Weird formatting of whitespace in between code and in-line comments

Summary

I tried to format this code:

fn foo() {
    [
        (Ty::U32, OperandKind::Reg, OperandKind::Immediate),  // i32, funcref, externref
        (Ty::U32, OperandKind::Slot, OperandKind::Immediate), // i32, f32, funcref, externref
        (Ty::U64, OperandKind::Reg, OperandKind::Slot),       // i32, i64, funcref, externref
        (Ty::U64, OperandKind::Reg, OperandKind::Immediate),  // i64
        (Ty::U64, OperandKind::Slot, OperandKind::Reg),       // i32, i64
        (Ty::U64, OperandKind::Slot, OperandKind::Slot),      // i32, i64, f32, f64, funcref, externref
        (Ty::U64, OperandKind::Slot, OperandKind::Immediate), // i64, f64
        (Ty::F32, OperandKind::Reg, OperandKind::Immediate),  // f32
        (Ty::F32, OperandKind::Reg, OperandKind::Slot),       // f32
        (Ty::F32, OperandKind::Slot, OperandKind::Reg),       // f32
        (Ty::F64, OperandKind::Reg, OperandKind::Slot),       // f64
        (Ty::F64, OperandKind::Slot, OperandKind::Reg),       // f64
    ];
}

Expected behavior

I expected to see this happen: trailing whitespace between code and in-line comments stays the same.

Actual behavior

Instead, this happened: whitespace between code and in-line comments is weirdly changed from 2 to 6 whitespaces.

fn foo() {
    [
        (Ty::U32, OperandKind::Reg, OperandKind::Immediate), // i32, funcref, externref
        (Ty::U32, OperandKind::Slot, OperandKind::Immediate), // i32, f32, funcref, externref
        (Ty::U64, OperandKind::Reg, OperandKind::Slot),      // i32, i64, funcref, externref
        (Ty::U64, OperandKind::Reg, OperandKind::Immediate), // i64
        (Ty::U64, OperandKind::Slot, OperandKind::Reg),      // i32, i64
        (Ty::U64, OperandKind::Slot, OperandKind::Slot), // i32, i64, f32, f64, funcref, externref
        (Ty::U64, OperandKind::Slot, OperandKind::Immediate), // i64, f64
        (Ty::F32, OperandKind::Reg, OperandKind::Immediate), // f32
        (Ty::F32, OperandKind::Reg, OperandKind::Slot),  // f32
        (Ty::F32, OperandKind::Slot, OperandKind::Reg),  // f32
        (Ty::F64, OperandKind::Reg, OperandKind::Slot),  // f64
        (Ty::F64, OperandKind::Slot, OperandKind::Reg),  // f64
    ];
}

Configuration

rustfmt.toml:

imports_granularity = "Crate"
imports_layout = "HorizontalVertical"
edition = "2021" # 2024 does not change anything
match_arm_leading_pipes = "Preserve"

rustfmt cli options used (if applicable):

cargo +nightly fmt

> cargo +nightly fmt --version --verbose
rustfmt 1.9.0-nightly (fb0a5a5a9c 2026-05-08)

Reproduce the formatting change with the provided Rust snippet, rustfmt.toml, and cargo +nightly fmt. Compare the expected and actual spacing around inline comments, then inspect rustfmt's comment and whitespace formatting behavior. Done means the behavior is understood and covered by a regression test that preserves the intended spacing.