为何std::ops::Shl::shl与<<不等？mutagen移位操作类型推断失败求助

Great question! The discrepancy boils down to how Rust handles type inference for direct trait method calls versus operator syntax, plus some subtle rules around integer literals and trait resolution. Let’s break this down clearly:

First, let’s unpack your test case #

Your code:

use std::ops::Shl;
fn main() {
    // Errors with: cannot multiply i32 to u32
    println!("{}", 1u32.shl(2) * 3);
}

If you swap the method call for the << operator (and adjust for precedence if needed), it would work as expected:

fn main() {
    // Works fine
    println!("{}", (1u32 << 2) * 3u32);
}

1. Type inference: Method calls vs. operators #

The core difference is in how Rust infers types for these two scenarios:

• Direct method call (1u32.shl(2)):
  Rust uses left-to-right type inference here. First, it infers the literal 2 as its default type: i32. It then looks for a Shl<i32> implementation for u32 (which exists in the standard library) and returns a u32 result. When you try to multiply this u32 by the i32 literal 3, Rust throws an error—since it doesn’t allow implicit coercion between integer types for arithmetic operations.

• Operator syntax (1u32 << 2):
  Operators get special treatment with bidirectional type inference. Rust doesn’t just lock in the literal’s default type; it considers the entire expression’s context to find a valid trait implementation. In this case, it sees the result will be used in a multiplication, so it automatically coerces the i32 literal 2 to u32 (matching the left-hand side). This makes the result of 1u32 << 2 a u32, which can multiply with 3u32 without issue.

2. Operator precedence (a common gotcha) #

Another thing to note: * has higher precedence than <<. So 1u32 << 2 * 3 is parsed as 1u32 << (2 * 3), not (1u32 << 2) * 3. If you tested the operator version without parentheses, you might not have hit the same multiplication conflict as your method call example—making it seem like the operator "works" when the method call doesn’t.

Fixing your mutagen scenario #

To resolve the type inference issue with mutagen’s shift replacement, you have a few options:

• Explicitly cast the right-hand side to match the left operand’s type:

  ::mutagen::ShlShift::shl(a, b as u32, mutation_count)
  

• Use turbofish syntax to specify the expected Rhs type for the trait:

  ::mutagen::ShlShift::shl::<u32>(a, b, mutation_count)
  

• Adjust your mutagen trait definition to mirror the standard library’s flexible Shl trait, which allows a separate Rhs type instead of tying it to Self:

  pub trait ShlShift<Rhs = Self> {
      type Output;
      fn shl(self, rhs: Rhs, mutation_count: usize) -> Self::Output;
  }
  

内容的提问来源于stack exchange，提问作者llogiq