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Previously, `assert_eq!(left, right,)` (respectively, `assert_ne!(left,
right,)`; note the trailing comma) would result in a confusing "requires
at least a format string argument" error. In reality, a format string is
optional, but the trailing comma puts us into the "match a token tree of
zero or more tokens" branch of the macro (in order to support the
optional format string), and passing the empty token tree into
`format_args!` results in the confusing error. If instead we match a
token tree of one or more tokens, we get a much more sensible
"unexpected end of macro invocation" error.
While we're here, fix up a stray space before a comma in the match
guards.
Resolves #39369.
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This teaches `format_args!` how to interpret format printf- and
shell-style format directives. This is used in cases where there are
unused formatting arguments, and the reason for that *might* be because
the programmer is trying to use the wrong kind of formatting string.
This was prompted by an issue encountered by simulacrum on the #rust IRC
channel. In short: although `println!` told them that they weren't using
all of the conversion arguments, the problem was in using printf-syle
directives rather than ones `println!` would undertand.
Where possible, `format_args!` will tell the programmer what they should
use instead. For example, it will suggest replacing `%05d` with `{:0>5}`,
or `%2$.*3$s` with `{1:.3$}`. Even if it cannot suggest a replacement,
it will explicitly note that Rust does not support that style of directive,
and direct the user to the `std::fmt` documentation.
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- Fixes #36164
- Part of #35233
- handles unknown fields
- uses UI-style tests
- update all related tests (cfail, ui, incremental)
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