Provide ability for caller to seed/re-use symbol table across compile jobs (#294)

In addition to allowing a later compilation job to detect collisions
with symbols defined in earlier jobs, this also gives the caller access
to an index of all symbols, which can be used to implement an efficient
resolver over all results of compilation.

Author: jhump

compiler.go

@@ -89,6 +89,15 @@ type Compiler struct {
 	// will be removed as soon as it's no longer needed. This can help reduce
 	// total memory usage for operations involving a large number of files.
 	RetainASTs bool
+
+	// If non-nil, the set of symbols already known. Any symbols in the current
+	// compilation will be added to it. If the compilation tries to redefine any
+	// of these symbols, it will be reported as a collision.
+	//
+	// This allows a large compilation to be split up into multiple, smaller
+	// operations and still be able to identify naming collisions and extension
+	// number collisions across all operations.
+	Symbols *linker.Symbols
 }
 
 // SourceInfoMode indicates how source code info is generated by a Compiler.
@@ -140,12 +149,16 @@ func (c *Compiler) Compile(ctx context.Context, files ...string) (linker.Files,
 
 	h := reporter.NewHandler(c.Reporter)
 
+	sym := c.Symbols
+	if sym == nil {
+		sym = &linker.Symbols{}
+	}
 	e := executor{
 		c:       c,
 		h:       h,
 		s:       semaphore.NewWeighted(int64(par)),
 		cancel:  cancel,
-		sym:     &linker.Symbols{},
+		sym:     sym,
 		results: map[string]*result{},
 	}