Report coverage `0` of dead blocks

Fixes: #84018

With `-Z instrument-coverage`, coverage reporting of dead blocks
(for example, blocks dropped because a conditional branch is dropped,
based on const evaluation) is now supported.

If `instrument-coverage` is enabled, `simplify::remove_dead_blocks()`
finds all dropped coverage `Statement`s and adds their `code_region`s as
`Unreachable` coverage `Statement`s to the `START_BLOCK`, so they are
still included in the coverage map.

Check out the resulting changes in the test coverage reports in this PR.

10 files changed, 54 insertions, 49 deletions

diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.async2.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.async2.txt
index 8a445433ab6..a8c79efd723 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.async2.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.async2.txt
@@ -12,6 +12,7 @@
    12|      1|    if b {
    13|      1|        println!("non_async_func println in block");
    14|      1|    }
+                   ^0
    15|      1|}
    16|       |
    17|       |// FIXME(#83985): The auto-generated closure in an async function is failing to include
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.conditions.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.conditions.txt
index 656a2659775..2d8a98a5d0c 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.conditions.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.conditions.txt
@@ -5,6 +5,7 @@
     5|      1|    if true {
     6|      1|        countdown = 10;
     7|      1|    }
+                   ^0
     8|       |
     9|       |    const B: u32 = 100;
    10|      1|    let x = if countdown > 7 {
@@ -24,6 +25,7 @@
    24|      1|    if true {
    25|      1|        countdown = 10;
    26|      1|    }
+                   ^0
    27|       |
    28|      1|    if countdown > 7 {
    29|      1|        countdown -= 4;
@@ -42,6 +44,7 @@
    41|      1|        if true {
    42|      1|            countdown = 10;
    43|      1|        }
+                       ^0
    44|       |
    45|      1|        if countdown > 7 {
    46|      1|            countdown -= 4;
@@ -54,13 +57,14 @@
    53|       |        } else {
    54|      0|            return;
    55|       |        }
-   56|       |    } // Note: closing brace shows uncovered (vs. `0` for implicit else) because condition literal
-   57|       |      // `true` was const-evaluated. The compiler knows the `if` block will be executed.
+   56|      0|    }
+   57|       |
    58|       |
    59|      1|    let mut countdown = 0;
    60|      1|    if true {
    61|      1|        countdown = 1;
    62|      1|    }
+                   ^0
    63|       |
    64|      1|    let z = if countdown > 7 {
                       ^0
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.doctest.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.doctest.txt
index 1b6bb9ff889..7ae0e978808 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.doctest.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.doctest.txt
@@ -9,7 +9,7 @@
     8|      1|//!     assert_eq!(1, 1);
     9|       |//! } else {
    10|       |//!     // this is not!
-   11|       |//!     assert_eq!(1, 2);
+   11|      0|//!     assert_eq!(1, 2);
    12|       |//! }
    13|      1|//! ```
    14|       |//!
@@ -84,7 +84,7 @@
    74|      1|    if true {
    75|      1|        assert_eq!(1, 1);
    76|       |    } else {
-   77|       |        assert_eq!(1, 2);
+   77|      0|        assert_eq!(1, 2);
    78|       |    }
    79|      1|}
    80|       |
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.drop_trait.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.drop_trait.txt
index fab5be41901..fe6a9e93cbf 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.drop_trait.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.drop_trait.txt
@@ -19,11 +19,11 @@
    19|      1|    if true {
    20|      1|        println!("Exiting with error...");
    21|      1|        return Err(1);
-   22|       |    }
-   23|       |
-   24|       |    let _ = Firework { strength: 1000 };
-   25|       |
-   26|       |    Ok(())
+   22|      0|    }
+   23|      0|
+   24|      0|    let _ = Firework { strength: 1000 };
+   25|      0|
+   26|      0|    Ok(())
    27|      1|}
    28|       |
    29|       |// Expected program output:
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generics.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generics.txt
index 7b38ffb87cb..8e8bc0fd189 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generics.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generics.txt
@@ -52,15 +52,15 @@
    30|      1|    if true {
    31|      1|        println!("Exiting with error...");
    32|      1|        return Err(1);
-   33|       |    } // The remaining lines below have no coverage because `if true` (with the constant literal
-   34|       |      // `true`) is guaranteed to execute the `then` block, which is also guaranteed to `return`.
-   35|       |      // Thankfully, in the normal case, conditions are not guaranteed ahead of time, and as shown
-   36|       |      // in other tests, the lines below would have coverage (which would show they had `0`
-   37|       |      // executions, assuming the condition still evaluated to `true`).
-   38|       |
-   39|       |    let _ = Firework { strength: 1000 };
-   40|       |
-   41|       |    Ok(())
+   33|      0|    }
+   34|      0|
+   35|      0|
+   36|      0|
+   37|      0|
+   38|      0|
+   39|      0|    let _ = Firework { strength: 1000 };
+   40|      0|
+   41|      0|    Ok(())
    42|      1|}
    43|       |
    44|       |// Expected program output:
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.loops_branches.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.loops_branches.txt
index 81d5c7d9034..5d572db7cc6 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.loops_branches.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.loops_branches.txt
@@ -9,23 +9,23 @@
     9|      1|    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
    10|      1|        if true {
    11|      1|            if false {
-   12|       |                while true {
-   13|       |                }
+   12|      0|                while true {
+   13|      0|                }
    14|      1|            }
-   15|      1|            write!(f, "error")?;
-                                            ^0
-   16|       |        } else {
-   17|       |        }
+   15|      1|            write!(f, "cool")?;
+                                           ^0
+   16|      0|        } else {
+   17|      0|        }
    18|       |
    19|     10|        for i in 0..10 {
    20|     10|            if true {
    21|     10|                if false {
-   22|       |                    while true {}
+   22|      0|                    while true {}
    23|     10|                }
-   24|     10|                write!(f, "error")?;
-                                                ^0
-   25|       |            } else {
-   26|       |            }
+   24|     10|                write!(f, "cool")?;
+                                               ^0
+   25|      0|            } else {
+   26|      0|            }
    27|       |        }
    28|      1|        Ok(())
    29|      1|    }
@@ -36,21 +36,21 @@
    34|       |impl std::fmt::Display for DisplayTest {
    35|      1|    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
    36|      1|        if false {
-   37|       |        } else {
+   37|      0|        } else {
    38|      1|            if false {
-   39|       |                while true {}
+   39|      0|                while true {}
    40|      1|            }
-   41|      1|            write!(f, "error")?;
-                                            ^0
+   41|      1|            write!(f, "cool")?;
+                                           ^0
    42|       |        }
    43|     10|        for i in 0..10 {
    44|     10|            if false {
-   45|       |            } else {
+   45|      0|            } else {
    46|     10|                if false {
-   47|       |                    while true {}
+   47|      0|                    while true {}
    48|     10|                }
-   49|     10|                write!(f, "error")?;
-                                                ^0
+   49|     10|                write!(f, "cool")?;
+                                               ^0
    50|       |            }
    51|       |        }
    52|      1|        Ok(())
diff --git a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.tight_inf_loop.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.tight_inf_loop.txt
index 5adeef7d085..2d4c57f451a 100644
--- a/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.tight_inf_loop.txt
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.tight_inf_loop.txt
@@ -1,6 +1,6 @@
     1|      1|fn main() {
     2|      1|    if false {
-    3|       |        loop {}
+    3|      0|        loop {}
     4|      1|    }
     5|      1|}
 
diff --git a/src/test/run-make-fulldeps/coverage/conditions.rs b/src/test/run-make-fulldeps/coverage/conditions.rs
index 8a2a0b53e58..057599d1b47 100644
--- a/src/test/run-make-fulldeps/coverage/conditions.rs
+++ b/src/test/run-make-fulldeps/coverage/conditions.rs
@@ -53,8 +53,8 @@ fn main() {
         } else {
             return;
         }
-    } // Note: closing brace shows uncovered (vs. `0` for implicit else) because condition literal
-      // `true` was const-evaluated. The compiler knows the `if` block will be executed.
+    }
+
 
     let mut countdown = 0;
     if true {
diff --git a/src/test/run-make-fulldeps/coverage/generics.rs b/src/test/run-make-fulldeps/coverage/generics.rs
index cbeda35d3b8..18b38868496 100644
--- a/src/test/run-make-fulldeps/coverage/generics.rs
+++ b/src/test/run-make-fulldeps/coverage/generics.rs
@@ -30,11 +30,11 @@ fn main() -> Result<(),u8> {
     if true {
         println!("Exiting with error...");
         return Err(1);
-    } // The remaining lines below have no coverage because `if true` (with the constant literal
-      // `true`) is guaranteed to execute the `then` block, which is also guaranteed to `return`.
-      // Thankfully, in the normal case, conditions are not guaranteed ahead of time, and as shown
-      // in other tests, the lines below would have coverage (which would show they had `0`
-      // executions, assuming the condition still evaluated to `true`).
+    }
+
+
+
+
 
     let _ = Firework { strength: 1000 };
 
diff --git a/src/test/run-make-fulldeps/coverage/loops_branches.rs b/src/test/run-make-fulldeps/coverage/loops_branches.rs
index 4d9bbad3367..7116ce47f4b 100644
--- a/src/test/run-make-fulldeps/coverage/loops_branches.rs
+++ b/src/test/run-make-fulldeps/coverage/loops_branches.rs
@@ -12,7 +12,7 @@ impl std::fmt::Debug for DebugTest {
                 while true {
                 }
             }
-            write!(f, "error")?;
+            write!(f, "cool")?;
         } else {
         }
 
@@ -21,7 +21,7 @@ impl std::fmt::Debug for DebugTest {
                 if false {
                     while true {}
                 }
-                write!(f, "error")?;
+                write!(f, "cool")?;
             } else {
             }
         }
@@ -38,7 +38,7 @@ impl std::fmt::Display for DisplayTest {
             if false {
                 while true {}
             }
-            write!(f, "error")?;
+            write!(f, "cool")?;
         }
         for i in 0..10 {
             if false {
@@ -46,7 +46,7 @@ impl std::fmt::Display for DisplayTest {
                 if false {
                     while true {}
                 }
-                write!(f, "error")?;
+                write!(f, "cool")?;
             }
         }
         Ok(())