Soft disable incremental

This disables incremental (i.e., -Cincremental) taking effect unless an
environment variable, RUSTC_FORCE_INCREMENTAL, is set to 1 in the environment of
the running rustc. Currently incremental causes errors for many users, and we do
not have an expectation of being able to quickly fix these errors in a
backportable way - so, for now, disable incremental entirely. The user can still
opt-in, but this way the majority of users merely get slower builds, not broken
builds.

compiler/rustc_session/src/config.rs

@@ -1885,7 +1885,12 @@ 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
+        };
 
     if debugging_opts.profile && incremental.is_some() {
         early_error(

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

@@ -161,6 +161,9 @@ 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/tools/compiletest/src/runtest.rs

@@ -229,7 +229,15 @@ pub fn run(config: Config, testpaths: &TestPaths, revision: Option<&str>) {
         print!("\n\n");
     }
     debug!("running {:?}", testpaths.file.display());
-    let props = TestProps::from_file(&testpaths.file, revision, &config);
+    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")));
 
     let cx = TestCx { config: &config, props: &props, testpaths, revision };
     create_dir_all(&cx.output_base_dir()).unwrap();
@@ -240,7 +248,11 @@ pub fn run(config: Config, testpaths: &TestPaths, revision: Option<&str>) {
         assert!(!props.revisions.is_empty(), "Incremental tests require revisions.");
         cx.init_incremental_test();
         for revision in &props.revisions {
-            let revision_props = TestProps::from_file(&testpaths.file, Some(revision), &config);
+            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")));
             let rev_cx = TestCx {
                 config: &config,
                 props: &revision_props,