Rollup merge of rust-lang#59208 - kenta7777:reduce-code-repetition, r=oli-obk

Reduce a Code Repetition Related to Bit Operation

This PR is related to [rust-lang#49937](rust-lang#49937).
Should I do more commits about [`FIXME(49937)`](https://github.com/rust-lang/rust/search?q=FIXME%2849937%29&unscoped_q=FIXME%2849937%29) in this PR?

Author: Centril

src/librustc_mir/build/matches/simplify.rs

@@ -114,14 +114,12 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         (Some(('\u{0000}' as u128, '\u{10FFFF}' as u128, Size::from_bits(32))), 0)
                     }
                     ty::Int(ity) => {
-                        // FIXME(49937): refactor these bit manipulations into interpret.
                         let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
                         let max = truncate(u128::max_value(), size);
                         let bias = 1u128 << (size.bits() - 1);
                         (Some((0, max, size)), bias)
                     }
                     ty::Uint(uty) => {
-                        // FIXME(49937): refactor these bit manipulations into interpret.
                         let size = Integer::from_attr(&tcx, UnsignedInt(uty)).size();
                         let max = truncate(u128::max_value(), size);
                         (Some((0, max, size)), 0)

src/librustc_mir/hair/pattern/_match.rs

@@ -172,7 +172,7 @@ use rustc::ty::{self, subst::SubstsRef, Ty, TyCtxt, TypeFoldable, Const};
 use rustc::ty::layout::{Integer, IntegerExt, VariantIdx, Size};
 
 use rustc::mir::Field;
-use rustc::mir::interpret::{ConstValue, Scalar};
+use rustc::mir::interpret::{ConstValue, Scalar, truncate};
 use rustc::util::common::ErrorReported;
 
 use syntax::attr::{SignedInt, UnsignedInt};
@@ -678,16 +678,14 @@ fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
             ]
         }
         ty::Int(ity) => {
-            // FIXME(49937): refactor these bit manipulations into interpret.
             let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
             let min = 1u128 << (bits - 1);
-            let max = (1u128 << (bits - 1)) - 1;
+            let max = min - 1;
             vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included)]
         }
         ty::Uint(uty) => {
-            // FIXME(49937): refactor these bit manipulations into interpret.
-            let bits = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size().bits() as u128;
-            let max = !0u128 >> (128 - bits);
+            let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
+            let max = truncate(u128::max_value(), size);
             vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included)]
         }
         _ => {