[Analyzer] Recommend string.ToLower{Invariant}().Contains/IndexOf/StartsWith("lowercasestring") be replaced by case-insensitive comparison

[stephentoub]

There's a significant amount of code that's apparently trying to perform case-insensitive comparisons by first lower-casing and then comparing against a lowercased string, e.g.
https://grep.app/search?q=.ToLower%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.StartsWith&filter[lang][0]=C%23
Less but still many for ToUpper with upper-case, e.g.
https://grep.app/search?q=.ToUpper%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.StartsWith&filter[lang][0]=C%23
and with the Invariant variations, e.g.
https://grep.app/search?q=.ToLowerInvariant%28%29.Contains&filter[lang][0]=C%23

We should write an analyzer that flags these patterns and recommends comparisons using a StringComparison instead.

Such analyzers will help avoid the allocations caused by ToLower/ToUpper calls.

Performance rules Category
Severity = suggestion

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichalPetryka]

Should the invariant variants be also changed? Ignore this, wanted to post it in #78607.

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Issue Details

---

There's a significant amount of code that's apparently trying to perform case-insensitive comparisons by first lower-casing and then comparing against a lowercased string, e.g.
https://grep.app/search?q=.ToLower%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.StartsWith&filter[lang][0]=C%23
Less but still many for ToUpper with upper-case, e.g.
https://grep.app/search?q=.ToUpper%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.StartsWith&filter[lang][0]=C%23
and with the Invariant variations, e.g.
https://grep.app/search?q=.ToLowerInvariant%28%29.Contains&filter[lang][0]=C%23

We should write an analyzer that flags these patterns and recommends comparisons using a StringComparer instead.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Runtime, code-analyzer, code-fixer
Milestone:	8.0.0

Tagging subscribers to this area: @dotnet/area-system-globalization
See info in area-owners.md if you want to be subscribed.

Issue Details

---

There's a significant amount of code that's apparently trying to perform case-insensitive comparisons by first lower-casing and then comparing against a lowercased string, e.g.
https://grep.app/search?q=.ToLower%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToLower%28%29.StartsWith&filter[lang][0]=C%23
Less but still many for ToUpper with upper-case, e.g.
https://grep.app/search?q=.ToUpper%28%29.Contains&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.IndexOf&filter[lang][0]=C%23
https://grep.app/search?q=.ToUpper%28%29.StartsWith&filter[lang][0]=C%23
and with the Invariant variations, e.g.
https://grep.app/search?q=.ToLowerInvariant%28%29.Contains&filter[lang][0]=C%23

We should write an analyzer that flags these patterns and recommends comparisons using a StringComparer instead.

Such analyzers will help avoid the allocations caused by ToLower/ToUpper calls.

Performance rules Category
Severity = suggestion

Author:	stephentoub
Assignees:	-
Labels:	area-System.Globalization, area-System.Runtime, code-analyzer, code-fixer
Milestone:	8.0.0