Auto merge of #48209 - kennytm:try-fix-48116, r=alexcrichton

Try to fix 48116 and 48192

The bug #48116 happens because of a misoptimization of the `import_path_to_string` function, where a `names` slice is empty but the `!names.is_empty()` branch is executed.

https://github.com/rust-lang/rust/blob/4d2d3fc5dadf894a8ad709a5860a549f2c0b1032/src/librustc_resolve/resolve_imports.rs#L1015-L1042

Yesterday, @eddyb had locally reproduced the bug, and [came across the `position` function](https://mozilla.logbot.info/rust-infra/20180214#c14296834) where the `assume()` call is found to be suspicious. We have *not* concluded that this `assume()` causes #48116, but given [the reputation of `assume()`](https://github.com/rust-lang/rust/pull/45501#issuecomment-340159627), this seems higher relevant. Here we try to see if commenting it out can fix the errors.

Later @alexcrichton has bisected and found a potential bug [in the LLVM side](https://github.com/rust-lang/rust/issues/48116#issuecomment-365624777). We are currently testing if reverting that LLVM commit is enough to stop the bug. If true, this PR can be reverted (keep the `assume()`) and we could backport the LLVM patch instead.

(This PR also includes an earlier commit from #48127 for help debugging ICE happening in compile-fail/parse-fail tests.)

The PR also reverts #48059, which seems to cause #48192.

r? @alexcrichton
cc @eddyb, @arthurprs (#47333)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index a87809dd7bc..bef085e17ea 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -250,6 +250,7 @@ impl<'test> TestCx<'test> {
     fn run_cfail_test(&self) {
         let proc_res = self.compile_test();
         self.check_if_test_should_compile(&proc_res);
+        self.check_no_compiler_crash(&proc_res);
 
         let output_to_check = self.get_output(&proc_res);
         let expected_errors = errors::load_errors(&self.testpaths.file, self.revision);
@@ -262,7 +263,6 @@ impl<'test> TestCx<'test> {
             self.check_error_patterns(&output_to_check, &proc_res);
         }
 
-        self.check_no_compiler_crash(&proc_res);
         self.check_forbid_output(&output_to_check, &proc_res);
     }