TRPL: Requested: explanation of Lifetimes on `impl`

[mcast]

I think this is an omission from TRPL, in the description of either Lifetimes or impl - requesting because I think any new user would benefit from it.

Having got a limited understanding of struct, lifetime and impl I wanted to write

pub struct Foo<'a> { ... }
impl Foo<'a> {
  pub fn new<'a>(...) -> Foo<'a> { ...}
}

thinking that the 'a on impl Foo defines it for... whatever is using it later.

I got the answer (thanks user22207 / made mcast/markdown-chapterise@9fcff539) and concluded that in

impl<'a> Foo<'b> {   // does not compile

I am defining 'a for the impl block, then using that definition at 'b (which must be 'a here, to compile) to describe the Foo.

The sort of explanatory example that would make sense to me would be like

// does not compile
struct Foo<'a, 'b> {
  bar: &'c str,
  baz: &'d str,
}

impl<'e, 'f> Foo<'g, 'h> {
  fn mkfoo<'i>(arg: &'j str) -> Foo<'k> { ... }
}

in which I avoid repeating a lifetime label so it's easier to describe

• which define and which consume
• which could be elided & when, and what that means
• what the described lifetime constraints will mean when we set e.g. 'a == 'c == 'e and so on, to make it compile
• relationships to lifetimes in enclosing blocks, if there could be any