RFC to restrict placing an identifier after a literal.

text/0000-future-proof-literal-suffixes.md

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+- Start Date: 2014-09-28
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Include identifiers immediately after literals in the literal token to
+allow future expansion, e.g. `"foo"bar` and a `1baz` are considered
+whole (but semantically invalid) tokens, rather than two separate
+tokens `"foo"`, `bar` and `1`, `baz` respectively. This allows future
+expansion of handling literals without risking breaking (macro) code.
+
+
+# Motivation
+
+Currently a few kinds of literals (integers and floats) can have a
+fixed set of suffixes and other kinds do not include any suffixes. The
+valid suffixes on numbers are:
+
+
+```text
+u, u8, u16, u32, u64
+i, i8, i16, i32, i64
+f32, f64
+```
+
+Most things not in this list are just ignored and treated as an
+entirely separate token (prefixes of `128` are errors: e.g. `1u12` has
+an error `"invalid int suffix"`), and similarly any suffixes on other
+literals are also separate tokens. For example:
+
+```rust
+#![feature(macro_rules)]
+
+// makes a tuple
+macro_rules! foo( ($($a: expr)*) => { ($($a, )+) } )
+
+fn main() {
+    let bar = "suffix";
+    let y = "suffix";
+
+    let t: (uint, uint) = foo!(1u256);
+    println!("{}", foo!("foo"bar));
+    println!("{}", foo!('x'y));
+}
+/*
+output:
+(1, 256)
+(foo, suffix)
+(x, suffix)
+*/
+```
+
+The compiler is eating the `1u` and then seeing the invalid suffix
+`256` and so treating that as a separate token, and similarly for the
+string and character literals. (This problem is only visible in
+macros, since that is the only place where two literals/identifiers can be placed
+directly adjacent.)
+
+This behaviour means we would be unable to expand the possibilities
+for literals after freezing the language/macros, which would be
+unfortunate, since [user defined literals in C++][cpp] are reportedly
+very nice, proposals for "bit data" would like to use types like `u1`
+and `u5` (e.g. [RFC PR 327][327]), and there are "fringe" types like
+[`f16`][f16], [`f128`][f128] and `u128` that have uses but are not
+common enough to warrant adding to the language now.
+
+[cpp]: http://en.cppreference.com/w/cpp/language/user_literal
+[327]: https://github.com/rust-lang/rfcs/pull/327
+[f16]: http://en.wikipedia.org/wiki/Half-precision_floating-point_format
+[f128]: https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format
+
+# Detailed design
+
+The tokenizer will have grammar `literal: raw_literal identifier?`
+where `raw_literal` covers strings, characters and numbers without
+suffixes (e.g. `"foo"`, `'a'`, `1`, `0x10`).
+
+Examples of "valid" literals after this change (that is, entities that
+will be consumed as a single token):
+
+```
+"foo"bar "foo"_baz
+'a'x 'a'_y
+
+15u16 17i18 19f20 21.22f23
+0b11u25 0x26i27 28.29e30f31
+
+123foo 0.0bar
+```
+
+Placing a space between the letter of the suffix and the literal will
+cause it to be parsed as two separate tokens, just like today. That is
+`"foo"bar` is one token, `"foo" bar` is two tokens.
+
+The example above would then be an error, something like:
+
+```rust
+    let t: (uint, uint) = foo!(1u256); // error: literal with unsupported size
+    println!("{}", foo!("foo"bar)); // error: literal with unsupported suffix
+    println!("{}", foo!('x'y)); // error: literal with unsupported suffix
+```
+
+The above demonstrates that numeric suffixes could be special cased
+to detect `u<...>` and `i<...>` to give more useful error messages.
+
+(The macro example there is definitely an error because it is using
+the incorrectly-suffixed literals as `expr`s. If it was only
+handling them as a token, i.e. `tt`, there is the possibility that it
+wouldn't have to be illegal, e.g. `stringify!(1u256)` doesn't have to
+be illegal because the `1u256` never occurs at runtime/in the type
+system.)
+
+# Drawbacks
+
+None beyond outlawing placing a literal immediately before a pattern,
+but the current behaviour can easily be restored with a space: `123u
+456`. (If a macro is using this for the purpose of hacky generalised
+literals, the unresolved question below touches on this.)
+
+# Alternatives
+
+Don't do this, or consider doing it for adjacent suffixes with an
+alternative syntax, e.g. `10'bar` or `10$bar`.
+
+# Unresolved questions
+
+- Should it be the parser or the tokenizer rejecting invalid suffixes?
+  This is effectively asking if it is legal for syntax extensions to
+  be passed the raw literals? That is, can a `foo` procedural syntax
+  extension accept and handle literals like `foo!(1u2)`?
+
+- Should this apply to all expressions, e.g. `(1 + 2)bar`?