Add coverage to continue statements

`continue` statements were missing coverage. This was particularly
noticeable in a match pattern that contained only a `continue`
statement, leaving the branch appear uncounted. This PR addresses the
problem and adds tests to prove it.

compiler/rustc_mir_build/src/build/scope.rs

@@ -618,6 +618,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         } else {
             assert!(value.is_none(), "`return` and `break` should have a destination");
+            // `continue` statements generate no MIR statement with the `continue` statement's Span,
+            // and the `InstrumentCoverage` statement will have no way to generate a coverage
+            // code region for the `continue` statement, unless we add a dummy `Assign` here:
+            let mut local_decl = LocalDecl::new(self.tcx.mk_unit(), span);
+            local_decl = local_decl.immutable();
+            let temp = self.local_decls.push(local_decl);
+            let temp_place = Place::from(temp);
+            self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(temp) });
+            self.cfg.push_assign_unit(block, source_info, temp_place, self.tcx);
+            self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageDead(temp) });
         }
 
         let region_scope = self.scopes.breakable_scopes[break_index].region_scope;

src/test/mir-opt/loop_test.main.SimplifyCfg-promote-consts.after.mir

@@ -8,6 +8,7 @@ fn main() -> () {
     let mut _4: !;                       // in scope 0 at $DIR/loop_test.rs:13:5: 16:6
     let mut _5: ();                      // in scope 0 at $DIR/loop_test.rs:6:1: 17:2
     let _6: i32;                         // in scope 0 at $DIR/loop_test.rs:14:13: 14:14
+    let _7: ();                          // in scope 0 at $DIR/loop_test.rs:15:9: 15:17
     scope 1 {
         debug x => _6;                   // in scope 1 at $DIR/loop_test.rs:14:13: 14:14
     }
@@ -42,6 +43,9 @@ fn main() -> () {
         StorageLive(_6);                 // scope 0 at $DIR/loop_test.rs:14:13: 14:14
         _6 = const 1_i32;                // scope 0 at $DIR/loop_test.rs:14:17: 14:18
         FakeRead(ForLet(None), _6);      // scope 0 at $DIR/loop_test.rs:14:13: 14:14
+        StorageLive(_7);                 // scope 1 at $DIR/loop_test.rs:15:9: 15:17
+        _7 = const ();                   // scope 1 at $DIR/loop_test.rs:15:9: 15:17
+        StorageDead(_7);                 // scope 1 at $DIR/loop_test.rs:15:9: 15:17
         StorageDead(_6);                 // scope 0 at $DIR/loop_test.rs:16:5: 16:6
         goto -> bb3;                     // scope 0 at $DIR/loop_test.rs:1:1: 1:1
     }

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.continue.txt

@@ -0,0 +1,75 @@
+    1|       |#![allow(unused_assignments, unused_variables)]
+    2|       |
+    3|      1|fn main() {
+    4|      1|    let is_true = std::env::args().len() == 1;
+    5|      1|
+    6|      1|    let mut x = 0;
+    7|     11|    for _ in 0..10 {
+                      ^10
+    8|     10|        match is_true {
+    9|       |            true => {
+   10|     10|                continue;
+   11|       |            }
+   12|      0|            _ => {
+   13|      0|                x = 1;
+   14|      0|            }
+   15|      0|        }
+   16|      0|        x = 3;
+   17|       |    }
+   18|     11|    for _ in 0..10 {
+                      ^10
+   19|     10|        match is_true {
+   20|      0|            false => {
+   21|      0|                x = 1;
+   22|      0|            }
+   23|       |            _ => {
+   24|     10|                continue;
+   25|       |            }
+   26|       |        }
+   27|      0|        x = 3;
+   28|       |    }
+   29|     11|    for _ in 0..10 {
+                      ^10
+   30|     10|        match is_true {
+   31|     10|            true => {
+   32|     10|                x = 1;
+   33|     10|            }
+   34|       |            _ => {
+   35|      0|                continue;
+   36|       |            }
+   37|       |        }
+   38|     10|        x = 3;
+   39|       |    }
+   40|     11|    for _ in 0..10 {
+                      ^10
+   41|     10|        if is_true {
+   42|     10|            continue;
+   43|      0|        }
+   44|      0|        x = 3;
+   45|       |    }
+   46|     11|    for _ in 0..10 {
+                      ^10
+   47|     10|        match is_true {
+   48|      0|            false => {
+   49|      0|                x = 1;
+   50|      0|            }
+   51|     10|            _ => {
+   52|     10|                let _ = x;
+   53|     10|            }
+   54|       |        }
+   55|     10|        x = 3;
+   56|       |    }
+   57|      1|    for _ in 0..10 {
+   58|      1|        match is_true {
+   59|      0|            false => {
+   60|      0|                x = 1;
+   61|      0|            }
+   62|       |            _ => {
+   63|      1|                break;
+   64|       |            }
+   65|       |        }
+   66|      0|        x = 3;
+   67|       |    }
+   68|       |    let _ = x;
+   69|      1|}
+

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.uses_crate.txt

@@ -19,12 +19,12 @@
    18|      2|    println!("used_only_from_bin_crate_generic_function with {:?}", arg);
    19|      2|}
   ------------------
-  | used_crate::used_only_from_bin_crate_generic_function::<&alloc::vec::Vec<i32>>:
+  | used_crate::used_only_from_bin_crate_generic_function::<&str>:
   |   17|      1|pub fn used_only_from_bin_crate_generic_function<T: Debug>(arg: T) {
   |   18|      1|    println!("used_only_from_bin_crate_generic_function with {:?}", arg);
   |   19|      1|}
   ------------------
-  | used_crate::used_only_from_bin_crate_generic_function::<&str>:
+  | used_crate::used_only_from_bin_crate_generic_function::<&alloc::vec::Vec<i32>>:
   |   17|      1|pub fn used_only_from_bin_crate_generic_function<T: Debug>(arg: T) {
   |   18|      1|    println!("used_only_from_bin_crate_generic_function with {:?}", arg);
   |   19|      1|}

src/test/run-make-fulldeps/coverage/continue.rs

@@ -0,0 +1,69 @@
+#![allow(unused_assignments, unused_variables)]
+
+fn main() {
+    let is_true = std::env::args().len() == 1;
+
+    let mut x = 0;
+    for _ in 0..10 {
+        match is_true {
+            true => {
+                continue;
+            }
+            _ => {
+                x = 1;
+            }
+        }
+        x = 3;
+    }
+    for _ in 0..10 {
+        match is_true {
+            false => {
+                x = 1;
+            }
+            _ => {
+                continue;
+            }
+        }
+        x = 3;
+    }
+    for _ in 0..10 {
+        match is_true {
+            true => {
+                x = 1;
+            }
+            _ => {
+                continue;
+            }
+        }
+        x = 3;
+    }
+    for _ in 0..10 {
+        if is_true {
+            continue;
+        }
+        x = 3;
+    }
+    for _ in 0..10 {
+        match is_true {
+            false => {
+                x = 1;
+            }
+            _ => {
+                let _ = x;
+            }
+        }
+        x = 3;
+    }
+    for _ in 0..10 {
+        match is_true {
+            false => {
+                x = 1;
+            }
+            _ => {
+                break;
+            }
+        }
+        x = 3;
+    }
+    let _ = x;
+}