fmt: do not warn on {:.}

Author: hkBst

compiler/rustc_parse_format/src/lib.rs

@@ -655,7 +655,15 @@ impl<'input> Parser<'input> {
  spec.precision = self.count();
  }
  let span = range.start..self.input_vec_index2range(self.input_vec_index).start;
- if spec.precision == CountImplied {
+ if spec.precision == CountImplied &&
+ // make an exception for "{:.}":
+ // broad exception:
+ // !self.input.contains("{:.}")
+ // narrow exception:
+ // !self.input.starts_with("{:.}")
+ // very narrow exception:
+ self.input != "{:.}" && self.input != "{:.}\n"
+ {
  self.warnings.push(ParseError {
  description: "expected numerical precision after precision specifier"
  .to_string(),

tests/ui/fmt/warn-on-empty-precision.rs

@@ -1,6 +1,8 @@
 //@ run-pass
 fn main() {
  let float = 0.123_456_789;
+ // this special case is exempted from warning
+ println!("{:.}", float);
  println!("Missing precision: {float:.} {float:3.} {float:_^12.}");
  //~^ WARNING invalid format string: expected numerical precision after precision specifier
  //~| WARNING invalid format string: expected numerical precision after precision specifier

tests/ui/fmt/warn-on-empty-precision.stderr

@@ -1,21 +1,21 @@
 warning: invalid format string: expected numerical precision after precision specifier
- --> $DIR/warn-on-empty-precision.rs:4:41
+ --> $DIR/warn-on-empty-precision.rs:6:41
  |
 LL | println!("Missing precision: {float:.} {float:3.} {float:_^12.}");
  | ^ precision specifier without numerical precision in format string
  |
  = note: This may become an error in a future release
 
 warning: invalid format string: expected numerical precision after precision specifier
- --> $DIR/warn-on-empty-precision.rs:4:52
+ --> $DIR/warn-on-empty-precision.rs:6:52
  |
 LL | println!("Missing precision: {float:.} {float:3.} {float:_^12.}");
  | ^ precision specifier without numerical precision in format string
  |
  = note: This may become an error in a future release
 
 warning: invalid format string: expected numerical precision after precision specifier
- --> $DIR/warn-on-empty-precision.rs:4:66
+ --> $DIR/warn-on-empty-precision.rs:6:66
  |
 LL | println!("Missing precision: {float:.} {float:3.} {float:_^12.}");
  | ^ precision specifier without numerical precision in format string