Allow using -Z fewer-names=no to retain value names

Change `-Z fewer-names` into an optional boolean flag and allow using it
to either discard value names when true or retain them when false,
regardless of other settings.

compiler/rustc_interface/src/tests.rs

@@ -547,7 +547,7 @@ macro_rules! tracked {
     tracked!(debug_macros, true);
     tracked!(dep_info_omit_d_target, true);
     tracked!(dual_proc_macros, true);
-    tracked!(fewer_names, true);
+    tracked!(fewer_names, Some(true));
     tracked!(force_overflow_checks, Some(true));
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));

compiler/rustc_session/src/options.rs

@@ -900,7 +900,7 @@ fn parse_target_feature(slot: &mut String, v: Option<&str>) -> bool {
         "emits a future-incompatibility report for lints (RFC 2834)"),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
         "emit a section containing stack size metadata (default: no)"),
-    fewer_names: bool = (false, parse_bool, [TRACKED],
+    fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),
     force_overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -734,12 +734,15 @@ pub fn panic_strategy(&self) -> PanicStrategy {
         self.opts.cg.panic.unwrap_or(self.target.panic_strategy)
     }
     pub fn fewer_names(&self) -> bool {
-        let more_names = self.opts.output_types.contains_key(&OutputType::LlvmAssembly)
-            || self.opts.output_types.contains_key(&OutputType::Bitcode)
-            // AddressSanitizer and MemorySanitizer use alloca name when reporting an issue.
-            || self.opts.debugging_opts.sanitizer.intersects(SanitizerSet::ADDRESS | SanitizerSet::MEMORY);
-
-        self.opts.debugging_opts.fewer_names || !more_names
+        if let Some(fewer_names) = self.opts.debugging_opts.fewer_names {
+            fewer_names
+        } else {
+            let more_names = self.opts.output_types.contains_key(&OutputType::LlvmAssembly)
+                || self.opts.output_types.contains_key(&OutputType::Bitcode)
+                // AddressSanitizer and MemorySanitizer use alloca name when reporting an issue.
+                || self.opts.debugging_opts.sanitizer.intersects(SanitizerSet::ADDRESS | SanitizerSet::MEMORY);
+            !more_names
+        }
     }
 
     pub fn unstable_options(&self) -> bool {

src/test/codegen/fewer-names.rs

@@ -0,0 +1,20 @@
+// no-system-llvm
+// compile-flags: -Coverflow-checks=no -O
+// revisions: YES NO
+// [YES]compile-flags: -Zfewer-names=yes
+// [NO] compile-flags: -Zfewer-names=no
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn sum(x: u32, y: u32) -> u32 {
+// YES-LABEL: define i32 @sum(i32 %0, i32 %1)
+// YES-NEXT:    %3 = add i32 %1, %0
+// YES-NEXT:    ret i32 %3
+
+// NO-LABEL: define i32 @sum(i32 %x, i32 %y)
+// NO-NEXT:  start:
+// NO-NEXT:    %z = add i32 %y, %x
+// NO-NEXT:    ret i32 %z
+    let z = x + y;
+    z
+}