split `redundant_type_annotations` into "redundant" and "unnecessary" flavors

[eric-seppanen]

Description

I enabled redundant_type_annotations on a large codebase, and found that it flags some code that needs improvement, but also a lot of things that seem fine as-is.

I agree with the lint that type annotations like this are redundant:

let socket: SocketAddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 8080);

let start: SystemTime = SystemTime::now();

... because it literally spells out the type twice.

I also agree that this type annotation is unnecessary:

let mut success: bool = true;

... because true and false are keywords, and there is no chance of confusion what type they are.

But I don't think the same thinking stretches to all type annotations, just because a function returns an unambiguous type that could have been inferred. Some examples:

// We see `len()` and assume `usize`, but `Metadata::len()` is different.
let file_size: u64 = file.metadata()?.len();

// I want to clarify that this is `PathBuf` and not  `&Path` or `OsString`.
let mut runner_dir: PathBuf = std::env::temp_dir();

// I want to clarify that that `DirEntry::file_name()` returns `OsString` and not  `&Path` or `PathBuf`.
let entry_name: OsString = entry.file_name();

// I want to remember that this is `ThreadRng` and not `SmallRng` or `OsRng`.
let mut rng: ThreadRng = rand::rng();

In these cases I would defer to the programmer, who probably added the type annotation to help them while debugging or refactoring, or as a readability aid: it may be easier for humans to understand/review the code if some of the types are visible.

Perhaps these should be handled as separate lints:

• redundant_type_annotations: only fires for "redundant" type annotations, where the type was already named in the rvalue.
• unnecessary_type_annotations: all the other cases where type annotations could be elided.

I didn't see any debate on this topic when the lint was created (#10570).

Version

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

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