Type inference of integer literals is inconsistenct with the reference

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
compilers, documentation

Research direction

Reproduce the two integer-literal examples and compare their behavior with the reference's integer-literals section. Review related issues #39255 and #40985, then reconcile the intended inference behavior with the documentation, including whether floating-point literals are affected.

Written by the indexing model from the issue text.

Description

A-types C-bug

Originally at https://github.com/rust-lang/rust/issues/41060

The reference currently says the following for integer literals

The type of an unsuffixed integer literal is determined by type inference:
[...]
If the program context under-constrains the type, it defaults to the signed 32-bit integer i32.

This does however not work for some inherent methods of signed integer types:

trait A {
    fn what_type(&self);
}


impl A for i16 {
    fn what_type(&self) {
        println!("i16");
    }
}

impl A for i32 {
    fn what_type(&self)  {
        println!("i32");
    }
}

fn main() {
    let z = 1;
    z.what_type();
    //z.is_positive(); // <- uncomment this line
}

As you can see, z is under-constrained in this case. The compiler does the right thing and defaults to type i32, so calling the trait method what_type() works.

If you uncomment the inherent method call to is_positive(), z still is under-constrained in pretty much the same way as before, however, the compiler fails to default to i32 and instead prints

error: no method named `is_positive` found for type `{integer}` in the current scope

Also note that the reference says

If an integer type can be uniquely determined from the surrounding program context, the unsuffixed integer literal has that type.

But the following example show that the order of statements is important:

fn one() { // compiles
    let a = 0;
    take_some(a);
    a.is_positive();
}

fn two() { // does not compile
    let a = 0;
    a.is_positive();
    take_some(a);
}

fn take_some(var: i32) {
    // do nothing
}

I think floating-point literals have the same issue, but I haven't checked.

rust-lang/rust#39255 and rust-lang/rust#40985 may be related.

Dominant language
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