Update rustc option for instrument-coverage (informalsystems#1377)

* chore: update rustc flag for instrument-coverage

In nightly, the compiler flag graduated from -Z to
-C (stable options).

* CI: link dead code for coverage

Throw in -C link-dead-code into the compiler flags for the
nightly-coverage CI job.

* Fix prelude imports

Remove imports that are reported as unused by the nightly compiler,
override the lint for the vec macro import that is actually used.

* Restore vec macro import in prelude

Override the lint more selectively, specifically for this macro
re-export.

.github/workflows/test.yml

@@ -110,7 +110,7 @@ jobs:
           components: llvm-tools-preview
       - run: cargo test-all-features
         env:
-          RUSTFLAGS: '-Zinstrument-coverage'
+          RUSTFLAGS: '-C instrument-coverage -C link-dead-code'
           LLVM_PROFILE_FILE: '%p-%m.profraw'
       - name: Install grcov
         run: |

config/src/prelude.rs

@@ -2,7 +2,6 @@
 // https://doc.rust-lang.org/src/alloc/prelude/v1.rs.html
 pub use alloc::{
     borrow::ToOwned,
-    boxed::Box,
     format,
     string::{String, ToString},
     vec,

light-client-verifier/src/prelude.rs

@@ -1,13 +1,10 @@
 // Re-export according to alloc::prelude::v1 because it is not yet stabilized
 // https://doc.rust-lang.org/src/alloc/prelude/v1.rs.html
+#[allow(unused_imports)]
+pub use alloc::vec;
 pub use alloc::{
-    borrow::ToOwned,
     boxed::Box,
-    format,
     string::{String, ToString},
-    vec,
     vec::Vec,
 };
-// will be included in 2021 edition.
-pub use core::convert::{TryFrom, TryInto};
 pub use core::prelude::v1::*;

proto/src/prelude.rs

@@ -1,11 +1,12 @@
 // Re-export according to alloc::prelude::v1 because it is not yet stabilized
 // https://doc.rust-lang.org/src/alloc/prelude/v1.rs.html
+
+#[allow(unused_imports)]
+pub use alloc::vec;
 pub use alloc::{
     borrow::ToOwned,
-    boxed::Box,
     format,
     string::{String, ToString},
-    vec,
     vec::Vec,
 };
 pub use core::prelude::v1::*;