Use early unwraps instead of bubbling up errors just to unwrap in the end

src/librustc_codegen_ssa/mir/constant.rs

@@ -49,36 +49,36 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         constant: Result<ty::Const<'tcx>, ErrorHandled>,
     ) -> (Bx::Value, Ty<'tcx>) {
         constant
-            .and_then(|c| {
+            .map(|c| {
                 let field_ty = c.ty.builtin_index().unwrap();
                 let fields = match c.ty.sty {
                     ty::Array(_, n) => n.unwrap_usize(bx.tcx()),
                     ref other => bug!("invalid simd shuffle type: {}", other),
                 };
-                let values: Result<Vec<_>, ErrorHandled> = (0..fields).map(|field| {
+                let values: Vec<_> = (0..fields).map(|field| {
                     let field = const_field(
                         bx.tcx(),
                         ty::ParamEnv::reveal_all(),
                         None,
                         mir::Field::new(field as usize),
                         c,
-                    )?;
+                    );
                     if let Some(prim) = field.val.try_to_scalar() {
                         let layout = bx.layout_of(field_ty);
                         let scalar = match layout.abi {
                             layout::Abi::Scalar(ref x) => x,
                             _ => bug!("from_const: invalid ByVal layout: {:#?}", layout)
                         };
-                        Ok(bx.scalar_to_backend(
+                        bx.scalar_to_backend(
                             prim, scalar,
                             bx.immediate_backend_type(layout),
-                        ))
+                        )
                     } else {
                         bug!("simd shuffle field {:?}", field)
                     }
                 }).collect();
-                let llval = bx.const_struct(&values?, false);
-                Ok((llval, c.ty))
+                let llval = bx.const_struct(&values, false);
+                (llval, c.ty)
             })
             .unwrap_or_else(|_| {
                 bx.tcx().sess.span_err(

src/librustc_mir/const_eval.rs

@@ -466,45 +466,42 @@ impl<'a, 'mir, 'tcx> interpret::Machine<'a, 'mir, 'tcx>
 }
 
 /// Projects to a field of a (variant of a) const.
+// this function uses `unwrap` copiously, because an already validated constant must have valid
+// fields and can thus never fail outside of compiler bugs
 pub fn const_field<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     variant: Option<VariantIdx>,
     field: mir::Field,
     value: ty::Const<'tcx>,
-) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
+) -> ty::Const<'tcx> {
     trace!("const_field: {:?}, {:?}", field, value);
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
-    let result = (|| {
-        // get the operand again
-        let op = ecx.const_to_op(value, None)?;
-        // downcast
-        let down = match variant {
-            None => op,
-            Some(variant) => ecx.operand_downcast(op, variant)?
-        };
-        // then project
-        let field = ecx.operand_field(down, field.index() as u64)?;
-        // and finally move back to the const world, always normalizing because
-        // this is not called for statics.
-        op_to_const(&ecx, field)
-    })();
-    result.map_err(|error| {
-        let err = error_to_const_error(&ecx, error);
-        err.report_as_error(ecx.tcx, "could not access field of constant");
-        ErrorHandled::Reported
-    })
+    // get the operand again
+    let op = ecx.const_to_op(value, None).unwrap();
+    // downcast
+    let down = match variant {
+        None => op,
+        Some(variant) => ecx.operand_downcast(op, variant).unwrap(),
+    };
+    // then project
+    let field = ecx.operand_field(down, field.index() as u64).unwrap();
+    // and finally move back to the const world, always normalizing because
+    // this is not called for statics.
+    op_to_const(&ecx, field).unwrap()
 }
 
+// this function uses `unwrap` copiously, because an already validated constant must have valid
+// fields and can thus never fail outside of compiler bugs
 pub fn const_variant_index<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     val: ty::Const<'tcx>,
-) -> EvalResult<'tcx, VariantIdx> {
+) -> VariantIdx {
     trace!("const_variant_index: {:?}", val);
     let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env);
-    let op = ecx.const_to_op(val, None)?;
-    Ok(ecx.read_discriminant(op)?.1)
+    let op = ecx.const_to_op(val, None).unwrap();
+    ecx.read_discriminant(op).unwrap().1
 }
 
 pub fn error_to_const_error<'a, 'mir, 'tcx>(

src/librustc_mir/hair/pattern/_match.rs

@@ -440,13 +440,7 @@ impl<'tcx> Constructor<'tcx> {
                 assert!(!adt.is_enum());
                 VariantIdx::new(0)
             }
-            &ConstantValue(c) => {
-                crate::const_eval::const_variant_index(
-                    cx.tcx,
-                    cx.param_env,
-                    c,
-                ).unwrap()
-            },
+            &ConstantValue(c) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
             _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
         }
     }

src/librustc_mir/hair/pattern/mod.rs

@@ -937,10 +937,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
         debug!("const_to_pat: cv={:#?} id={:?}", cv, id);
         let adt_subpattern = |i, variant_opt| {
             let field = Field::new(i);
-            let val = const_field(
-                self.tcx, self.param_env,
-                variant_opt, field, cv,
-            ).expect("field access failed");
+            let val = const_field(self.tcx, self.param_env, variant_opt, field, cv);
             self.const_to_pat(instance, val, id, span)
         };
         let adt_subpatterns = |n, variant_opt| {
@@ -979,9 +976,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
                 PatternKind::Wild
             }
             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
-                let variant_index = const_variant_index(
-                    self.tcx, self.param_env, cv
-                ).expect("const_variant_index failed");
+                let variant_index = const_variant_index(self.tcx, self.param_env, cv);
                 let subpatterns = adt_subpatterns(
                     adt_def.variants[variant_index].fields.len(),
                     Some(variant_index),