Multiple Iterator generic parameters confuse the type inference

[TDplai]

rust-analyzer gets confused about IntoIterator::Item types when multiple generic parameters implement IntoIterator.

Example:

pub fn item_type_confusion<'a, I: IntoIterator<Item = &'a mut usize>, I2: IntoIterator>(it: I) {
    for a in it {
        // rust-analyzer reports error: cannot mutate immutable variable `a`
        *a += 1;
    }
}

This error is only reported in rust-analyzer. The function compiles without errors. Furthermore, neither cargo check nor cargo clippy report any errors.

The hover-text on a is a: {unknown}, which shows that rust-analyzer was unable to infer the type of a.

This occurs even when I and I2 have the same Item type:

pub fn analyzer_confusion<
    'a,
    I: IntoIterator<Item = &'a mut usize>,
    I2: IntoIterator<Item = &'a mut usize>,
>(
    i1: I,
) {
    for a in i1 {
        // rust-analyzer reports error: cannot mutate immutable variable `a`
        *a += 1;
    }
}

If I coerce a into an &mut usize, the error goes away:

pub fn item_type_confusion<'a, I: IntoIterator<Item = &'a mut usize>, I2: IntoIterator>(it: I) {
    for a in it {
        let _: &mut usize = a;
        // no error reported
        *a += 1;
    }
}

rust-analyzer does recognise that a is an &mut usize when printing a type error, but still claims that *a is immutable:

pub fn item_type_confusion<'a, I: IntoIterator<Item = &'a mut usize>, I2: IntoIterator>(it: I) {
    for a in it {
        // rust-analyzer reports the expected error: expected `u64`, found `&mut usize`
        let _: u64 = a;
        // rust-analyzer reports error: cannot mutate immutable variable `a`
        *a += 1;
    }
}

Furthermore, if I2: IntoIterator is removed, the error goes away:

pub fn item_type_confusion<'a, I: IntoIterator<Item = &'a mut usize>, I2>(it: I) {
    for a in it {
        // no error reported
        *a += 1;
    }
}

rust-analyzer version: rust-analyzer 1.72.0-nightly (8084f39 2023-06-25)

rustc version: rustc 1.72.0-nightly (8084f397c 2023-06-25)

relevant settings:
CARGO_HOME is set to $HOME/.local/share/cargo
RUSTUP_HOME is set to $HOME/.local/share/rustup
RUSTC_WRAPPER is set to sccache
rust-analyzer settings are:

["rust-analyzer"] = {
	cargo = {
		features = "all",
	},
	checkOnSave = {
		command = "clippy",
		allFeatures = true,
		extraArgs = { "--", "-W", "clippy::pedantic" },
	},
},

(this is copied verbatim from my Neovim configuration)

[TDplai]

In further testing, if I coerce a into &mut _, rust-analyzer is able to infer the usize part.

pub fn analyzer_confusion<'a, I: IntoIterator<Item = &'a mut usize>, I2: IntoIterator>(i1: I) {
    for a in i1 {
        let _: &mut _ = a;
        // no error reported
        *a += 1;
    }
}

[lowr]

Thanks for the report -- this is a known issue that needs to be solved in an upstream library: rust-lang/chalk#750. Closing as a duplicate of #10653.