Detect likely attempts to use interpolation in string literals #100584

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1. estebank

   added

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

   P-lowLow priorityLow priority

   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.

   A-suggestion-diagnosticsArea: Suggestions generated by the compiler applied by `cargo fix`Area: Suggestions generated by the compiler applied by `cargo fix`

   D-papercutDiagnostics: An error or lint that needs small tweaks.Diagnostics: An error or lint that needs small tweaks.

   D-newcomer-roadblockDiagnostics: Confusing error or lint; hard to understand for new users.Diagnostics: Confusing error or lint; hard to understand for new users.

   D-terseDiagnostics: An error or lint that doesn't give enough information about the problem at hand.Diagnostics: An error or lint that doesn't give enough information about the problem at hand.

   on Aug 15, 2022
2. lyming2007 commented on Aug 16, 2022

   

   lyming2007

   on Aug 16, 2022

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   Do you mean, we should emit like this?

   warning: unused variable: `x`
    --> main.rs:1:8
     |
   1 | fn foo(x: &str) {
     |        let _ = "{x}";
     |                     ======= x needs to be inside of a format!() invocation
     |
     = note: `#[warn(unused_variables)]` on by default
   
   
3. estebank commented on Aug 16, 2022

   

   estebank

   on Aug 16, 2022

   ContributorAuthor

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   Ideally, it would look something like this:

   warning: unused variable: `x`
    --> main.rs:1:8
     |
   1 | fn foo(x: &str) {
     |        ^ unused variable
   2 |        let _ = "{x}";
     |                  - you might have meant to use string interpolation in this string literal
   help: string interpolation only works in `format!` invocations
     |
   2 |        let _ = format!("{x}");
     |                ++++++++     +
     |
     = note: `#[warn(unused_variables)]` on by default
   
4. lyming2007 commented on Aug 16, 2022

   

   lyming2007

   on Aug 16, 2022

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   @rustbot claim
5. rustbot

   assigned

   lyming2007

   on Aug 16, 2022
6. lyming2007 commented on Aug 24, 2022

   

   lyming2007

   on Aug 24, 2022

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   cat main.rs
   fn foo(xyza: &str) {
       let _ = "{xyza}";
   }
   
   
   fn main() {
     foo("aaa");
   }
   

   Current output is:

   warning: unused variable: `xyza`
    --> main.rs:1:8
     |
   1 | fn foo(xyza: &str) {
     |        ^^^^ help: if this is intentional, prefix it with an underscore: `_xyza`
     |
     = note: `#[warn(unused_variables)]` on by default
   
   warning: 1 warning emitted
   
   

   With my fix https://github.com/lyming2007/rust/pull/new/issue-100584

   The output looks like:

   warning: unused variable: `xyza`
    --> main.rs:1:8
     |
   1 | fn foo(xyza: &str) {
     |        ^^^^
     |        |
     |        unused variable
     |        help: if this is intentional, prefix it with an underscore: `_xyza`
   2 |     let _ = "{xyza}";
     |             -------- you might have meant to use string interpolation in this string literal
     |
     = note: `#[warn(unused_variables)]` on by default
     = help: string interpolation only works in `format!` invocations
   
   warning: 1 warning emitted
   

   There are still 2 issues for me.
   #1 I am not able to emit something like

    |
   2 |        let _ = format!("{x}");
     |                ++++++++     +
     |
   

   please give me an example, then I can look how the compiler deal with it.

   #2 I can't upload a specific test case for this issue, because the test script will opt out the unused variable. Here is the script I got:

   /home/lym/rust3/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc /home/lym/rust3/rust/src/test/ui/type/issue-100584.rs -Zthreads=1 --target=x86_64-unknown-linux-gnu --error-format json --json future-incompat -Ccodegen-units=1 -Zui-testing -Zdeduplicate-diagnostics=no -Cstrip=debuginfo --emit metadata -C prefer-dynamic --out-dir /home/lym/rust3/rust/build/x86_64-unknown-linux-gnu/test/ui/type/issue-100584 -A unused -Crpath -Cdebuginfo=0 -Lnative=/home/lym/rust3/rust/build/x86_64-unknown-linux-gnu/native/rust-test-helpers -L /home/lym/rust3/rust/build/x86_64-unknown-linux-gnu/test/ui/type/issue-100584/auxiliary
   

   So it will always output nothing to stderr and make the test failed.
   @estebank please advise
7. estebank commented on Aug 24, 2022

   

   estebank

   on Aug 24, 2022

   ContributorAuthor

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   You'd have to use:

   err.multipart_suggestion(
       vec![
           (lit_expr.span.shrink_to_lo(), "format!(".to_string()),
           (lit_expr.span.shrink_to_hi(), ")".to_string()),
       ],
       "if you meant to use string interpolation in the string literal, use the `format!` marco",
       Applicability::MachineApplicable,
   );

   Have you run python3 x.py test src/test/ui/type/issue-100584.rs --bless?
8. lyming2007 commented on Aug 29, 2022

   

   lyming2007

   on Aug 29, 2022

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   thanks. please see my fix:
   #100941
   @estebank
9. estebank

   closed this as completedon Sep 29, 2023