rust-lang / rust-lang/rust

`unused_qualifications` lint confused by `pub use Foo::*`

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A-lints C-bug L-unused_qualifications T-compiler
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Rust
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Description

I tried enabling unused_qualifications for the compiler itself. It mostly works, but gets confused by pub use Foo::* forms.

Here's an example from compiler/rustc_ast/src/token.rs, which has this:

pub use LitKind::*;

followed by this:

pub enum LitKind { 
    Bool, // AST only, must never appear in a `Token`
    Byte,
    Char,
    Integer, // e.g. `1`, `1u8`, `1f32`
    Float,   // e.g. `1.`, `1.0`, `1e3f32`
    Str,
    StrRaw(u8), // raw string delimited by `n` hash symbols
    ByteStr,
    ByteStrRaw(u8), // raw byte string delimited by `n` hash symbols
    CStr,
    CStrRaw(u8),
    Err(ErrorGuaranteed),
}   

It emits a bogus error for every variant of the enum. Here's one example:

error: unnecessary qualification
   --> compiler/rustc_ast/src/token.rs:160:10
    |
160 |   pub enum LitKind {
    |  __________^
161 | |     Bool, // AST only, must never appear in a `Token`
162 | |     Byte,
163 | |     Char,
164 | |     Integer, // e.g. `1`, `1u8`, `1f32`
    | |___________^
    |
help: remove the unnecessary path segments
    |
160 - pub enum LitKind {
161 -     Bool, // AST only, must never appear in a `Token`
162 -     Byte,
163 -     Char,
164 -     Integer, // e.g. `1`, `1u8`, `1f32`
160 + pub enum Integer, // e.g. `1`, `1u8`, `1f32`
    |

There is no qualification present, and the help suggestion is nonsense.

I was unable to reproduce this behaviour in a standalone test case.

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Research direction

The reported case is in compiler/rustc_ast/src/token.rs, around pub use LitKind::* and the LitKind enum. Start by reproducing the unused_qualifications diagnostic with this example, then trace how the glob re-export affects the reported span and suggestion. Done means the example produces no bogus qualification error or nonsensical help text.

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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