Planning Eclipse4.8 JEP286

Planning page for JDT Implementation of Local Variable Type Inference (JEP 286) for Photon (4.8)

Main tracking bug: JEP286 Bug

Compiler changes

Changes regarding the compiler: Compiler Bug

Grammar changes

Since var is not a keyword, but a reserved type name AND we need to be backwards compatible anyway, it should be handled in type resolution instead of grammar. Local variable type inference should be denoted by a bit AST node and type binding.

Type checking

• Determine the type of the var declaration
• Determine upwards/downwards type projection for type
• Fallback to "Object" type in case of error

Error reports

• Errors where local variable type cannot be inferred:
  • Cannot use 'var' on variable without initializer
  • Lambda expression needs an explicit target-type
  • Variable initializer is 'null'
  • Inferred type is 'void'
  • When method reference needs an explicit target-type.
  • array initializer needs an explicit target-type
• Error when defining a type called 'var':

This should be a configurable 'warning'/'info' for Java 9 and earlier.

Code generation

Shouldn't be much work since the stack frame would be filled in with the bound type anyway.

Java Search support

Changes regarding the Search: Search Bug

DOM support

Changes regarding the DOM: DOM Bug

Content Assist support

• Top level bug at https://bugs.eclipse.org/bugs/show_bug.cgi?id=529556
• var to be included in completion - https://bugs.eclipse.org/bugs/show_bug.cgi?id=529552
• type. completions if the type is var - tracked via https://bugs.eclipse.org/bugs/show_bug.cgi?id=529556
• name of the variable? Default camel case type to a little alphabet prefixed name will not work here - alternatives (low prio).
• changes to priority of proposals ?

UI Changes

• Switch ASTs to JLS10 - bug 530298, bug 531142.

Code hover

• Show inferred type of variable - bug 531041.

Preferences

• Allow JDT UI to target Java 10 in Compiler Preferences - bug 529432
• Clean up option to prefer 'var' over specific types (to control the irritants generated by the compiler) - [based on need - low prio].

Quick assists

• Convert 'var' into the resolved type - bug 531039.
• Convert explicit typed local variable into 'var', like changing "String coolIde = "Eclipse";" into "var coolIde = "Eclipse";" It should also include: Convert diamond operator to var, like fixing "List names = new LinkedList<>()" into "var names = new LinkedList()" - bug 531040.

Code templates

• New templates with 'var' in for loops and introduce new variable - [can be added if useful - need examples - low prio].