Add coverage script

.gitignore

@@ -26,3 +26,5 @@ deltachat-ffi/html
 deltachat-ffi/xml
 
 .rsynclist
+
+coverage/

ci_scripts/coverage.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+set -eu
+
+if ! which grcov 2>/dev/null 1>&2; then
+    echo >&2 '`grcov` not found. Check README at https://github.com/mozilla/grcov for setup instructions.'
+    echo >&2 'Run `cargo install grcov` to build `grcov` from source.'
+    exit 1
+fi
+
+# Allow `-Z` flags without using nightly Rust.
+export RUSTC_BOOTSTRAP=1
+
+# We are using `-Zprofile` instead of source-based coverage [1]
+# (`-Zinstrument-coverage`) due to a bug resulting in empty reports [2].
+#
+# [1] https://blog.rust-lang.org/inside-rust/2020/11/12/source-based-code-coverage.html
+# [2] https://github.com/mozilla/grcov/issues/595
+
+export CARGO_INCREMENTAL=0
+export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
+export RUSTDOCFLAGS="-Cpanic=abort"
+cargo clean
+cargo build
+cargo test
+
+grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing -o ./coverage/