add option to enable "careful mode" inspired by cargo-careful

* based on https://github.com/RalfJung/cargo-careful/
* disables default `-Zbuild-std` from the previous PR on `build-std`

src/options.rs

@@ -88,7 +88,13 @@ pub struct BuildOptions {
     /// Currently this conflicts with coverage instrumentation but -Zbuild-std enables detecting
     /// more bugs so this option defaults to true, but when using `cargo fuzz coverage` it
     /// defaults to false.
-    pub build_std: Option<bool>,
+    pub build_std: bool,
+
+    #[arg(short, long = "careful")]
+    /// enable "careful" mode: inspired by https://github.com/RalfJung/cargo-careful, this enables
+    /// building the fuzzing harness along with the standard library (implies --build-std) with
+    /// debug assertions and extra const UB and init checks.
+    pub careful_mode: bool,
 
     #[arg(long = "target", default_value(crate::utils::default_target()))]
     /// Target triple of the fuzz target
@@ -217,7 +223,8 @@ mod test {
             no_default_features: false,
             all_features: false,
             features: None,
-            build_std: None,
+            build_std: false,
+            careful_mode: false,
             sanitizer: Sanitizer::Address,
             triple: String::from(crate::utils::default_target()),
             unstable_flags: Vec::new(),

src/options/coverage.rs

@@ -27,7 +27,7 @@ pub struct Coverage {
 
 impl RunCommand for Coverage {
     fn run_command(&mut self) -> Result<()> {
-        if self.build.build_std.unwrap_or(false) {
+        if self.build.build_std {
             bail!(
                 "-Zbuild-std is currently incompatible with -Zinstrument-coverage, \
                 see https://github.com/rust-lang/wg-cargo-std-aware/issues/63"

src/project.rs

@@ -156,9 +156,10 @@ impl FuzzProject {
         for flag in &build.unstable_flags {
             cmd.arg("-Z").arg(flag);
         }
-        if let Sanitizer::Memory = build.sanitizer {
-            cmd.arg("-Z").arg("build-std");
-        } else if build.build_std.unwrap_or(true) && !build.coverage {
+
+        if (matches!(build.sanitizer, Sanitizer::Memory) || build.build_std || build.careful_mode)
+            && !build.coverage
+        {
             cmd.arg("-Z").arg("build-std");
         }
 
@@ -196,10 +197,14 @@ impl FuzzProject {
                 sanitizer = build.sanitizer
             )),
         }
+
+        if build.careful_mode {
+            rustflags.push_str(" -Zextra-const-ub-checks -Zstrict-init-checks --cfg careful");
+        }
         if build.triple.contains("-linux-") {
             rustflags.push_str(" -Cllvm-args=-sanitizer-coverage-stack-depth");
         }
-        if !build.release || build.debug_assertions {
+        if !build.release || build.debug_assertions || build.careful_mode {
             rustflags.push_str(" -Cdebug-assertions");
         }
         if build.triple.contains("-msvc") {