Accommodate simple cases of arith-overflow in `rustc` related crates.

src/librustc_trans/trans/adt.rs

@@ -778,7 +778,9 @@ fn load_discr(bcx: Block, ity: IntType, ptr: ValueRef, min: Disr, max: Disr)
     assert!(bits <= 64);
     let  bits = bits as uint;
     let mask = (-1u64 >> (64 - bits)) as Disr;
-    if (max + 1) & mask == min & mask {
+    // For a (max) discr of -1, max will be `-1 as usize`, which overflows.
+    // However, that is fine here (it would still represent the full range),
+    if (max.wrapping_add(1)) & mask == min & mask {
         // i.e., if the range is everything.  The lo==hi case would be
         // rejected by the LLVM verifier (it would mean either an
         // empty set, which is impossible, or the entire range of the
@@ -787,7 +789,7 @@ fn load_discr(bcx: Block, ity: IntType, ptr: ValueRef, min: Disr, max: Disr)
     } else {
         // llvm::ConstantRange can deal with ranges that wrap around,
         // so an overflow on (max + 1) is fine.
-        LoadRangeAssert(bcx, ptr, min, (max+1), /* signed: */ True)
+        LoadRangeAssert(bcx, ptr, min, (max.wrapping_add(1)), /* signed: */ True)
     }
 }
 

src/librustc_typeck/astconv.rs

@@ -205,9 +205,9 @@ pub fn opt_ast_region_to_region<'tcx>(
 
                                 if len == 2 && i == 0 {
                                     m.push_str(" or ");
-                                } else if i == len - 2 {
+                                } else if i + 2 == len {
                                     m.push_str(", or ");
-                                } else if i != len - 1 {
+                                } else if i + 1 != len {
                                     m.push_str(", ");
                                 }
                             }

src/libsyntax/ext/deriving/encodable.rs

@@ -181,7 +181,6 @@ fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
         Struct(ref fields) => {
             let emit_struct_field = cx.ident_of("emit_struct_field");
             let mut stmts = Vec::new();
-            let last = fields.len() - 1;
             for (i, &FieldInfo {
                     name,
                     ref self_,
@@ -204,6 +203,7 @@ fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
                                                  lambda));
 
                 // last call doesn't need a try!
+                let last = fields.len() - 1;
                 let call = if i != last {
                     cx.expr_try(span, call)
                 } else {