Append .0 to unsuffixed float if it would otherwise become int token

[dtolnay]

Previously the unsuffixed f32/f64 constructors of proc_macro::Literal would create literal tokens that are definitely not a float:

Literal::f32_unsuffixed(10.0)  // 10
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10
Literal::f64_suffixed(10.0)    // 10f64

Notice that the 10 are actually integer tokens if you were to reparse them, not float tokens.

This diff updates Literal::f32_unsuffixed and Literal::f64_unsuffixed to produce tokens that unambiguously parse as a float. This matches longstanding behavior of the proc-macro2 crate's implementation of these APIs dating back at least 3.5 years, so it's likely an unobjectionable behavior.

Literal::f32_unsuffixed(10.0)  // 10.0
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10.0
Literal::f64_suffixed(10.0)    // 10f64

Fixes dtolnay/syn#1085.

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

From reading the standard library code it looks like to_string should never produce an exponential form, so this shouldn't result in something like 1e100.0.

The Debug impl for floats does exactly what we need (10.0) for small floats, but it does produce exponential forms for large floats.

Too bad fn float_to_decimal_common_shortest in library\core\src\fmt\float.rs is private, it can do exactly what we need here (although in theory it can be made public, but unstable and hidden, for library/proc_macro).

It's actually unfortunate that there's no public way to "print float as a float" in the standard library.
I wonder why the Display impl doesn't use the minimal precision 1 by default.

[dtolnay]

I added a test involving 1e100 so that at least something breaks if to_string starts turning it into 1e100.0, though I don't expect that change to ever get made.

[petrochenkov]

@dtolnay
r=me after reading (and optionally answering) the comment #90297 (comment).

[dtolnay]

@bors r=petrochenkov

[bors]

📌 Commit 1f98077 has been approved by petrochenkov

[bors]

⌛ Testing commit 1f98077 with merge 7276a6a...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 7276a6a to master...

[rust-timer]

Finished benchmarking commit (7276a6a): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression