
+error[E0061]: this function takes 1 argument but 0 arguments were supplied
+ --> main.rs:2:5
+  |
+2 |     is_true::query();
+  |     ^^^^^^^^^^^^^^-- an argument of type `bool` is missing
+  |
+note: function defined here
+ --> /TEST_DIR/is_true.rs:1:8
+  |
+1 | pub fn query(_: bool) {}
+  |        ^^^^^
+help: provide the argument
+  |
+2 |     is_true::query(/* bool */);
+  |                   ~~~~~~~~~~~~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0061`.

tests/run-make/remap-path-prefix-inconsistent/is_true.rs

@@ -0,0 +1 @@
+pub fn query(_: bool) {}

tests/run-make/remap-path-prefix-inconsistent/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    is_true::query();
+}