Code coverage (#529)

Adds `make coverage` (cargo-llvm-cov over the full workspace including
revive-llvm-builder), a new "Code coverage" book chapter that briefly
explains how to run it, and a PR workflow that runs coverage and
comments on the PR (it updates the same comment if it already exists)
when the `measure code coverage` label is applied (see e.g. [this
comment](https://github.com/paritytech/revive/pull/529#issuecomment-4966136441)).

Building instrumented LLVM and generating an LLVM C++ coverage report is
also possible, see accompanying [PR description
#575](https://github.com/paritytech/revive/pull/575) for more details.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <bigcyrill@hotmail.com>
Co-authored-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Co-authored-by: xermicus <cyrill@parity.io>
Co-authored-by: elle-j <linnea@parity.io>
Co-authored-by: LJ <81748770+elle-j@users.noreply.github.com>

crates/llvm-builder/src/platforms/shared.rs

@@ -8,7 +8,8 @@ use std::path::Path;
 use std::process::Command;
 
 /// The build options shared by all platforms.
-pub const SHARED_BUILD_OPTS: [&str; 21] = [
+/// `LLVM_ENABLE_ZLIB` is emitted by [`shared_build_opts_coverage`].
+pub const SHARED_BUILD_OPTS: [&str; 20] = [
     "-DPACKAGE_VENDOR='Parity Technologies'",
     "-DCMAKE_BUILD_WITH_INSTALL_RPATH=1",
     "-DLLVM_BUILD_DOCS='Off'",
@@ -18,7 +19,6 @@ pub const SHARED_BUILD_OPTS: [&str; 21] = [
     "-DLLVM_ENABLE_DOXYGEN='Off'",
     "-DLLVM_ENABLE_SPHINX='Off'",
     "-DLLVM_ENABLE_OCAMLDOC='Off'",
-    "-DLLVM_ENABLE_ZLIB='Off'",
     "-DLLVM_ENABLE_ZSTD='Off'",
     "-DLLVM_ENABLE_LIBXML2='Off'",
     "-DLLVM_ENABLE_BINDINGS='Off'",
@@ -188,10 +188,18 @@ pub fn shared_build_opts_tests(enabled: bool) -> Vec<String> {
 
 /// The code coverage build options shared by all platforms.
 pub fn shared_build_opts_coverage(enabled: bool) -> Vec<String> {
-    vec![format!(
-        "-DLLVM_BUILD_INSTRUMENTED_COVERAGE='{}'",
-        if enabled { "On" } else { "Off" },
-    )]
+    vec![
+        format!(
+            "-DLLVM_BUILD_INSTRUMENTED_COVERAGE='{}'",
+            if enabled { "On" } else { "Off" },
+        ),
+        // zlib is required to be able to read the compressed profiles that
+        // instrumented resolc binaries emit.
+        format!(
+            "-DLLVM_ENABLE_ZLIB='{}'",
+            if enabled { "FORCE_ON" } else { "Off" },
+        ),
+    ]
 }
 
 /// Use of compiler cache (ccache) to speed up the build process.