Fix substitution parts having a shifted underline in some cases

[ShE3py]

If two suggestions parts are side by side, the underline's offset:
(WIP PR as an example, not yet pushed)

error: expected a pattern, found an expression
 --> ./main.rs:4:9
  |
4 |         1 + 2 => 3
  |         ^^^^^ arbitrary expressions are not allowed in patterns
  |
help: check the value in an arm guard
  |
4 |         n if n == 1 + 2 => 3
  |         ~     +++++++++++++

The emitter didn't take into account that the string had shrunk/grown if two substitution parts were side-by-side (surprisingly, there was only one case in the ui testsuite.)

help: check the value in an arm guard
  |
4 |         n if n == 1 + 2 => 3
  |         ~ +++++++++++++

@rustbot label +A-suggestion-diagnostics

[rustbot]

r? @fee1-dead

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[fee1-dead]

If two suggestions parts are side by side, the underline's offset: (WIP PR as an example, not yet pushed)

error: expected a pattern, found an expression
 --> ./main.rs:4:9
  |
4 |         1 + 2 => 3
  |         ^^^^^ arbitrary expressions are not allowed in patterns
  |
help: check the value in an arm guard
  |
4 |         n if n == 1 + 2 => 3
  |         ~     +++++++++++++

Please include this as an UI test.

@rustbot author

[ShE3py]

I'm still working on PR #123877 which makes this suggestion, here is the blessed UI tests after this PR: a580d10

The suggestion is emitted here:

rust/compiler/rustc_parse/src/parser/item.rs

Lines 2450 to 2455 in a580d10

	err.subdiagnostic(&that.parser.dcx(), UnexpectedExpressionInPatternArmSugg::CreateGuard {
	ident_span,
	pat_hi: arm.pat.span.shrink_to_hi(),
	ident,
	expr: expr.clone(),
	});

rust/compiler/rustc_parse/src/errors.rs

Lines 2450 to 2461 in a580d10

	CreateGuard {
	/// The span of the `PatKind:Err` to be transformed into a `PatKind::Ident`.
	#[suggestion_part(code = "{ident}")]
	ident_span: Span,
	/// The end of the match arm's pattern.
	#[suggestion_part(code = " if {ident} == {expr}")]
	pat_hi: Span,
	/// The suggested identifier.
	ident: String,
	/// `ident_span`'s snippet.
	expr: String,
	},

@rustbot review

[fee1-dead]

Alright, thanks!
@bors r+ rollup

[bors]

📌 Commit b52e4bd has been approved by fee1-dead

It is now in the queue for this repository.

[bors]

⌛ Testing commit b52e4bd with merge 35f5b73...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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[fee1-dead]

@bors retry: appears spurious

[fee1-dead]

@bors retry