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mark-i-m opened this issue Dec 2, 2018 · 1 comment
Open

Unhelpful error message when multiple lifetimes have the same name #56423

mark-i-m opened this issue Dec 2, 2018 · 1 comment
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A-diagnostics Area: Messages for errors, warnings, and lints A-lifetimes Area: Lifetimes / regions C-enhancement Category: An issue proposing an enhancement or a PR with one. D-confusing Diagnostics: Confusing error or lint that should be reworked. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@mark-i-m
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mark-i-m commented Dec 2, 2018

I was just looking for some lifetime errors for a project, and I came across the following example:

https://stackoverflow.com/questions/24847331/rust-lifetime-error-expected-concrete-lifetime-but-found-bound-lifetime

The solution in that thread is correct, but the error message is somewhat unhelpful with today's stable and nightly compilers:

error[E0308]: method not compatible with trait
  --> src/l3.rs:19:5
   |
19 |     fn to_c(&self, r: &'a Ref) -> Container<'a> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected type `fn(&l3::ContainerB<'a>, &'a l3::Ref) -> l3::Container<'a>`
              found type `fn(&l3::ContainerB<'a>, &'a l3::Ref) -> l3::Container<'a>`
note: the lifetime 'a as defined on the method body at 19:5...
  --> src/l3.rs:19:5
   |
19 |     fn to_c(&self, r: &'a Ref) -> Container<'a> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...does not necessarily outlive the lifetime 'a as defined on the impl at 17:6
  --> src/l3.rs:17:6
   |
17 | impl<'a> ToC for ContainerB<'a> {
   |      ^^

error[E0308]: method not compatible with trait
  --> src/l3.rs:19:5
   |
19 |     fn to_c(&self, r: &'a Ref) -> Container<'a> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected type `fn(&l3::ContainerB<'a>, &'a l3::Ref) -> l3::Container<'a>`
              found type `fn(&l3::ContainerB<'a>, &'a l3::Ref) -> l3::Container<'a>`
note: the lifetime 'a as defined on the impl at 17:6...
  --> src/l3.rs:17:6
   |
17 | impl<'a> ToC for ContainerB<'a> {
   |      ^^
note: ...does not necessarily outlive the lifetime 'a as defined on the method body at 19:5
  --> src/l3.rs:19:5
   |
19 |     fn to_c(&self, r: &'a Ref) -> Container<'a> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

Basically the problem is that there are two lifetimes called 'a and the "expected" and "found" types use different 'as.
@Centril Centril added the A-diagnostics Area: Messages for errors, warnings, and lints label Dec 2, 2018
@estebank estebank added A-lifetimes Area: Lifetimes / regions T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 26, 2019
@estebank
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CC #48812

@estebank estebank added D-confusing Diagnostics: Confusing error or lint that should be reworked. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. labels Nov 5, 2019
@crlf0710 crlf0710 added the C-enhancement Category: An issue proposing an enhancement or a PR with one. label Jun 11, 2020
teddywing added a commit to teddywing/fastcgi-conduit that referenced this issue Jun 28, 2020
@teddywing
FastCgiRequest: Start implementing conduit::RequestExt
81402cc 
Was getting this error about the lifetimes in the trait impl:

    error[E0308]: method not compatible with trait
       --> src/lib.rs:137:4
        |
    137 |    fn host(&'a self) -> conduit::Host<'a> {
        |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
        |
        = note: expected fn pointer `fn(&'a FastCgiRequest<'static>) -> conduit::Host<'a>`
                   found fn pointer `fn(&'a FastCgiRequest<'static>) -> conduit::Host<'a>`
    note: the lifetime `'a` as defined on the method body at 137:4...
       --> src/lib.rs:137:4
        |
    137 |    fn host(&'a self) -> conduit::Host<'a> {
        |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    note: ...does not necessarily outlive the lifetime `'a` as defined on the impl at 124:6
       --> src/lib.rs:124:6
        |
    124 | impl<'a> conduit::RequestExt for FastCgiRequest {
        |      ^^

    error[E0308]: method not compatible with trait
       --> src/lib.rs:137:4
        |
    137 |    fn host(&'a self) -> conduit::Host<'a> {
        |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
        |
        = note: expected fn pointer `fn(&'a FastCgiRequest<'static>) -> conduit::Host<'a>`
                   found fn pointer `fn(&'a FastCgiRequest<'static>) -> conduit::Host<'a>`
    note: the lifetime `'a` as defined on the impl at 124:6...
       --> src/lib.rs:124:6
        |
    124 | impl<'a> conduit::RequestExt for FastCgiRequest {
        |      ^^
    note: ...does not necessarily outlive the lifetime `'a` as defined on the method body at 137:4
       --> src/lib.rs:137:4
        |
    137 |    fn host(&'a self) -> conduit::Host<'a> {
        |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Learned from these resources that the problem arose because the `impl`
uses an `'a` lifetime, but the lifetime on the `host()` method need to
be a different one:

rust-lang/rust#56423
https://stackoverflow.com/questions/24847331/rust-lifetime-error-expected-concrete-lifetime-but-found-bound-lifetime/24848424#24848424
@estebank estebank removed their assignment Oct 4, 2021
@fmease fmease mentioned this issue May 8, 2023
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A-diagnostics Area: Messages for errors, warnings, and lints A-lifetimes Area: Lifetimes / regions C-enhancement Category: An issue proposing an enhancement or a PR with one. D-confusing Diagnostics: Confusing error or lint that should be reworked. D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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@crlf0710 @Centril @estebank @mark-i-m