Fix the primary span of redundant_pub_crate when flagging nameless items

[fmease]

Found this while reviewing PR rust-lang/rust#138384: See rust-lang/rust#138384 (comment) in which I suggested a FIXME to be added that I'm now fixing in this PR.

---

Before this PR, redundant_pub_crate computed a broken primary Span when flagging items (hir::Items) that never bear a name (ForeignMod, GlobalAsm, Impl, Use(UseKind::Glob), Use(UseKind::ListStem)). Namely, it created a span whose high byte index is DUMMY_SP.hi() which is quite broken (DUMMY_SP is synonymous with 0..0 wrt. the entire SourceMap meaning it points at the start of the very first source file in the SourceMap).

Why did this happen? Before PR rust-lang/rust#138384, the offending line looked like let span = item.span.with_hi(item.ident.span.hi());. For nameless items, item.ident used to be set to Ident(sym::empty, DUMMY_SP). This is where the DUMMY_SP came from.

The code means to compute a "shorter item span" that doesn't include the "body" of items, only the "head" (similar to TyCtxt::def_span).

Examples of Clippy's butchered output on master

#![deny(clippy::redundant_pub_crate)]

mod m5 {
    pub mod m5_1 {}
    pub(crate) use m5_1::*;
}

error: pub(crate) import inside private module
 --> file.rs:1:1
  |
1 | / #![deny(clippy::redundant_pub_crate)]
2 | |
3 | | mod m5 {
4 | |     pub mod m5_1 {}
5 | |     pub(crate) use m5_1::*;
  | |    ^---------- help: consider using: `pub`
  | |____|
  |

Or if the SourceMap contains multiple files (notice how it leaks clippy.toml!):

error: pub(crate) import inside private module
 --> /home/fmease/programming/rust/clippy/clippy.toml:1:1
  |
1 | / avoid-breaking-exported-api = false
2 | |
3 | | check-inconsistent-struct-field-initializers = true
... |
6 | |
  | |_^
  |
 ::: file.rs:6:5
  |
6 |       pub(crate) use m5_1::{*}; // Glob
  |       ---------- help: consider using: `pub`
  |

---

Note: Currently, the only nameless item kind that can also have a visibility is Use(UseKind::{Glob,ListStem}). Thus I'm just falling back to the entire item's Span which wouldn't be that verbose. However, in the future Rust will feature impl restrictions (like pub(crate) impl Trait for Type {}, see RFC 3323 and rust-lang/rust#106074). Using item.span for these would be quite bad (it would include all associated items). Should I add a FIXME?

---

changelog: [redundant_pub_crate]: Fix the code highlighting for nameless items.

[rustbot]

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[fmease]

Output on master for comparison

warning: pub(crate) import inside private module
 --> file.rs:1:1
  |
1 | / #![warn(clippy::redundant_pub_crate)]
2 | |
3 | | mod m5 {
4 | |     pub mod m5_1 {}
5 | |     // Test that the primary span isn't butchered for item kinds that don't have an ident.
6 | |     pub(crate) use m5_1::*; //~ redundant_pub_crate
  | |    ^---------- help: consider using: `pub`
  | |____|
  |
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pub_crate
note: the lint level is defined here
 --> file.rs:1:9
  |
1 | #![warn(clippy::redundant_pub_crate)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: pub(crate) import inside private module
 --> file.rs:1:1
  |
1 | / #![warn(clippy::redundant_pub_crate)]
2 | |
3 | | mod m5 {
4 | |     pub mod m5_1 {}
... |
7 | |     #[rustfmt::skip]
8 | |     pub(crate) use m5_1::{*}; //~ redundant_pub_crate
  | |_____----------___________^
  |       |
  |       help: consider using: `pub`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pub_crate

[Manishearth]

Good catch! lintcheck fails but it doesn't seem to be your fault

(unclear if the build queue cares about lintcheck)