Fix mismatched_lifetime_syntaxes suggestions for hidden path lifetimes

[reddevilmidzy]

close: #154493

r? shepmaster

cc: @chenyukang

[chenyukang]

it's better to keep only one the same lifetime is hidden here here?

[reddevilmidzy]

Ah okay, I missed that comment—thanks.

[reddevilmidzy]

Would you prefer showing the same lifetime is hidden here only once even in cases like this?

struct Foo<'a, 'b> {
    ptr1: &'a str,
    ptr2: &'b str,
}

fn missing(_: &str) -> (Foo, Foo) {
    //~^ ERROR: hiding a lifetime that's elided elsewhere is confusing
    todo!()
}

   |
LL | fn missing(_: &str) -> (Foo, Foo) {
   |               ^^^^      ^^^  ^^^
   |               |         |
   |               |         the same lifetime is hidden here
   |               the lifetime is elided here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
note: the lint level is defined here
  --> $DIR/hello.rs:2:9
   |
LL | #![deny(mismatched_lifetime_syntaxes)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use `'_` for type paths
   |
LL | fn missing(_: &str) -> (Foo<'_, '_>, Foo<'_, '_>) {
   |                            ++++++++     ++++++++

[chenyukang]

i'm neutral for this, since there are two struct type here.
it only seems weird for this:

it's perfect if we show as something like the same 2 lifetimes are hidden here?

[reddevilmidzy]

Ah, good. Then I will modify it to emit summarized message only within the same struct.

[chenyukang]

it's not valid to use span as key here, repeated metavariable expansions can share the same source span, which makes this merge distinct hidden-path occurrences and produces suggestions like Pair<'a, 'a, 'a, 'a> instead of Pair<'a, 'a>.

You can test it with this test case:

#![deny(mismatched_lifetime_syntaxes)]

struct Pair<'a, 'b>(&'a u8, &'b u8);

macro_rules! repeated {
    ($($pair:ident),+ ; $middle:ty) => {
        ($($pair),+, $middle, $($pair),+)
        //~^ ERROR hiding or eliding a lifetime that's named elsewhere is confusing
    };
}

fn repeated_macro<'a>(x: &'a u8) -> repeated!(Pair, Pair; &'_ u8) {
    (Pair(x, x), Pair(x, x), x, Pair(x, x), Pair(x, x))
}

fn main() {}

right fix should be:

macro_rules! repeated {
    ($($pair:ident),+ ; $middle:ty) => {
        ($($pair<'a, 'a>),+, $middle, $($pair<'a, 'a>),+)
        //~^ ERROR hiding or eliding a lifetime that's named elsewhere is confusing
    };
}

fn repeated_macro<'a>(x: &'a u8) -> repeated!(Pair, Pair; &'a u8) {
    (Pair(x, x), Pair(x, x), x, Pair(x, x), Pair(x, x))
}

[reddevilmidzy]

Changed Span to HirId!

[reddevilmidzy]

To keep it similar to the other code above, I made it output the same lifetimes are \ the same lifetime is.

[chenyukang]

Thanks!
@bors r=chenyukang

[rust-bors]

📌 Commit 8cef68a has been approved by chenyukang

It is now in the queue for this repository.