Soft-disable incremental compilation

This disables incremental compilation by default and adds a snippet to the
compiler release notes explaining the rationale and encouraging testing.

RELEASES.md

@@ -18,6 +18,21 @@ Compiler
 - [Warn when a `#[test]`-like built-in attribute macro is present multiple times.][91172]
 - [Add support for riscv64gc-unknown-freebsd][91284]
 - [Stabilize `-Z emit-future-incompat` as `--json future-incompat`][91535]
+- [Soft disable incremental compilation][94124]
+
+This release disables incremental compilation, unless the user has explicitly
+opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable.
+This is due to a known and relatively frequently occurring bug in incremental
+compilation, which causes builds to issue internal compiler errors. This
+particular bug is already fixed on nightly, but that fix has not yet rolled out
+to stable and is deemed too risky for a direct stable backport.
+
+As always, we encourage users to test with nightly and report bugs so that we
+can track failures and fix issues earlier.
+
+See [94124] for more details.
+
+[94124]: https://github.com/rust-lang/rust/issues/94124
 
 Libraries
 ---------

compiler/rustc_session/src/config.rs

@@ -2102,7 +2102,12 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
 
     check_thread_count(&debugging_opts, error_format);
 
-    let incremental = cg.incremental.as_ref().map(PathBuf::from);
+    let incremental =
+        if std::env::var_os("RUSTC_FORCE_INCREMENTAL").map(|v| v == "1").unwrap_or(false) {
+            cg.incremental.as_ref().map(PathBuf::from)
+        } else {
+            None
+        };
 
     let assert_incr_state =
         parse_assert_incr_state(&debugging_opts.assert_incr_state, error_format);

src/doc/rustc/src/codegen-options/index.md

@@ -160,6 +160,9 @@ to save information after compiling a crate to be reused when recompiling the
 crate, improving re-compile times. This takes a path to a directory where
 incremental files will be stored.
 
+Note that this option currently does not take effect unless
+`RUSTC_FORCE_INCREMENTAL=1` in the environment.
+
 ## inline-threshold
 
 This option lets you set the default threshold for inlining a function. It

src/test/run-make/dep-graph/Makefile

@@ -5,7 +5,9 @@
 # Just verify that we successfully run and produce dep graphs when requested.
 
 all:
-	RUST_DEP_GRAPH=$(TMPDIR)/dep-graph $(RUSTC) \
+	RUST_DEP_GRAPH=$(TMPDIR)/dep-graph \
+    RUSTC_FORCE_INCREMENTAL=1 \
+    $(RUSTC) \
         -Cincremental=$(TMPDIR)/incr \
         -Zquery-dep-graph -Zdump-dep-graph foo.rs
 	test -f $(TMPDIR)/dep-graph.txt

src/test/run-make/incremental-session-fail/Makefile

@@ -8,6 +8,7 @@ all:
 	# Make it so that rustc will fail to create a session directory.
 	touch $(SESSION_DIR)
 	# Check exit code is 1 for an error, and not 101 for ICE.
+	RUSTC_FORCE_INCREMENTAL=1 \
 	$(RUSTC) foo.rs --crate-type=rlib -C incremental=$(SESSION_DIR) > $(OUTPUT_FILE) 2>&1; [ $$? -eq 1 ]
 	$(CGREP) "Could not create incremental compilation crate directory" < $(OUTPUT_FILE)
 	# -v tests are fragile, hopefully this text won't change

src/tools/compiletest/src/runtest.rs

@@ -130,6 +130,15 @@ pub fn run(config: Config, testpaths: &TestPaths, revision: Option<&str>) {
     }
     debug!("running {:?}", testpaths.file.display());
     let mut props = TestProps::from_file(&testpaths.file, revision, &config);
+
+    // Currently, incremental is soft disabled unless this environment
+    // variable is set. A bunch of our tests assume it's enabled, though - so
+    // just enable it for our tests.
+    //
+    // This is deemed preferable to ignoring those tests; we still want to test
+    // incremental somewhat, as users can opt in to it.
+    props.rustc_env.push((String::from("RUSTC_FORCE_INCREMENTAL"), String::from("1")));
+
     if props.incremental {
         props.incremental_dir = Some(incremental_dir(&config, testpaths));
     }
@@ -146,6 +155,12 @@ pub fn run(config: Config, testpaths: &TestPaths, revision: Option<&str>) {
         assert!(!props.revisions.is_empty(), "Incremental tests require revisions.");
         for revision in &props.revisions {
             let mut revision_props = TestProps::from_file(&testpaths.file, Some(revision), &config);
+
+            // See above - need to enable it explicitly for now.
+            revision_props
+                .rustc_env
+                .push((String::from("RUSTC_FORCE_INCREMENTAL"), String::from("1")));
+
             revision_props.incremental_dir = props.incremental_dir.clone();
             let rev_cx = TestCx {
                 config: &config,
@@ -1630,7 +1645,17 @@ impl<'test> TestCx<'test> {
     /// Returns whether or not it is a dylib.
     fn build_auxiliary(&self, source_path: &str, aux_dir: &Path) -> bool {
         let aux_testpaths = self.compute_aux_test_paths(source_path);
-        let aux_props = self.props.from_aux_file(&aux_testpaths.file, self.revision, self.config);
+        let mut aux_props =
+            self.props.from_aux_file(&aux_testpaths.file, self.revision, self.config);
+
+        // Currently, incremental is soft disabled unless this environment
+        // variable is set. A bunch of our tests assume it's enabled, though - so
+        // just enable it for our tests.
+        //
+        // This is deemed preferable to ignoring those tests; we still want to test
+        // incremental somewhat, as users can opt in to it.
+        aux_props.rustc_env.push((String::from("RUSTC_FORCE_INCREMENTAL"), String::from("1")));
+
         let aux_output = TargetLocation::ThisDirectory(self.aux_output_dir_name());
         let aux_cx = TestCx {
             config: self.config,