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base repository: rust-lang/rust
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  • 1 commit
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Commits on May 15, 2025

  1. Permit duplicate macro imports 

    Consider a crate that depends on both `serde` (without the `derive`
feature) and `serde_derive`, and imports `serde::Serialize` (a trait)
and `serde_derive::Serialize` (a macro). Then, imagine some other crate
in a build graph depends on `serde` *with* the `derive` feature; they
import both the macro and trait simultaneously with `use
serde::Serialize`. If duplicate imports of the same item are always
forbidden, these crates cannot co-exist in the same build-graph; the
former crate will fail to build, as its first import (which will now
also import the `Serialize` macro) conflicts with its second import.

This build hazard is confusing — the author of the second crate had no
idea that their dependence on the `derive` feature might be problematic
for other crates. The author of the first crate can mitigate the hazard
by only glob-importing from proc-macro crates, but glob imports run
against many's personal preference and tooling affordances (e.g.,
`rust-analyzer`'s auto-import feature).

We mitigate this hazard across the ecosystem by permitting duplicate
imports of macros. We don't limit this exception to proc macros, as it
should not be a breaking change to rewrite a proc macro into a
by-example macro. Although it would be semantically unproblematic to
permit *all* duplicate imports (not just those of macros), other kinds
of imports have not, in practice, posed the same hazard, and there might
be cases we'd like to warn-by-default against. For now, we only permit
duplicate macro imports.

See https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Allowing.20same-name.20imports.20of.20the.20same.20item/near/516777221
    @jswrenn
    jswrenn committed May 15, 2025
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