gopls/go/analysis/doc/suggested

1	# Suggested Fixes in the Analysis Framework
2
3	## The Purpose of Suggested Fixes
4
5	The analysis framework is planned to add a facility to output
6	suggested fixes. Suggested fixes in the analysis framework
7	are meant to address two common use cases. The first is the
8	natural use case of allowing the user to quickly fix errors or issues
9	pointed out by analyzers through their editor or analysis tool.
10	An editor, when showing a diagnostic for an issue, can propose
11	code to fix that issue. Users can accept the proposal and have
12	the editor apply the fix for them. The second case is to allow
13	for defining refactorings. An analyzer meant to perform a
14	refactoring can produce suggested fixes equivalent to the diff
15	of the refactoring. Then, an analysis driver meant to apply
16	refactorings can automatically apply all the diffs that
17	are produced by the analysis as suggested fixes.
18
19	## Proposed Suggested Fix API
20
21	Suggested fixes will be defined using the following structs:
22
23	```go
24	// A SuggestedFix is a code change associated with a Diagnostic that a user can choose
25	// to apply to their code. Usually the SuggestedFix is meant to fix the issue flagged
26	// by the diagnostic.
27	type SuggestedFix struct {
28	// A description for this suggested fix to be shown to a user deciding
29	// whether to accept it.
30	Message string
31	TextEdits []TextEdit
32	}
33
34	// A TextEdit represents the replacement of the code between Pos and End with the new text.
35	type TextEdit struct {
36	// For a pure insertion, End can either be set to Pos or token.NoPos.
37	Pos token.Pos
38	End token.Pos
39	NewText []byte
40	}
41	```
42
43	A suggested fix needs a message field so it can specify what it will do.
44	Some analyses may not have clear cut fixes, and a suggested fix may need
45	to provide additional information to help users specify whether they
46	should be added.
47
48	Suggested fixes are allowed to make multiple
49	edits in a file, because some logical changes may affect otherwise
50	unrelated parts of the AST.
51
52	A TextEdit specifies a Pos and End: these will usually be the Pos
53	and End of an AST node that will be replaced.
54
55	Finally, the replacements themselves are represented as []bytes.
56
57
58	Suggested fixes themselves will be added as a field in the
59	Diagnostic struct:
60
61	```go
62
63	type Diagnostic struct {
64	...
65	SuggestedFixes []SuggestedFix // this is an optional field
66	}
67
68	```
69
70	### Requirements for SuggestedFixes
71
72	SuggestedFixes will be required to conform to several requirements:
73
74	* TextEdits for a SuggestedFix should not overlap.
75	* TextEdits for SuggestedFixes should not contain edits for other packages.
76	* Each TextEdit should apply to a single file.
77
78	These requirements guarantee that suggested fixes can be cleanly applied.
79	Because a driver may only analyze, or be able to modify, the current package,
80	we restrict edits to the current package. In general this restriction should
81	not be a big problem for users because other packages might not belong to the
82	same module and so will not be safe to modify in a singe change.
83
84	On the other hand, analyzers will not be required to produce gofmt-compliant
85	code. Analysis drivers will be expected to apply gofmt to the results of
86	a SuggestedFix application.
87
88	## SuggestedFix integration points
89
90	### ```checker -fix```
91
92	Singlechecker and multichecker have the ```-fix``` flag, which will automatically
93	apply all fixes suggested by their analysis or analyses. This is intended to
94	be used primarily by refactoring tools, because in general, like diagnostics,
95	suggested fixes will need to be examined by a human who can decide whether
96	they are relevant.
97
98	### gopls
99
100	Suggested fixes have been integrated into ```gopls```, and editors can choose
101	to display the suggested fixes to the user as they type, so that they can be
102	accepted to fix diagnostics immediately.
103
104	### Code Review Tools (Future Work)
105
106	Suggested fixes can be integrated into programs that are integrated with
107	code review systems to suggest fixes that users can apply from their code review tools.
108
109	## Alternatives
110
111	### Performing transformations directly on the AST
112
113	Even though it may be more convenient
114	for authors of refactorings to perform transformations directly on
115	the AST, allowing mutations on the AST would mean that a copy of the AST
116	would need to be made every time a transformation was produced, to avoid
117	having transformations interfere with each other.
118
119	This is primarily an issue with the current design of the Go AST and
120	it's possible that a new future version of the AST might make this a more
121	viable option.
122
123	### Supplying AST nodes directly
124
125	Another possibility would be for SuggestedFixes to supply the replacement
126	ASTs directly. There is one primary limitation to this: that because
127	comments to ASTs specify their location using token.Pos values, it's very
128	difficult to place any comments in the right place.
129
130	In general, it's also more difficult to generate the AST structures for
131	some code than to generate the text for that code. So we prefer to allow
132	the flexibility to do the latter.
133
134	Because users can call ```format.Node``` to produce the text for any
135	AST node, users will always be able to produce a SuggestedFix from AST
136	nodes. In future, we may choose to add a convenience method that does this for users.
137

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