Tracking Issue for improving suggestion rendering in diagnostics

[estebank]

This is a tracking issue for the prioritizing "verbose" or "hidden" suggestions over "inline" (the default given by span_suggestion suggestions.

Inline suggestions have several issues that make them less readable than verbose ones:

• I've seen people get confused by them
• Small differences caused by typos are much better seen on the verbose output, where the highlighting now points at the exact character, instead of having to visually notice the different text in non-aligned places
• The inline output was the original implementation, but since we've introduced the diff output which is clearer on what we expect the user to write
• For additions, things like "add a type parameter: " are even more unclear
• Relying on the inline suggestions have lead us to use worse spans on purpose because otherwise the rendering would be bad (like skipping whitespace in some cases) and has a tendency to make suggestion mistakes harder to notice (broken suggestions making it in because we couldn't tell during review)
• When a single suggestion is in the diagnostic, the default renders inline, but if there are two, then both get rendered as verbose, which makes the output sometimes sensitive to what was emitted and can cause two similar errors to look very different.

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Steps

• Audit all cases of span_suggestion uses.
• Adjust individual cases to be either span_suggestion_verbose, span_suggestion_hidden or span_suggestion_tool_only
• Change span_suggestion to span_suggestion_inline and span_suggestion_verbose to span_suggestion

Implementation history

• Make some suggestions "verbose" #137343
• Include whitespace in "remove |" suggestion and make it hidden #137872
• Structured suggestion for extern crate foo when foo isn't resolved in import #128151
• Fix "missing match arm body" suggestion involving ! #137435
• Fix incorrect cfg structured suggestion and make suggestion verbose #137773
• Tweak auto trait errors #137831
• Tweak invalid RTN errors #137824
• Make inline suggestions no longer be the default #127282

https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/Why.20are.20we.20making.20suggestions.20verbose.3F

[compiler-errors]

This work is also enabled by:

• Fix presentation of purely "additive" replacement suggestion parts #136958
• More sophisticated span trimming for suggestions #137348

which improve the presentation of verbose suggestions when they're "purely additive", which should help reduce the impact of this work.