rust-lang / rust-lang/rust

Compiler can't infer &str type when &String is inserted first in a collection with tuples

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A-inference T-compiler
Dominant language
Rust
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Description

Description: compiler can't infer &str type when String is inserted first in a collection with tuples.

Steps to reproduce: create a new array and fill it with tuples, where one of the elements is a string literal and one is a variable of type &String. Depending one the order, it might or might not compile because type can't be inferred.

let s = &String::from("foo");

let arr1 = [(1, s), (2, "bar")];  // This line contains a compilation error 
let arr2 = [(2, "bar"), (1, s)];  // Does compile; inferred type is [(i32, &str); 2]

Error message:

error[E0308]: mismatched types
   |
   |     let arr1 = [(1, s), (2, "bar")];
   |                             ^^^^^ expected `&String`, found `&str`
   |
   = note: expected reference `&String`
              found reference `&'static str`

Rust compiler version: rustc 1.80.1 (3f5fd8dd4 2024-08-06)

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Research direction

Start by compiling the provided arr1 and arr2 reproductions with rustc 1.80.1 and compare their inferred types and diagnostics. Investigate the compiler's type inference for arrays containing tuples with &String and string literals; done means both orderings compile consistently with the intended common element type and have regression coverage.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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