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, 15 insertions, 9 deletions

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index aacbbd5058e..ac390313a67 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -1246,15 +1246,18 @@ macro_rules! iterator {
             {
                 // The addition might panic on overflow
                 // Use the len of the slice to hint optimizer to remove result index bounds check.
-                let n = make_slice!(self.ptr, self.end).len();
+                let _n = make_slice!(self.ptr, self.end).len();
                 self.try_fold(0, move |i, x| {
                     if predicate(x) { Err(i) }
                     else { Ok(i + 1) }
                 }).err()
-                    .map(|i| {
-                        unsafe { assume(i < n) };
-                        i
-                    })
+                    // // FIXME(#48116/#45964):
+                    // // This assume() causes misoptimization on LLVM 6.
+                    // // Commented out until it is fixed again.
+                    // .map(|i| {
+                    //     unsafe { assume(i < n) };
+                    //     i
+                    // })
             }
 
             #[inline]
@@ -1271,10 +1274,13 @@ macro_rules! iterator {
                     if predicate(x) { Err(i) }
                     else { Ok(i) }
                 }).err()
-                    .map(|i| {
-                        unsafe { assume(i < n) };
-                        i
-                    })
+                    // // FIXME(#48116/#45964):
+                    // // This assume() causes misoptimization on LLVM 6.
+                    // // Commented out until it is fixed again.
+                    // .map(|i| {
+                    //     unsafe { assume(i < n) };
+                    //     i
+                    // })
             }
         }