Auto merge of #114843 - Zalathar:test-coverage-map, r=oli-obk

coverage: Explicitly test the coverage maps produced by codegen/LLVM

Our existing coverage tests verify the output of end-to-end coverage reports, but we don't have any way to test the specific mapping information (code regions and their associated counters) that are emitted by `rustc_codegen_llvm` and LLVM. That makes it harder to to be confident in changes that would modify those mappings (whether deliberately or accidentally).

This PR addresses that by adding a new `coverage-map` test suite that does the following:
- Compiles test files to LLVM IR assembly (`.ll`)
- Feeds those IR files to a custom tool (`src/tools/coverage-dump`) that extracts and decodes coverage mappings, and prints them in a more human-readable format
- Checks the output of that tool against known-good snapshots

---

I recommend excluding the last commit while reviewing the main changes, because that last commit is just ~40 test files copied over from `tests/run-coverage`, plus their blessed coverage-map snapshots and a readme file. Those snapshots aren't really intended to be checked by hand; they're mostly there to increase the chances that an unintended change to coverage maps will be observable (even if it requires relatively specific circumstances to manifest).

1 files changed, 41 insertions, 0 deletions

diff --git a/tests/coverage-map/status-quo/unused.rs b/tests/coverage-map/status-quo/unused.rs
new file mode 100644
index 00000000000..d985af13547
--- /dev/null
+++ b/tests/coverage-map/status-quo/unused.rs
@@ -0,0 +1,41 @@
+#![allow(dead_code, unused_assignments, unused_must_use, unused_variables)]
+
+fn foo<T>(x: T) {
+    let mut i = 0;
+    while i < 10 {
+        i != 0 || i != 0;
+        i += 1;
+    }
+}
+
+fn unused_template_func<T>(x: T) {
+    let mut i = 0;
+    while i < 10 {
+        i != 0 || i != 0;
+        i += 1;
+    }
+}
+
+fn unused_func(mut a: u32) {
+    if a != 0 {
+        a += 1;
+    }
+}
+
+fn unused_func2(mut a: u32) {
+    if a != 0 {
+        a += 1;
+    }
+}
+
+fn unused_func3(mut a: u32) {
+    if a != 0 {
+        a += 1;
+    }
+}
+
+fn main() -> Result<(), u8> {
+    foo::<u32>(0);
+    foo::<f32>(0.0);
+    Ok(())
+}