Incorrect constraints on const generic parameters

[afetisov]

Environment

• IntelliJ Rust plugin version: 0.4.149.3939-211
• Rust toolchain version: 1.53.0 (53cb7b09b 2021-06-17) x86_64-unknown-linux-gnu
• IDE name and version: CLion 2021.1.2 (CL-211.7442.42)
• Operating system: Linux 5.4.0-74-generic
• Macro expansion engine: new
• Name resolution engine: new

Problem description

The constraints on const generic parameters are incorrectly inferred when only some of the parameters are generic.

Steps to reproduce

The code:

struct Foo<const N: u8, const S: bool>(u32);

impl<const N: u8> From<u32> for Foo<N, true> {
                                    // ^^^^ error
    fn from(val: u32) -> Self { Foo(val) }
}

Error: mismatched types [E0308] expected u8, found bool .

The constraint applied to S is always that of the preceding type. For example:

struct Foo<const N: usize, const S: u32>(u32);

impl<const N: usize> From<u32> for Foo<N, 1u32> {
                                       // ^^^^ error
    fn from(val: u32) -> Self { Foo(val) }
}

fn foo<const N: usize>() -> Foo<N, 1u32> {
                                // ^^^^ error
    Foo(0)
}

Error: mismatched types [E0308] expected usize, found u32.

No error occurs if an impl block has either no or all specified generic parameters. E.g.:

impl<const N: usize, const S: u32> From<bool> for Foo<N, S> {
    fn from(_: bool) -> Self {
        Foo(0)
    }
}

No error.

impl From<bool> for Foo<0usize, 1u32> {
    fn from(_: bool) -> Self {
        Foo(0)
    }
}

No error.

Extra error

The following error is probably related:

struct Foo<const N: usize, const S: u32>(u32);
fn foo<const S: u32>() -> Foo<1usize, S> {
                                   // ^ error
    Foo(0)
}

Error: Type arguments must be declared prior to const arguments.