Amend RFC2603 to allow identifiers to start with a digit.

Author: eddyb

text/2603-rust-symbol-name-mangling-v0.md

@@ -506,7 +506,7 @@ mod gödel {
 would be mangled as:
 
 ```
-_RNvNtNtC7mycrateu8gdel_Fqa6escher4bach
+_RNvNtNtC7mycrateu8gdel_5qa6escher4bach
                  <-------->
               Unicode component
 ```
@@ -613,10 +613,10 @@ compiler generates mangled names.
 
 The syntax of mangled names is given in extended Backus-Naur form:
 
- - Non-terminals are within angle brackets (as in `<name-prefix>`)
+ - Non-terminals are within angle brackets (as in `<path>`)
  - Terminals are within quotes (as in `"_R"`),
- - Optional parts are in brackets (as in `[<decimal>]`),
- - Repetition (zero or more times) is signified by curly braces (as in `{<name-prefix>}`)
+ - Optional parts are in brackets (as in `[<disambiguator>]`),
+ - Repetition (zero or more times) is signified by curly braces (as in `{<type>}`)
  - Comments are marked with `//`.
 
 Mangled names conform to the following grammar:
@@ -641,11 +641,13 @@ Mangled names conform to the following grammar:
 <impl-path> = [<disambiguator>] <path>
 
 // The <decimal-number> is the length of the identifier in bytes.
-// <bytes> is the identifier itself and must not start with a decimal digit.
+// <bytes> is the identifier itself, and it's optionally preceded by "_",
+// to separate it from its length - this "_" is mandatory if the <bytes>
+// starts with a decimal digit, or "_", in order to keep it unambiguous.
 // If the "u" is present then <bytes> is Punycode-encoded.
 <identifier> = [<disambiguator>] <undisambiguated-identifier>
 <disambiguator> = "s" <base-62-number>
-<undisambiguated-identifier> = ["u"] <decimal-number> <bytes>
+<undisambiguated-identifier> = ["u"] <decimal-number> ["_"] <bytes>
 
 // Namespace of the identifier in a (nested) path.
 // It's an a-zA-Z character, with a-z reserved for implementation-internal
@@ -775,29 +777,22 @@ and, for now, only define a mangling for integer values.
 ### Punycode Identifiers
 
 Punycode generates strings of the form `([[:ascii:]]+-)?[[:alnum:]]+`.
-This is problematic for two reasons:
+This is problematic because of the `-` character, which is not in the
+supported character set; Punycode uses it to separate the ASCII part
+(if it exists), from the base-36 encoding of the non-ASCII characters.
 
-- Generated strings can contain a `-` character; which is not in the
-  supported character set.
-- Generated strings can start with a digit; which makes them clash
-  with the byte-count prefix of the `<identifier>` production.
-
-For these reasons, vanilla Punycode string are further encoded during mangling:
-
-- The `-` character is simply replaced by a `_` character.
-- The part of the Punycode string that encodes the non-ASCII characters
-  is a base-36 number, using `[a-z0-9]` as its "digits". We want to get
-  rid of the decimal digits in there, so we simply remap `0-9` to `A-J`.
+For this reasons, we deviate from vanilla Punycode, by replacing
+the `-` character with a `_` character.
 
 Here are some examples:
 
 | Original        | Punycode        | Punycode + Encoding |
 |-----------------|-----------------|---------------------|
-| føø             | f-5gaa          | f_Fgaa              |
-| α_ω             | _-ylb7e         | __ylbHe             |
-| 铁锈             | n84amf          | nIEamf              |
-| 🤦              | fq9h            | fqJh                |
-| ρυστ            | 2xaedc          | Cxaedc              |
+| føø             | f-5gaa          | f_5gaa              |
+| α_ω             | _-ylb7e         | __ylb7e             |
+| 铁锈             | n84amf          | n84amf              |
+| 🤦              | fq9h            | fq9h                |
+| ρυστ            | 2xaedc          | 2xaedc              |
 
 With this post-processing in place the Punycode strings can be treated
 like regular identifiers and need no further special handling.
@@ -1154,3 +1149,4 @@ pub static QUUX: u32 = {
 - Resolve question of complex constant data.
 - Add a recommended resolution for open question around Punycode identifiers.
 - Add a recommended resolution for open question around encoding function parameter types.
+- Allow identifiers to start with a digit.