Do not generate assumes for plain integer casts

src/librustc_codegen_llvm/mir/rvalue.rs

@@ -304,7 +304,9 @@ pub fn codegen_rvalue_operand(&mut self,
                                 // then `i1 1` (i.e. E::B) is effectively `i8 -1`.
                                 signed = !scalar.is_bool() && s;
 
-                                if scalar.valid_range.end() > scalar.valid_range.start() {
+                                let er = scalar.valid_range_exclusive(bx.cx);
+                                if er.end != er.start &&
+                                   scalar.valid_range.end() > scalar.valid_range.start() {
                                     // We want `table[e as usize]` to not
                                     // have bound checks, and this is the most
                                     // convenient place to put the `assume`.

src/librustc_target/abi/mod.rs

@@ -596,7 +596,16 @@ pub struct Scalar {
     pub value: Primitive,
 
     /// Inclusive wrap-around range of valid values, that is, if
-    /// min > max, it represents min..=u128::MAX followed by 0..=max.
+    /// start > end, it represents `start..=max_value()`,
+    /// followed by `0..=end`.
+    ///
+    /// That is, for an i8 primitive, a range of `254..=2` means following
+    /// sequence:
+    ///
+    ///    254 (-2), 255 (-1), 0, 1, 2
+    ///
+    /// This is intended specifically to mirror LLVM’s `!range` metadata,
+    /// semantics.
     // FIXME(eddyb) always use the shortest range, e.g. by finding
     // the largest space between two consecutive valid values and
     // taking everything else as the (shortest) valid range.

src/test/codegen/no-assumes-on-casts.rs

@@ -0,0 +1,29 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type = "lib"]
+
+// compile-flags: -Cno-prepopulate-passes
+
+// CHECK-LABEL: fna
+#[no_mangle]
+pub fn fna(a: i16) -> i32 {
+    a as i32
+// CHECK-NOT: assume
+// CHECK: sext
+}
+
+// CHECK-LABEL: fnb
+#[no_mangle]
+pub fn fnb(a: u16) -> u32 {
+    a as u32
+// CHECK-NOT: assume
+// CHECK: zext
+}