move error allocation test to error.rs

Author: RalfJung

src/librustc/mir/interpret/error.rs

@@ -354,7 +354,7 @@ pub enum UnsupportedOpInfo<'tcx> {
     Unsupported(String),
 
     /// When const-prop encounters a situation it does not support, it raises this error.
-    /// This must not allocate for performance reasons.
+    /// This must not allocate for performance reasons (hence `str`, not `String`).
     ConstPropUnsupported(&'tcx str),
 
     // -- Everything below is not categorized yet --
@@ -612,3 +612,19 @@ impl fmt::Debug for InterpError<'_> {
         }
     }
 }
+
+impl InterpError<'_> {
+    /// Some errors allocate to be created as they contain free-from strings.
+    /// And sometiems we want to be sure that did not happen as it is a
+    /// waste of resources.
+    pub fn allocates(&self) -> bool {
+        match self {
+            InterpError::MachineStop(_)
+            | InterpError::Unsupported(UnsupportedOpInfo::Unsupported(_))
+            | InterpError::Unsupported(UnsupportedOpInfo::ValidationFailure(_))
+            | InterpError::UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
+            | InterpError::UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_)) => true,
+            _ => false,
+        }
+    }
+}

src/librustc_mir/transform/const_prop.rs

@@ -404,32 +404,15 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         let r = match f(self) {
             Ok(val) => Some(val),
             Err(error) => {
-                use rustc::mir::interpret::{
-                    InterpError::*, UndefinedBehaviorInfo, UnsupportedOpInfo,
-                };
-                match error.kind {
-                    MachineStop(_) => bug!("ConstProp does not stop"),
-
-                    // Some error shouldn't come up because creating them causes
-                    // an allocation, which we should avoid. When that happens,
-                    // dedicated error variants should be introduced instead.
-                    // Only test this in debug builds though to avoid disruptions.
-                    Unsupported(UnsupportedOpInfo::Unsupported(_))
-                    | Unsupported(UnsupportedOpInfo::ValidationFailure(_))
-                    | UndefinedBehavior(UndefinedBehaviorInfo::Ub(_))
-                    | UndefinedBehavior(UndefinedBehaviorInfo::UbExperimental(_))
-                        if cfg!(debug_assertions) =>
-                    {
-                        bug!("const-prop encountered allocating error: {:?}", error.kind);
-                    }
-
-                    Unsupported(_)
-                    | UndefinedBehavior(_)
-                    | InvalidProgram(_)
-                    | ResourceExhaustion(_) => {
-                        // Ignore these errors.
-                    }
-                }
+                // Some errors shouldn't come up because creating them causes
+                // an allocation, which we should avoid. When that happens,
+                // dedicated error variants should be introduced instead.
+                // Only test this in debug builds though to avoid disruptions.
+                debug_assert!(
+                    !error.kind.allocates(),
+                    "const-prop encountered allocating error: {}",
+                    error
+                );
                 None
             }
         };