std.fmt: check result types in parseInt functions

[ilumys]

The parseInt() family of functions do not perform type checking thus accepting erroneous inputs for result types.

Sample subject case, built with zig 0.15.0-dev.869+640a13065:

const std = @import("std");

pub fn main() !void {
    const i = "1234";

    // selectively run with any of the following; error is the same
    _ = try std.fmt.parseInt(f32, i, 10);
    // _ = try std.fmt.parseIntWithGenericCharacter(f32, u8, i, 10);
    // _ = try std.fmt.parseUnsigned(f32, i, 10);
}

Outputs:

/home/user/zig/lib/std/fmt.zig:1636:41: error: access of union field 'int' while field 'float' is active
    const Accumulate = std.meta.Int(info.int.signedness, @max(8, info.int.bits));
                                    ~~~~^~~~
/home/user/zig/lib/std/builtin.zig:572:18: note: union declared here
pub const Type = union(enum) {
                 ^~~~~
referenced by:
    parseIntWithGenericCharacter__anon_23506: /home/user/zig/lib/std/fmt.zig:1529:47
    parseInt__anon_23379: /home/user/zig/lib/std/fmt.zig:1518:40
    6 reference(s) hidden; use '-freference-trace=8' to see all references
/home/user/zig/lib/std/fmt.zig:1636:41: error: access of union field 'int' while field 'float' is active
    const Accumulate = std.meta.Int(info.int.signedness, @max(8, info.int.bits));
                                    ~~~~^~~~
/home/user/zig/lib/std/builtin.zig:572:18: note: union declared here
pub const Type = union(enum) {
                 ^~~~~

This PR checks that the given input (T or Result) is an integer and ouputs a compiler error when this is not true. The specific form of error output was taken from prior art in parseFloat(). While the code will not compile without this change, it is desirable to make this a clear compiler error, both to prevent processing of invalid inputs and improve user experience.

Output with patch:

/home/user/zig/lib/std/fmt.zig:1521:9: error: Cannot parse an integer into a non-integer point type.
        @compileError("Cannot parse an integer into a non-integer point type.");
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    main: test/t.zig:7:29
    callMain [inlined]: /home/user/zig/lib/std/start.zig:681:37
    callMainWithArgs [inlined]: /home/user/zig/lib/std/start.zig:641:20
    posixCallMainAndExit: /home/user/zig/lib/std/start.zig:596:36
    2 reference(s) hidden; use '-freference-trace=6' to see all references

parseIntWithSign() is not exported from the container but I believe its inclusion has merit. Type check is compile- time and an assertion of expected input type(s); docs (which, to be fair, are largely taken from other parseInt() fn's) ease readability of source code.

[andrewrk]

Thanks for the patch. Apologies but I'm going to keep it in limbo until I can get some conflicting work done.