Auto merge of #127234 - ZhuUx:inlined-expr, r=davidtwco,Zalathar

[Coverage][MCDC] Group mcdc tests and fix panic when generating mcdc code for inlined expressions.

### Changes

1. Group all mcdc tests to one directory.
2. Since mcdc instruments different mappings for boolean expressions with normal branch coverage as #125766 introduces, it would be better also trace branch coverage results in mcdc tests.
3. So far rustc does not call `CoverageInfoBuilderMethods::init_coverage` for inlined functions. As a result, it could panic if it tries to instrument mcdc statements for inlined functions due to uninitialized cond bitmaps. We can reproduce this issue by current nightly rustc and [the test](https://github.com/rust-lang/rust/pull/127234/files#diff-c81af6bf4869aa42f5c7334e3e86344475de362f673f54ce439ec75fcb5ac3e5) with flag `--release`. This patch fixes it.

1 files changed, 103 insertions, 0 deletions

diff --git a/tests/coverage/mcdc/if.rs b/tests/coverage/mcdc/if.rs
new file mode 100644
index 00000000000..d8e6b61a9d5
--- /dev/null
+++ b/tests/coverage/mcdc/if.rs
@@ -0,0 +1,103 @@
+#![feature(coverage_attribute)]
+//@ edition: 2021
+//@ min-llvm-version: 18
+//@ compile-flags: -Zcoverage-options=mcdc
+//@ llvm-cov-flags: --show-branches=count --show-mcdc
+
+fn mcdc_check_neither(a: bool, b: bool) {
+    if a && b {
+        say("a and b");
+    } else {
+        say("not both");
+    }
+}
+
+fn mcdc_check_a(a: bool, b: bool) {
+    if a && b {
+        say("a and b");
+    } else {
+        say("not both");
+    }
+}
+
+fn mcdc_check_b(a: bool, b: bool) {
+    if a && b {
+        say("a and b");
+    } else {
+        say("not both");
+    }
+}
+
+fn mcdc_check_both(a: bool, b: bool) {
+    if a && b {
+        say("a and b");
+    } else {
+        say("not both");
+    }
+}
+
+fn mcdc_check_tree_decision(a: bool, b: bool, c: bool) {
+    // This expression is intentionally written in a way
+    // where 100% branch coverage indicates 100% mcdc coverage.
+    if a && (b || c) {
+        say("pass");
+    } else {
+        say("reject");
+    }
+}
+
+fn mcdc_check_not_tree_decision(a: bool, b: bool, c: bool) {
+    // Contradict to `mcdc_check_tree_decision`,
+    // 100% branch coverage of this expression does not mean indicates 100% mcdc coverage.
+    if (a || b) && c {
+        say("pass");
+    } else {
+        say("reject");
+    }
+}
+
+fn mcdc_nested_if(a: bool, b: bool, c: bool) {
+    if a || b {
+        say("a or b");
+        if b && c {
+            say("b and c");
+        }
+    } else {
+        say("neither a nor b");
+    }
+}
+
+#[coverage(off)]
+fn main() {
+    mcdc_check_neither(false, false);
+    mcdc_check_neither(false, true);
+
+    mcdc_check_a(true, true);
+    mcdc_check_a(false, true);
+
+    mcdc_check_b(true, true);
+    mcdc_check_b(true, false);
+
+    mcdc_check_both(false, true);
+    mcdc_check_both(true, true);
+    mcdc_check_both(true, false);
+
+    mcdc_check_tree_decision(false, true, true);
+    mcdc_check_tree_decision(true, true, false);
+    mcdc_check_tree_decision(true, false, false);
+    mcdc_check_tree_decision(true, false, true);
+
+    mcdc_check_not_tree_decision(false, true, true);
+    mcdc_check_not_tree_decision(true, true, false);
+    mcdc_check_not_tree_decision(true, false, false);
+    mcdc_check_not_tree_decision(true, false, true);
+
+    mcdc_nested_if(true, false, true);
+    mcdc_nested_if(true, true, true);
+    mcdc_nested_if(true, true, false);
+}
+
+#[coverage(off)]
+fn say(message: &str) {
+    core::hint::black_box(message);
+}