match expression falling through?

Question

Update: this is an old bug solved in 1.12

---

Here is some contrived but simple pattern matching example (demo):

fn main() {
    let x = 'a';

    match x {
            'a'...'b' if false => {
                println!("one");
            },

            'a' => {
                println!("two");
            },

            'a'...'b' => {
                println!("three");
            },

            _ => panic!("what?")
    }
}

When I run it, I get three as output. Why is this happening? Shouldn't the second branch match first?

Answer 1

Looking at the LLVM IR in Debug, it is already flawed, so this is definitely a bug in rustc; we'll use the curated IR below to check what's going on.

So, %x is assigned 'a' (97 in ASCII), and %10 is assigned the result of x >= 'a' and x <= 'b'; if this is true, we go to match_case, otherwise to compare_next. match_case redirect to cond, which redirects to case_body2 which prints "three".

In theory, we would have wanted to go to case_body1 (printing "two"), from cond7, from match_case4, from compare_next. But compare_next is only reached if x is not in ['a', 'b'] according to the IR.

This clearly looks like a bug.

; Function Attrs: uwtable
define internal void @_ZN4main20h4f7b0d7962de19d8eaaE() unnamed_addr #0 {
entry-block:
  %x = alloca i32
  ; [...]
  store i32 97, i32* %x
  %7 = load i32* %x, !range !0
  %8 = icmp uge i32 %7, 97
  %9 = icmp ule i32 %7, 98
  %10 = and i1 %8, %9
  br i1 %10, label %match_case, label %compare_next

case_body:                                        ; preds = %next6, %next
  ; println!("one")
  br label %case_body8

case_body1:                                       ; preds = %cond7
  ; println!("two")
  br label %case_body10

case_body2:                                       ; preds = %cond
  ; println!("three")
  br label %case_body15

case_body3:                                       ; preds = %match_else
  ; panic!("what")
  unreachable

match_else:                                       ; preds = %compare_next5
  br label %case_body3

match_case:                                       ; preds = %entry-block
  br i1 true, label %cond, label %next

compare_next:                                     ; preds = %entry-block
  %16 = icmp eq i32 %7, 97
  br i1 %16, label %match_case4, label %compare_next5

next:                                             ; preds = %match_case
  br label %case_body

cond:                                             ; preds = %match_case
  br label %case_body2

match_case4:                                      ; preds = %compare_next
  br i1 true, label %cond7, label %next6

; [...]

cond7:                                            ; preds = %match_case4
  br label %case_body1

; [...]
}