Bounds checking for float literals

[catamorphism]

As per a FIXME in syntax::parse::lexer, check (maybe in the parser, definitely somewhere earlier than trans) that 32-bit and 64-bit floating-point literals are in the right range.

[z0w0]

How exactly can I implement this in the parser? LIT_FLOAT takes an ident, so would I make LIT_FLOAT take an f64 and then parse it using f32:: for explicitly marked f32 suffixes and f64:: for f64 suffixes and then raise an error if there is an error parsing them? Where/how is this handled for ints?

[emberian]

See also #4220. @z0w0 it isn't handled for any literal, that's part of the problem.

[thestinger]

This is still not implemented.

[catamorphism]

De-milestoning, we agreed it's not a major concern.

[pnkfelix]

cc me

[huonw]

Triage, no change.

[emberian]

Unmarking as easy. Correct string floatification isn't trivial, and Rust doesn't have anything for it today.

[steveklabnik]

Today, we have

hello.rs:2:18: 2:60 warning: literal out of range for f32, #[warn(overflowing_literals)] on by default
hello.rs:2     let x: f32 = 1000000000000000000000000000000000000000.0;
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So seems good!