Tracking issue for analyzers/fixers to convert existing code to collection expressions.

[CyrusNajmabadi]

Cases to support:

This will be a large set of work items. So i'm breaking things into pieces to make it easier to review. The pieces are at least:

• Convert arrays and implicit array creation with initializers over to collection expressions. e.g. int[] x = { 1, 2, 3 } => int[] x = [1, 2, 3];. Initial support for a set of analyzers/fixers to update existing code to use collection expressions. #69118
• Update our existing 'use collection initializer' analyzer/fixer to detect 'AddRange' calls and update to ... e.g. x.Add(1); x.AddRange(y) => [1, ..y]. Update "use collection initializer" to use a collection-expression if the user prefers that form. #69219
• Direct calls to methods like Array.Empty<int>() and ImmutableXXX<int>.Empty. Add analyzer/fixer to update expressions like Array.Empty<T>() over to using the [] collection expression. #69279
• Detect if (x) y.Add(z) => .. x ? [z] : []. Add support for generating the .. x ? [y] : [] pattern in a collection-expression #69280
• Convert collection initializers. e.g. new List<int>{ 1, 2, 3 } => [1, 2, 3]. Add support for converting existing collection-initializers over to collection expressions. #69321
• Update our existing 'use collection initializer analyzer/fixer to detect a foreach+Add call and update to ... Add support for converting existing collection-initializers over to collection expressions. #69321
• Update our existing 'use collection initializer' analyzer/fixer to detect when it can jump from the starting point directly to an collection expression. Add support for converting existing collection-initializers over to collection expressions. #69321
• Convert stackalloc'ed spans. Add analyzer/fixer to recommend updating stackalloc expressions to use collection expressions. #69414
• Convert arrays that are constructed, and then assigned with statements over to collection expressions. e.g. var x = new string[3]; x[0] = ""; ... Support converting arrays, whose elements are assigned afterwards, to a collection expression. #69415
• Convert collections that have the 'CollectionBuilderAttribute'. Look for usages of things like ImmutableArray.Create(1, 2, 3) => [1, 2, 3]. Add analyzer/fixer to suggest changing code like ImmutableArray.Create(1, 2, 3) to [1, 2, 3] #69473
• Convert collections that have the 'CollectionBuilderAttribute'. Look for usages of things like ImmutableArray.CreateBuilder(); builder.Add ... Support Add/AddRange/etc. Add analyzer/fixer to suggest changing code like ImmutableArray.CreateBuilder<int>(); ... to a collection expression. #69500
• Convert arrays converted to spans with .AsSpan(). Add analyzer/fixer to suggest changing code fluent collection building to a collection expression #69580
• Convert collections created with linq-like helpers. a.Concat(b).ToImmutableArray() => [..a, ..b]. Add analyzer/fixer to suggest changing code fluent collection building to a collection expression #69580

Stretch cases:

• Inline arrays. Likely not impactful enough to warrant special fixes here.

The intent here is to also provide different diagnostic IDs for varying classes of these. For example "for arrays", "for spans", etc. We may want to revise this as appropriate as we continue with this.

Relates to test plan #66418 (collection expressions compiler feature)

[CyrusNajmabadi]

Closing out. This work item is complete. Further work can come in as bugs.