Auto merge of rust-lang#912 - fitzgen:only-run-analyses-if-needed, r=emilio

Only run analyses when we are going to use their results

Currently, there are various analyses related to deriving various traits that we unconditionally run. However, if we aren't going to derive those traits in codegen, then computing whether or not we can derive the traits is wasteful.

r? @emilio

Author: bors-servo

src/ir/context.rs

@@ -1804,7 +1804,9 @@ impl<'ctx> BindgenContext<'ctx> {
     /// Compute whether we can derive debug.
     fn compute_cannot_derive_debug(&mut self) {
         assert!(self.cannot_derive_debug.is_none());
-        self.cannot_derive_debug = Some(analyze::<CannotDeriveDebug>(self));
+        if self.options.derive_debug {
+            self.cannot_derive_debug = Some(analyze::<CannotDeriveDebug>(self));
+        }
     }
 
     /// Look up whether the item with `id` can
@@ -1821,7 +1823,9 @@ impl<'ctx> BindgenContext<'ctx> {
     /// Compute whether we can derive default.
     fn compute_cannot_derive_default(&mut self) {
         assert!(self.cannot_derive_default.is_none());
-        self.cannot_derive_default = Some(analyze::<CannotDeriveDefault>(self));
+        if self.options.derive_default {
+            self.cannot_derive_default = Some(analyze::<CannotDeriveDefault>(self));
+        }
     }
 
     /// Look up whether the item with `id` can
@@ -1844,7 +1848,9 @@ impl<'ctx> BindgenContext<'ctx> {
     /// Compute whether we can derive hash.
     fn compute_cannot_derive_hash(&mut self) {
         assert!(self.cannot_derive_hash.is_none());
-        self.cannot_derive_hash = Some(analyze::<CannotDeriveHash>(self));
+        if self.options.derive_hash {
+            self.cannot_derive_hash = Some(analyze::<CannotDeriveHash>(self));
+        }
     }
 
     /// Look up whether the item with `id` can