[NLL] Lifetimes errors in a function can mask later errors in same function #96331

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1. marmeladema

   added

   A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   on Apr 23, 2022
2. marmeladema commented on Apr 23, 2022

   

   marmeladema

   on Apr 23, 2022

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   @rustbot label +A-NLL +NLL-diagnostics
3. rustbot

   added

   A-NLLArea: Non-lexical lifetimes (NLL)Area: Non-lexical lifetimes (NLL)

   NLL-diagnosticsWorking towards the "diagnostic parity" goalWorking towards the "diagnostic parity" goal

   on Apr 23, 2022
4. m-ou-se commented on May 17, 2022

   

   m-ou-se

   on May 17, 2022

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   Reduced test case:

   fn b<'a, 'b, 'c>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize) {
       *x = *y;
       *z = *y;
   }

   Rustc output:

   error[E0623]: lifetime mismatch
    --> src/main.rs:2:10
     |
   1 | fn b<'a, 'b, 'c>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize) {
     |                          ---------          --------- these two types are declared with different lifetimes...
   2 |     *x = *y;
     |          ^^ ...but data from `y` flows into `x` here
   
   error[E0623]: lifetime mismatch
    --> src/main.rs:3:10
     |
   1 | fn b<'a, 'b, 'c>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize) {
     |                                             ---------          ---------
     |                                             |
     |                                             these two types are declared with different lifetimes...
   2 |     *x = *y;
   3 |     *z = *y;
     |          ^^ ...but data from `y` flows into `z` here
   
   error: aborting due to 2 previous errors
   

   Rustc output with -Z borrowck=mir:

   error: lifetime may not live long enough
    --> src/main.rs:2:5
     |
   1 | fn b<'a, 'b, 'c>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize) {
     |      --  -- lifetime `'b` defined here
     |      |
     |      lifetime `'a` defined here
   2 |     *x = *y;
     |     ^^^^^^^ assignment requires that `'b` must outlive `'a`
     |
     = help: consider adding the following bound: `'b: 'a`
   
   error: aborting due to previous error
   

   It doesn't give an error about the *z = *y; line.

   However, if you remove the *x = *y; line, it doesn't succeed. It then produces the right error:

   error: lifetime may not live long enough
    --> src/main.rs:2:5
     |
   1 | fn b<'a, 'b, 'c>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize) {
     |          --  -- lifetime `'c` defined here
     |          |
     |          lifetime `'b` defined here
   2 |     *z = *y;
     |     ^^^^^^^ assignment requires that `'b` must outlive `'c`
     |
     = help: consider adding the following bound: `'b: 'c`
   

   So, this doesn't seem like a case where invalid code is accepted. Just a case where diagnostics are incomplete.
5. m-ou-se commented on May 17, 2022

   

   m-ou-se

   on May 17, 2022

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   When using two unrelated variables/lifetimes, it does produce both errors:

   fn b<'a, 'b, 'c, 'd>(x: &mut &'a isize, y: &mut &'b isize, z: &mut &'c isize, z2: &mut &'d isize) {
       *x = *y;
       *z = *z2;
   }
   
6. m-ou-se commented on May 17, 2022

   

   m-ou-se

   on May 17, 2022

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   When doing *x = *y; *x = *y; (the same thing twice), you only get the error once. It seems that in the new mode, that 'deduplicating' effect also applies to *x = *y; *z = *y;.

   (*x = *y; *y = *z; (last assignment flipped) does produce two errors with and without nll.)
7. jackh726 commented on May 17, 2022

   

   jackh726

   on May 17, 2022

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   Thanks @m-ou-se for looking at this!

   I think based on this, it should be okay to move forward without "fixing" this. Seems like only a diagnostics thing instead of accidently allowing more code.

   👍React with 👍1Reacted by Mara Bos
8. jackh726

   changed the title [-][NLL] Investigate behavior difference with non-nll for `region-multiple-lifetime-bounds-on-fns-where-clause.rs` test[/-] [+][NLL] Lifetimes errors in a function can mask later errors in same function[/+] on May 21, 2022