[Parse] Provide better diagnostics for func 1() {}.

It is apparently a common mistake for beginners to start the names of functions
etc. with numbers, and before this patch the diagnostic wasn't specific about
the problem. It seems likely that most instances of `func 1(...` will be
mistakes in this vein, so this patch specifically diagnoses that case, and also
parses the number as the identifier to avoid follow on errors about top-level
closures (from the {}) and other invalid syntax.

Fixes rdar://problem/32316666 .

Author: huonw

include/swift/AST/DiagnosticsParse.def

@@ -182,6 +182,9 @@ ERROR(expected_decl,none,
       "expected declaration", ())
 ERROR(expected_identifier_in_decl,none,
       "expected identifier in %0 declaration", (StringRef))
+ERROR(number_cant_start_decl_name,none,
+      "%0 name can only start with a letter or underscore, not a number",
+      (StringRef))
 ERROR(expected_identifier_after_case_comma,none,
       "expected identifier after comma in enum 'case' declaration", ())
 ERROR(decl_redefinition,none,

lib/Parse/ParseDecl.cpp

@@ -2766,6 +2766,23 @@ parseIdentifierDeclName(Parser &P, Identifier &Result, SourceLoc &Loc,
 
   P.checkForInputIncomplete();
 
+  if (P.Tok.is(tok::integer_literal) || P.Tok.is(tok::floating_literal) ||
+      (P.Tok.is(tok::unknown) && isdigit(P.Tok.getText()[0]))) {
+    // Per rdar://problem/32316666, using numbers for identifiers is a common
+    // error for beginners, so it's worth handling this in a special way.
+    P.diagnose(P.Tok, diag::number_cant_start_decl_name, DeclKindName);
+
+    // Pretend this works as an identifier, which shouldn't be observable since
+    // actual uses of it will hit random other errors, e.g. `1()` won't be
+    // callable.
+    Result = P.Context.getIdentifier(P.Tok.getText());
+    Loc = P.Tok.getLoc();
+    P.consumeToken();
+
+    // We recovered, so this is a success.
+    return makeParserSuccess();
+  }
+
   if (P.Tok.isKeyword()) {
     P.diagnose(P.Tok, diag::keyword_cant_be_identifier, P.Tok.getText());
     P.diagnose(P.Tok, diag::backticks_to_escape)
@@ -4708,7 +4725,12 @@ Parser::parseDeclFunc(SourceLoc StaticLoc, StaticSpellingKind StaticSpelling,
   Token NameTok = Tok;
   SourceLoc NameLoc;
 
-  if (Tok.is(tok::identifier) || Tok.isKeyword()) {
+  if (Tok.isAny(tok::identifier, tok::integer_literal, tok::floating_literal,
+                tok::unknown) ||
+      Tok.isKeyword()) {
+    // This non-operator path is quite accepting of what tokens might be a name,
+    // because we're aggressive about recovering/providing good diagnostics for
+    // beginners.
     ParserStatus NameStatus =
         parseIdentifierDeclName(*this, SimpleName, NameLoc, "function",
                                 tok::l_paren, tok::arrow, tok::l_brace,

test/Parse/number_identifier_errors.swift

@@ -0,0 +1,74 @@
+// RUN: %target-typecheck-verify-swift
+
+// Per rdar://problem/32316666 , it is a common mistake for beginners
+// to start a function name with a number, so it's worth
+// special-casing the diagnostic to make it clearer.
+
+func 1() {}
+// expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+func 2.0() {}
+// expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+func 3func() {}
+// expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+// expected-error@-2 {{expected a digit after integer literal prefix}}
+
+protocol 4 {
+  // expected-error@-1 {{protocol name can only start with a letter or underscore, not a number}}
+  associatedtype 5
+  // expected-error@-1 {{associatedtype name can only start with a letter or underscore, not a number}}
+}
+protocol 6.0 {
+  // expected-error@-1 {{protocol name can only start with a letter or underscore, not a number}}
+  associatedtype 7.0
+  // expected-error@-1 {{associatedtype name can only start with a letter or underscore, not a number}}
+}
+protocol 8protocol {
+  // expected-error@-1 {{protocol name can only start with a letter or underscore, not a number}}
+  // expected-error@-2 {{expected a digit after integer literal prefix}}
+  associatedtype 9associatedtype
+  // expected-error@-1 {{associatedtype name can only start with a letter or underscore, not a number}}
+  // expected-error@-2 {{expected a digit after integer literal prefix}}
+}
+
+typealias 10 = Int
+// expected-error@-1 {{typealias name can only start with a letter or underscore, not a number}}
+typealias 11.0 = Int
+// expected-error@-1 {{typealias name can only start with a letter or underscore, not a number}}
+typealias 12typealias = Int
+// expected-error@-1 {{typealias name can only start with a letter or underscore, not a number}}
+// expected-error@-2 {{expected a digit after integer literal prefix}}
+
+struct 13 {}
+// expected-error@-1 {{struct name can only start with a letter or underscore, not a number}}
+struct 14.0 {}
+// expected-error@-1 {{struct name can only start with a letter or underscore, not a number}}
+struct 15struct {}
+// expected-error@-1 {{struct name can only start with a letter or underscore, not a number}}
+// expected-error@-2 {{expected a digit after integer literal prefix}}
+
+enum 16 {}
+// expected-error@-1 {{enum name can only start with a letter or underscore, not a number}}
+enum 17.0 {}
+// expected-error@-1 {{enum name can only start with a letter or underscore, not a number}}
+enum 18enum {}
+// expected-error@-1 {{enum name can only start with a letter or underscore, not a number}}
+// expected-error@-2 {{expected a digit in floating point exponent}}
+
+class 19 {
+  // expected-error@-1 {{class name can only start with a letter or underscore, not a number}}
+  func 20() {}
+  // expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+}
+class 21.0 {
+  // expected-error@-1 {{class name can only start with a letter or underscore, not a number}}
+  func 22.0() {}
+  // expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+}
+
+class 23class {
+  // expected-error@-1 {{class name can only start with a letter or underscore, not a number}}
+  // expected-error@-2 {{expected a digit after integer literal prefix}}
+  func 24method() {}
+  // expected-error@-1 {{function name can only start with a letter or underscore, not a number}}
+  // expected-error@-2 {{expected a digit after integer literal prefix}}
+}