Remove ability for plugins to register a MIR pass

In recent months there have been a few different people investigating how to make a plugin that
registers a MIR-pass – one that isn’t intended to be eventually merged into rustc proper.

The interface to register MIR passes was added primarily for miri (& later was
found to make prototyping of rustc-proper MIR passes a tiny bit faster). Since miri does not use
this interface anymore it seems like a good time to remove this "feature".

For prototyping purposes a similar interface can be added by developers themselves in their custom
rustc build.

src/librustc_driver/driver.rs

@@ -604,7 +604,7 @@ pub fn phase_2_configure_and_expand<F>(sess: &Session,
 
     let whitelisted_legacy_custom_derives = registry.take_whitelisted_custom_derives();
     let Registry { syntax_exts, early_lint_passes, late_lint_passes, lint_groups,
-                   llvm_passes, attributes, mir_passes, .. } = registry;
+                   llvm_passes, attributes, .. } = registry;
 
     sess.track_errors(|| {
         let mut ls = sess.lint_store.borrow_mut();
@@ -620,7 +620,6 @@ pub fn phase_2_configure_and_expand<F>(sess: &Session,
         }
 
         *sess.plugin_llvm_passes.borrow_mut() = llvm_passes;
-        sess.mir_passes.borrow_mut().extend(mir_passes);
         *sess.plugin_attributes.borrow_mut() = attributes.clone();
     })?;
 

src/librustc_plugin/registry.rs

@@ -13,8 +13,6 @@
 use rustc::lint::{EarlyLintPassObject, LateLintPassObject, LintId, Lint};
 use rustc::session::Session;
 
-use rustc::mir::transform::MirMapPass;
-
 use syntax::ext::base::{SyntaxExtension, NamedSyntaxExtension, NormalTT, IdentTT};
 use syntax::ext::base::MacroExpanderFn;
 use syntax::symbol::Symbol;
@@ -53,9 +51,6 @@ pub struct Registry<'a> {
     #[doc(hidden)]
     pub late_lint_passes: Vec<LateLintPassObject>,
 
-    #[doc(hidden)]
-    pub mir_passes: Vec<Box<for<'pcx> MirMapPass<'pcx>>>,
-
     #[doc(hidden)]
     pub lint_groups: HashMap<&'static str, Vec<LintId>>,
 
@@ -81,7 +76,6 @@ impl<'a> Registry<'a> {
             lint_groups: HashMap::new(),
             llvm_passes: vec![],
             attributes: vec![],
-            mir_passes: Vec::new(),
             whitelisted_custom_derives: Vec::new(),
         }
     }
@@ -157,11 +151,6 @@ impl<'a> Registry<'a> {
         self.lint_groups.insert(name, to.into_iter().map(|x| LintId::of(x)).collect());
     }
 
-    /// Register a MIR pass
-    pub fn register_mir_pass(&mut self, pass: Box<for<'pcx> MirMapPass<'pcx>>) {
-        self.mir_passes.push(pass);
-    }
-
     /// Register an LLVM pass.
     ///
     /// Registration with LLVM itself is handled through static C++ objects with