Tweak parse_check_cfg.

Instead of constructing the `CheckCfg` up front and then modifying it,
construct the three components and then put them together at the end.

This change resulted in the compiler noticing that one
`exhaustive_names` assignment was dead, so I commented that one out.

Author: nnethercote

compiler/rustc_interface/src/interface.rs

@@ -132,9 +132,9 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
     rustc_span::create_default_session_if_not_set_then(move |_| {
         // If any --check-cfg is passed then exhaustive_values and exhaustive_names
         // are enabled by default.
-        let exhaustive_names = !specs.is_empty();
-        let exhaustive_values = !specs.is_empty();
-        let mut check_cfg = CheckCfg { exhaustive_names, exhaustive_values, ..CheckCfg::default() };
+        let mut exhaustive_names = !specs.is_empty();
+        let mut exhaustive_values = !specs.is_empty();
+        let mut expecteds = FxHashMap::default();
 
         let mut old_syntax = None;
         for s in specs {
@@ -162,17 +162,16 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
                             if meta_item.has_name(sym::names) {
                                 // defaults are flipped for the old syntax
                                 if old_syntax == None {
-                                    check_cfg.exhaustive_names = false;
-                                    check_cfg.exhaustive_values = false;
+                                    //exhaustive_names = false; // overwritten immediately below
+                                    exhaustive_values = false;
                                 }
                                 old_syntax = Some(true);
 
-                                check_cfg.exhaustive_names = true;
+                                exhaustive_names = true;
                                 for arg in args {
                                     if arg.is_word() && arg.ident().is_some() {
                                         let ident = arg.ident().expect("multi-segment cfg key");
-                                        check_cfg
-                                            .expecteds
+                                        expecteds
                                             .entry(ident.name.to_string())
                                             .or_insert(ExpectedValues::Any);
                                     } else {
@@ -182,16 +181,15 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
                             } else if meta_item.has_name(sym::values) {
                                 // defaults are flipped for the old syntax
                                 if old_syntax == None {
-                                    check_cfg.exhaustive_names = false;
-                                    check_cfg.exhaustive_values = false;
+                                    exhaustive_names = false;
+                                    exhaustive_values = false;
                                 }
                                 old_syntax = Some(true);
 
                                 if let Some((name, values)) = args.split_first() {
                                     if name.is_word() && name.ident().is_some() {
                                         let ident = name.ident().expect("multi-segment cfg key");
-                                        let expected_values = check_cfg
-                                            .expecteds
+                                        let expected_values = expecteds
                                             .entry(ident.name.to_string())
                                             .and_modify(|expected_values| match expected_values {
                                                 ExpectedValues::Some(_) => {}
@@ -232,7 +230,7 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
                                         );
                                     }
                                 } else if args.is_empty() {
-                                    check_cfg.exhaustive_values = true;
+                                    exhaustive_values = true;
                                 } else {
                                     expected_error();
                                 }
@@ -322,11 +320,10 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
                                         error!("`cfg(any())` can only be provided in isolation");
                                     }
 
-                                    check_cfg.exhaustive_names = false;
+                                    exhaustive_names = false;
                                 } else {
                                     for name in names {
-                                        check_cfg
-                                            .expecteds
+                                        expecteds
                                             .entry(name.to_string())
                                             .and_modify(|v| match v {
                                                 ExpectedValues::Some(v)
@@ -366,7 +363,7 @@ pub fn parse_check_cfg(handler: &EarlyErrorHandler, specs: Vec<String>) -> Check
             }
         }
 
-        check_cfg
+        CheckCfg { exhaustive_names, exhaustive_values, expecteds }
     })
 }