Auto merge of #25013 - pnkfelix:span_to_lines-oflo, r=huonw

Guard against overflow in `codemap::span_to_lines`.

(Revised/expanded version of PR #24976)

Make `span_to_lines` to return a `Result`.

In `diagnostic`, catch `Err` from `span_to_lines` and print `"(unprintable span)"` instead.

----

There a number of recent issues that report the bug here.  See e.g. #24761 and #24954.

This change *might* fix them. However, that is *not* its main goal. The main goals are:

 1. Make it possible for callers to recover from an error here, and

 2. Insert a more conservative check, in that we are also checking that the files match up.

----

As a drive-by, fix #24997 , which was causing my attempts to `make check-stage1` on an `--enable-debug` build to fail.

1 files changed, 24 insertions, 6 deletions

diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index f0accef92ce..56dee303487 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -631,8 +631,14 @@ fn size_from_ptr<T>(_: *const T) -> usize {
 }
 
 
-// Use macro to be generic over const/mut
-macro_rules! slice_offset {
+// Use macros to be generic over const/mut
+//
+// They require non-negative `$by` because otherwise the expression
+// `(ptr as usize + $by)` would interpret `-1` as `usize::MAX` (and
+// thus trigger a panic when overflow checks are on).
+
+// Use this to do `$ptr + $by`, where `$by` is non-negative.
+macro_rules! slice_add_offset {
     ($ptr:expr, $by:expr) => {{
         let ptr = $ptr;
         if size_from_ptr(ptr) == 0 {
@@ -643,6 +649,18 @@ macro_rules! slice_offset {
     }};
 }
 
+// Use this to do `$ptr - $by`, where `$by` is non-negative.
+macro_rules! slice_sub_offset {
+    ($ptr:expr, $by:expr) => {{
+        let ptr = $ptr;
+        if size_from_ptr(ptr) == 0 {
+            transmute(ptr as usize - $by)
+        } else {
+            ptr.offset(-$by)
+        }
+    }};
+}
+
 macro_rules! slice_ref {
     ($ptr:expr) => {{
         let ptr = $ptr;
@@ -672,7 +690,7 @@ macro_rules! iterator {
                         None
                     } else {
                         let old = self.ptr;
-                        self.ptr = slice_offset!(self.ptr, 1);
+                        self.ptr = slice_add_offset!(self.ptr, 1);
                         Some(slice_ref!(old))
                     }
                 }
@@ -714,7 +732,7 @@ macro_rules! iterator {
                     if self.end == self.ptr {
                         None
                     } else {
-                        self.end = slice_offset!(self.end, -1);
+                        self.end = slice_sub_offset!(self.end, 1);
                         Some(slice_ref!(self.end))
                     }
                 }
@@ -776,7 +794,7 @@ impl<'a, T> Iter<'a, T> {
     fn iter_nth(&mut self, n: usize) -> Option<&'a T> {
         match self.as_slice().get(n) {
             Some(elem_ref) => unsafe {
-                self.ptr = slice_offset!(elem_ref as *const _, 1);
+                self.ptr = slice_add_offset!(elem_ref as *const _, 1);
                 Some(slice_ref!(elem_ref))
             },
             None => {
@@ -849,7 +867,7 @@ impl<'a, T> IterMut<'a, T> {
     fn iter_nth(&mut self, n: usize) -> Option<&'a mut T> {
         match make_mut_slice!(T => &'a mut [T]: self.ptr, self.end).get_mut(n) {
             Some(elem_ref) => unsafe {
-                self.ptr = slice_offset!(elem_ref as *mut _, 1);
+                self.ptr = slice_add_offset!(elem_ref as *mut _, 1);
                 Some(slice_ref!(elem_ref))
             },
             None => {