coverage: Add CLI support for -Zcoverage-options=condition

compiler/rustc_session/src/config.rs

@@ -159,7 +159,23 @@ pub enum CoverageLevel {
     Block,
     /// Also instrument branch points (includes block coverage).
     Branch,
-    /// Instrument for MC/DC. Mostly a superset of branch coverage, but might
+    /// Same as branch coverage, but also adds branch instrumentation for
+    /// certain boolean expressions that are not directly used for branching.
+    ///
+    /// For example, in the following code, `b` does not directly participate
+    /// in a branch, but condition coverage will instrument it as its own
+    /// artificial branch:
+    /// ```
+    /// # let (a, b) = (false, true);
+    /// let x = a && b;
+    /// //           ^ last operand
+    /// ```
+    ///
+    /// This level is mainly intended to be a stepping-stone towards full MC/DC
+    /// instrumentation, so it might be removed in the future when MC/DC is
+    /// sufficiently complete, or if it is making MC/DC changes difficult.
+    Condition,
+    /// Instrument for MC/DC. Mostly a superset of condition coverage, but might
     /// differ in some corner cases.
     Mcdc,
 }

compiler/rustc_session/src/options.rs

@@ -395,7 +395,7 @@ mod desc {
     pub const parse_optimization_fuel: &str = "crate=integer";
     pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str = parse_bool;
-    pub const parse_coverage_options: &str = "`block` | `branch` | `mcdc`";
+    pub const parse_coverage_options: &str = "`block` | `branch` | `condition` | `mcdc`";
     pub const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
     pub const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
@@ -961,6 +961,7 @@ mod parse {
             match option {
                 "block" => slot.level = CoverageLevel::Block,
                 "branch" => slot.level = CoverageLevel::Branch,
+                "condition" => slot.level = CoverageLevel::Condition,
                 "mcdc" => slot.level = CoverageLevel::Mcdc,
                 _ => return false,
             }

compiler/rustc_session/src/session.rs

@@ -353,6 +353,11 @@ impl Session {
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Branch
     }
 
+    pub fn instrument_coverage_condition(&self) -> bool {
+        self.instrument_coverage()
+            && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Condition
+    }
+
     pub fn instrument_coverage_mcdc(&self) -> bool {
         self.instrument_coverage()
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Mcdc