Reland - 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.

Note, this PR relands an earlier, reverted PR that failed when compiling
generators. The prior issues with generators has been resolved and a new
test was added to prevent future regressions.

Check out the resulting changes to test coverage of dead blocks in the
test coverage reports in this PR.

12 files changed, 116 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 322f5681b3f..dc06a485a8f 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|       |
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.generator.txt b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generator.txt
new file mode 100644
index 00000000000..0fb3808ff2e
--- /dev/null
+++ b/src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.generator.txt
@@ -0,0 +1,32 @@
+    1|       |#![feature(generators, generator_trait)]
+    2|       |
+    3|       |use std::ops::{Generator, GeneratorState};
+    4|       |use std::pin::Pin;
+    5|       |
+    6|       |// The following implementation of a function called from a `yield` statement
+    7|       |// (apparently requiring the Result and the `String` type or constructor)
+    8|       |// creates conditions where the `generator::StateTransform` MIR transform will
+    9|       |// drop all `Counter` `Coverage` statements from a MIR. `simplify.rs` has logic
+   10|       |// to handle this condition, and still report dead block coverage.
+   11|      1|fn get_u32(val: bool) -> Result<u32, String> {
+   12|      1|    if val { Ok(1) } else { Err(String::from("some error")) }
+                                          ^0
+   13|      1|}
+   14|       |
+   15|      1|fn main() {
+   16|      1|    let is_true = std::env::args().len() == 1;
+   17|      1|    let mut generator = || {
+   18|      1|        yield get_u32(is_true);
+   19|      1|        return "foo";
+   20|      1|    };
+   21|       |
+   22|      1|    match Pin::new(&mut generator).resume(()) {
+   23|      1|        GeneratorState::Yielded(Ok(1)) => {}
+   24|      0|        _ => panic!("unexpected return from resume"),
+   25|       |    }
+   26|      1|    match Pin::new(&mut generator).resume(()) {
+   27|      1|        GeneratorState::Complete("foo") => {}
+   28|      0|        _ => panic!("unexpected return from resume"),
+   29|       |    }
+   30|      1|}
+
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/generator.rs b/src/test/run-make-fulldeps/coverage/generator.rs
new file mode 100644
index 00000000000..4319991021e
--- /dev/null
+++ b/src/test/run-make-fulldeps/coverage/generator.rs
@@ -0,0 +1,30 @@
+#![feature(generators, generator_trait)]
+
+use std::ops::{Generator, GeneratorState};
+use std::pin::Pin;
+
+// The following implementation of a function called from a `yield` statement
+// (apparently requiring the Result and the `String` type or constructor)
+// creates conditions where the `generator::StateTransform` MIR transform will
+// drop all `Counter` `Coverage` statements from a MIR. `simplify.rs` has logic
+// to handle this condition, and still report dead block coverage.
+fn get_u32(val: bool) -> Result<u32, String> {
+    if val { Ok(1) } else { Err(String::from("some error")) }
+}
+
+fn main() {
+    let is_true = std::env::args().len() == 1;
+    let mut generator = || {
+        yield get_u32(is_true);
+        return "foo";
+    };
+
+    match Pin::new(&mut generator).resume(()) {
+        GeneratorState::Yielded(Ok(1)) => {}
+        _ => panic!("unexpected return from resume"),
+    }
+    match Pin::new(&mut generator).resume(()) {
+        GeneratorState::Complete("foo") => {}
+        _ => panic!("unexpected return from resume"),
+    }
+}
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(())