rust - 尝试为结构设置带有生存期的返回值时，由于需求冲突，因此无法为autoref推断适当的生存期

Question

pub struct SumAggregator<'ctx> {
    input: i32,
    state: Cell<Option<IntValue<'ctx>>>,
}

impl<'ctx> SumAggregator<'ctx> {
    pub fn new() -> SumAggregator<'ctx> {
        SumAggregator {
            input: 0,
            state: Cell::new(None),
        }
    }
}

impl<'ctx> Aggregator for SumAggregator<'ctx> {
    fn init(&self, generator: &Generator, layout: &mut Layout, idx: i32) {
        let col_type = generator.context.i32_type();
        self.state.set(Some(col_type.const_int(0, true)));
        generator.build_debug("initialized state value:", self.state.get().unwrap().as_basic_value_enum());
    }

    fn process(&self, val: IntValue) {
        unimplemented!()
    }
}

上面是引发此错误的核心代码。 col_type.const_int(0, true)可以返回带有'ctx生命周期的 IntValue。当我尝试将此值设置为我的结构时，发生此错误。我是Rust的新手。据我所知，只有引用可能会导致生命周期问题。但是，在我的用例中，我只想放置一个值而不是对struct的引用(即使该值具有生存期)。
这是错误堆栈:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/operator/groupby/mod.rs:40:42
   |
40 |         let col_type = generator.context.i32_type();
   |                                          ^^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #3 defined on the method body at 37:5...
  --> src/operator/groupby/mod.rs:37:5
   |
37 |     fn init(&self, generator: &Generator, layout: &mut Layout, idx: i32) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
  --> src/operator/groupby/mod.rs:40:24
   |
40 |         let col_type = generator.context.i32_type();
   |                        ^^^^^^^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'ctx` as defined on the impl at 36:6...
  --> src/operator/groupby/mod.rs:36:6
   |
36 | impl<'ctx> Aggregator for SumAggregator<'ctx> {
   |      ^^^^
note: ...so that the expression is assignable
  --> src/operator/groupby/mod.rs:41:24
   |
41 |         self.state.set(Some(col_type.const_int(0, true)));
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected `Option<inkwell::values::IntValue<'ctx>>`
              found `Option<inkwell::values::IntValue<'_>>`

Answer 1

问题在于SumAggregator希望拥有一个至少与自己生命周期一样长的IntValue。但是，在init函数中，您尝试为它提供一个IntValue，它仅在init函数结束之前有效。
您可以通过指定IntValue的引用的生存期来确保Generator的生存期足够长:

impl<'ctx> Aggregator for SumAggregator<'ctx> {
    fn init(&self, generator: &'ctx Generator, layout: &mut Layout, idx: i32) {
        // col_type is &'ctx IntType
        let col_type = generator.context.i32_type();

        // int_value is &'ctx IntValue
        let int_value = col_type.const_int(0, true)

        // putting &'ctx IntValue into &'ctx IntValue
        self.state.set(Some(int_value));

        generator.build_debug("initialized state value:", self.state.get().unwrap().as_basic_value_enum());
    }

    ...
}

如果不这样做，它与以下内容相同:

impl<'ctx> Aggregator for SumAggregator<'ctx> {
    fn init<'a, 'b>(&self, generator: &'a Generator, layout: &'b mut Layout, idx: i32) {
        // col_type is &'a IntType
        let col_type = generator.context.i32_type();

        // int_value is &'a IntValue
        let int_value = col_type.const_int(0, true)

        // trying to put &'a IntValue into &'ctx IntValue
        self.state.set(Some(int_value));

        generator.build_debug("initialized state value:", self.state.get().unwrap().as_basic_value_enum());
    }

    ...
}