lifetime constraints cross-interfere between method definitions

[dwrensha]

$ rustc --version
rustc 0.13.0-nightly (c46812f65 2014-10-19 00:47:18 +0000)

I expect this program to compile successfully:

// lifetimes.rs

pub struct ListBuilder<'a> {
    data : &'a [u8]
}

pub struct StructBuilder<'a> {
    data : &'a [u8]
}

impl <'a> ListBuilder <'a> {
    pub fn get_struct_element(&self, index : uint) -> StructBuilder<'a> {
        StructBuilder { data: self.data.slice(index, index + 4) }
    }
}

pub trait FromStructBuilder<'a> {
    fn new(structBuilder : StructBuilder<'a>) -> Self;
}

pub struct ConcreteListBuilder<'a, T> {
    pub builder : ListBuilder<'a>
}

impl <'a, T : FromStructBuilder<'a>> ConcreteListBuilder<'a, T> {
    pub fn get(&self, index : uint) -> T {
        FromStructBuilder::new(self.builder.get_struct_element(index))
    }
}

// Compilation succeeds if I comment out this impl.
impl <'a, T : FromStructBuilder<'a>> ConcreteListBuilder<'a, T> {
    pub fn get2(&self, index : uint) -> T {
        FromStructBuilder::new(self.builder.get_struct_element(index))
    }
}

pub fn main () {}

Instead, it gives me this error:

$ rustc lifetimes.rs 
lifetimes.rs:34:32: 34:44 error: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
lifetimes.rs:34         FromStructBuilder::new(self.builder.get_struct_element(index))
                                               ^~~~~~~~~~~~
lifetimes.rs:33:43: 35:6 note: first, the lifetime cannot outlive the lifetime 'a as defined on the block at 33:42...
lifetimes.rs:33     pub fn get2(&self, index : uint) -> T {
lifetimes.rs:34         FromStructBuilder::new(self.builder.get_struct_element(index))
lifetimes.rs:35     }
lifetimes.rs:34:32: 34:44 note: ...so that types are compatible (expected `&ListBuilder<'_>`, found `&ListBuilder<'a>`)
lifetimes.rs:34         FromStructBuilder::new(self.builder.get_struct_element(index))
                                               ^~~~~~~~~~~~
lifetimes.rs:26:42: 28:6 note: but, the lifetime must be valid for the lifetime 'a as defined on the block at 26:41...
lifetimes.rs:26     pub fn get(&self, index : uint) -> T {
lifetimes.rs:27         FromStructBuilder::new(self.builder.get_struct_element(index))
lifetimes.rs:28     }
lifetimes.rs:34:9: 34:31 note: ...so that trait type parameters matches those specified on the impl (expected `FromStructBuilder<'_>`, found `FromStructBuilder<'a>`)
lifetimes.rs:34         FromStructBuilder::new(self.builder.get_struct_element(index))
                        ^~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

The program successfully compiles if I comment out the definition of the get2() method.

This kind of thing used to work; e.g. the program compiles successfully at the commit 9b80efd.

cc @nikomatsakis

[dwrensha]

Fixed via #18235.