Add -C link-dead-code option r=alexcrichton

Turning gc-sections off improves code coverage based for tools which
use DWARF debugging information (like kcov). Otherwise dead code is
stripped and kcov returns a coverage percentage that doesn't reflect
reality.

src/librustc/session/config.rs

@@ -507,6 +507,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "system linker to link outputs with"),
     link_args: Option<Vec<String>> = (None, parse_opt_list,
         "extra arguments to pass to the linker (space separated)"),
+    link_dead_code: bool = (false, parse_bool,
+        "let the linker strip dead coded (turning it on can be used for code coverage)"),
     lto: bool = (false, parse_bool,
         "perform LLVM link-time optimizations"),
     target_cpu: Option<String> = (None, parse_opt_string,

src/librustc_trans/back/link.rs

@@ -976,7 +976,9 @@ fn link_args(cmd: &mut Linker,
 
     // Try to strip as much out of the generated object by removing unused
     // sections if possible. See more comments in linker.rs
-    cmd.gc_sections(dylib);
+    if !sess.opts.cg.link_dead_code {
+        cmd.gc_sections(dylib);
+    }
 
     let used_link_args = sess.cstore.used_link_args();
 

src/test/run-make/codegen-options-parsing/Makefile

@@ -22,3 +22,10 @@ all:
 	$(RUSTC) -C lto=foo dummy.rs 2>&1 | \
 		grep 'codegen option `lto` takes no value'
 	$(RUSTC) -C lto dummy.rs
+
+	# Should not link dead code...
+	$(RUSTC) -Z print-link-args dummy.rs 2>&1 | \
+		grep -e '--gc-sections\|-dead_strip\|/OPT:REF,ICF'
+	# ... unless you specifically ask to keep it
+	$(RUSTC) -Z print-link-args -C link-dead-code dummy.rs 2>&1 | \
+		(! grep -e '--gc-sections\|-dead_strip\|/OPT:REF,ICF')