Rollup merge of rust-lang#64974 - ecstatic-morse:generic-graphviz-zebra, r=petrochenkov

Fix zebra-striping in generic dataflow visualization

A small formatting improvement to rust-lang#64828.

Prior to this, the background color of the first row of the table for each basic block changed seemingly at random. You can see this in [basic block rust-lang#5](rust-lang#64828 (comment)) under "New table". Now it is always light.

This also updates the example table to match the current output.

Author: tmandry

src/librustc_mir/dataflow/generic/graphviz.rs

@@ -165,25 +165,25 @@ where
         block: BasicBlock,
     ) -> io::Result<()> {
         //   Sample output:
-        //   +-+--------------------------------------------------+
-        // A |                         bb4                        |
-        //   +-+----------------------------------+---------------+
-        // B |               MIR                  |     STATE     |
-        //   +-+----------------------------------+---------------+
-        // C | | (on entry)                       | {_0,_2,_3}    |
-        //   +-+----------------------------------+---------------+
-        // D |0|  0: StorageLive(_7)              |               |
-        //   +-+----------------------------------+---------------+
-        //   |1|  1: StorageLive(_8)              |               |
-        //   +-+----------------------------------+---------------+
-        //   |2|  2: _8 = &mut _1                 | +_8           |
-        //   +-+----------------------------------+---------------+
-        // E |T| _7 = const Foo::twiddle(move _8) | -_8           |
-        //   +-+----------------------------------+---------------+
-        // F | | (on unwind)                      | {_0,_2,_3,_7} |
-        //   +-+----------------------------------+---------------+
-        //   | | (on successful return)           | +_7           |
-        //   +-+----------------------------------+---------------+
+        //   +-+-----------------------------------------------+
+        // A |                      bb4                        |
+        //   +-+----------------------------------+------------+
+        // B |                MIR                 |   STATE    |
+        //   +-+----------------------------------+------------+
+        // C | | (on entry)                       | {_0,_2,_3} |
+        //   +-+----------------------------------+------------+
+        // D |0| StorageLive(_7)                  |            |
+        //   +-+----------------------------------+------------+
+        //   |1| StorageLive(_8)                  |            |
+        //   +-+----------------------------------+------------+
+        //   |2| _8 = &mut _1                     | +_8        |
+        //   +-+----------------------------------+------------+
+        // E |T| _4 = const Foo::twiddle(move _2) | -_2        |
+        //   +-+----------------------------------+------------+
+        // F | | (on unwind)                      | {_0,_3,_8} |
+        //   +-+----------------------------------+------------+
+        //   | | (on successful return)           | +_4        |
+        //   +-+----------------------------------+------------+
 
         write!(
             w,
@@ -211,6 +211,7 @@ where
         )?;
 
         // C: Entry state
+        self.bg = Background::Light;
         self.results.seek_to_block_start(block);
         self.write_row_with_curr_state(w, "", "(on entry)")?;
         self.prev_state.overwrite(self.results.get());