Rework force-frame-pointer

This reworks the force-frame-pointer PR to explicitly only consider the
value of the flag if it is provided, and use a target default otherwise.

Something that was tried but not kept was renaming the flag to
`frame-pointer`, because for flag `frame-pointer=no`, there is no
guarante, that LLVM will elide *all* the frame pointers; oposite of what
the literal reading of the flag would suggest.

src/librustc/session/config.rs

@@ -1053,8 +1053,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
          2 = full debug info with variable and type information"),
     opt_level: Option<String> = (None, parse_opt_string, [TRACKED],
         "optimize with possible levels 0-3, s, or z"),
-    force_frame_pointers: bool = (false, parse_bool, [TRACKED],
-        "force frame pointers to be used"),
+    force_frame_pointers: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "force use of the frame pointers"),
     debug_assertions: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "explicitly enable the cfg(debug_assertions) directive"),
     inline_threshold: Option<usize> = (None, parse_opt_uint, [TRACKED],
@@ -2968,7 +2968,7 @@ mod tests {
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
         opts = reference.clone();
-        opts.cg.force_frame_pointers = true;
+        opts.cg.force_frame_pointers = Some(false);
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
         opts = reference.clone();

src/librustc/session/mod.rs

@@ -658,8 +658,11 @@ impl Session {
     }
 
     pub fn must_not_eliminate_frame_pointers(&self) -> bool {
-        self.opts.debuginfo != DebugInfoLevel::NoDebugInfo
-            || !self.target.target.options.eliminate_frame_pointer
+        if let Some(x) = self.opts.cg.force_frame_pointers {
+            x
+        } else {
+            !self.target.target.options.eliminate_frame_pointer
+        }
     }
 
     /// Returns the symbol name for the registrar function,

src/test/codegen/force-frame-pointers.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 //
-// compile-flags: -C no-prepopulate-passes -C force-frame-pointers
+// compile-flags: -C no-prepopulate-passes -C force-frame-pointers=y
 
 #![crate_type="lib"]