`Assert` MIR terminators are printed incorrectly

[jonas-schievink]

fn f(slice: &[u8]) -> u8 {
    (*slice)[0]
}

fn f(_1: &[u8]) -> u8 {
    debug slice => _1;                   // in scope 0 at src/main.rs:1:6: 1:11
    let mut _0: u8;                      // return place in scope 0 at src/main.rs:1:23: 1:25
    let _2: usize;                       // in scope 0 at src/main.rs:2:14: 2:15
    let mut _3: usize;                   // in scope 0 at src/main.rs:2:5: 2:16
    let mut _4: bool;                    // in scope 0 at src/main.rs:2:5: 2:16

    bb0: {
        StorageLive(_2);                 // bb0[0]: scope 0 at src/main.rs:2:14: 2:15
        _2 = const 0usize;               // bb0[1]: scope 0 at src/main.rs:2:14: 2:15
                                         // ty::Const
                                         // + ty: usize
                                         // + val: Value(Scalar(0x0000000000000000))
                                         // mir::Constant
                                         // + span: src/main.rs:2:14: 2:15
                                         // + literal: Const { ty: usize, val: Value(Scalar(0x0000000000000000)) }
        _3 = Len((*_1));                 // bb0[2]: scope 0 at src/main.rs:2:5: 2:16
        _4 = Lt(_2, _3);                 // bb0[3]: scope 0 at src/main.rs:2:5: 2:16
        assert(move _4, "index out of bounds: the len is move _3 but the index is _2") -> bb1; // bb0[4]: scope 0 at src/main.rs:2:5: 2:16
    }

    bb1: {
        _0 = (*_1)[_2];                  // bb1[0]: scope 0 at src/main.rs:2:5: 2:16
        StorageDead(_2);                 // bb1[1]: scope 0 at src/main.rs:3:1: 3:2
        return;                          // bb1[2]: scope 0 at src/main.rs:3:2: 3:2
    }
}

Note how it says "index out of bounds: the len is move _3 but the index is _2". The formatting arguments are inlined into the message.

It should either mirror the format! string syntax, or at least use braces around the arguments so that they don't look like they're part of the string itself.

This issue has been assigned to @robojumper via this comment.

[jonas-schievink]

The code for formatting the Assert terminator is here:

rust/src/librustc_middle/mir/mod.rs

Lines 1409 to 1417 in 1406186

	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	use AssertKind::*;
	match self {
	BoundsCheck { ref len, ref index } => {
	write!(f, "index out of bounds: the len is {:?} but the index is {:?}", len, index)
	}
	_ => write!(f, "{}", self.description()),
	}
	}

The printed message just needs to be changed so that the result looks closer to how the write! macro itself is invoked. Something like this maybe:

assert(move _4, "index out of bounds: the len is {} but the index is", move _3, _2)

[robojumper]

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