Summary

As soon as there is some type complexity involved the lint seems to fail to see the possible const.
At first, I suspected generics and lifetimes on the struct, but they don't seem to be the main reason as its also reproducible without. Having generic things like Vec<Generic> seems to be part of the issue.
Interestingly it's not the entire issue as my second example highlights.

I'm not sure if I can further pinpoint the root cause. When more is known please update the issue title to reflect this.

Lint Name

missing_const_for_fn

Reproducer

pub struct TabsState {
    pub titles: Vec<String>,
}

impl TabsState {
    #[must_use]
    pub fn new(titles: Vec<String>) -> Self {
        Self { titles }
    }

    #[must_use]
    pub fn empty() -> Self {
        Self { titles: Vec::new() }
    }
}

I expected to see this happen:

The lint should hint both methods to be possible as const.

Instead, this happened:

Only the second method is marked to be possible as const.

Another way where a Vec seems to prevent this lint from working is this:

pub struct Other {
    pub text: String,
    pub vec: Vec<String>,
}

impl Other {
    pub fn new(text: String) -> Self {
        let vec = Vec::new();
        Self { text, vec }
    }
}

The lint should also detect this one, but it doesn't. Removing the vec from the struct and new function and this is correctly suggested. Using text: usize instead of text: String also works. So somehow this combination results in not detecting this possible const.

Version

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: x86_64-unknown-linux-gnu
release: 1.77.2
LLVM version: 17.0.6